NQMA-EXP-04 Au+Au around the Balance Energy 120INDRA-ALADIN Collaboration
Authors: Lukasik, J.; Trautmann, W.
Keywords/PACS: 25.70.Mn; 25.70.Pq; 25.40.Sc
NQMA-EXP-05 Isotopic Scaling and the Symmetry Energy in Spectator Fragmentation 121INDRA-ALADIN Collaboration
Experiment Proposal Number: S201
Authors: Le Fevre, A.; Botvina, A.; Trautmann, W.
Keywords/PACS: 25.70.Mn; 25.70.Pq; 24.10.Pa
NQMA-EXP-06 Gross Properties of Multifragment Decay at Relativistic Energies 122Authors: Sfienti, C.; ALADiN2000 Collaboration
NQMA-EXP-07 Flow and stopping in Ni+Ni and Pb+Pb at 0.4, 0.8 and 1.16A GeV 123FOPI CollaborationAuthors: Xiao, Z. G.; Andronic, A.; Hartmann, O. N.; Herrmann, N.; Hildenbrand, K. D.; Hong, B.;Kim, Y. J.; Kirejczyk, M.; Koczon, P.; Leifels, Y.; Reisdorf, W.; Schuttauf, A.; Tyminski, Z.
NQMA-EXP-08 Strangeness production with π−- beam at 1.15 GeV/c 124FOPI CollaborationAuthors: Benabderrahmane, M. L.; Cordier, E.; Herrmann, N.; Leifels, Y.; Mangiarotti, A.; Mer-schmeyer, M.Keywords/PACS: 25.80.-e; 25.80.Hp; 25.75.Dw
NQMA-EXP-09 Neutral strange particle production in Ni+Ni collisions at 1.93 AGeV 125FOPI Collaboration
Authors: Lopez, X.; Merschmeyer, M.
Keywords/PACS: 25.75.-q; 25.75.Dw
NQMA-EXP-00
115
NQMA-EXP-10 Analysis of Λ-Deuteron Correlations in Ni+Ni reactions 127FOPI Collaboration
Authors: Herrmann, N.; Benabderrahmane, M. L.; Cordier, E.; Mangiarotti, A.; Merschmeyer, M.
NQMA-EXP-11 Search for K Nuclear Clusters in Nuclear Collisions with FOPI 128FOPI Collaboration
NQMA-EXP-12 Azimuthal angular emission patterns of K+ and K− from Au+Au at 1.5 AGeV 129KAOS CollaborationAuthors: Ploskon, M.; Grosse, E.; Forster, A.; Koczon, P.; Kohlmeyer, B.; Lang, S.; Naumann, L.;Oeschler, H.; Scheinast, W.; Schmah, A.; Schuck, T.; Schwab, E.; Senger, P.; Shin, Y.; Strobele, H.;Sturm, C.; Uhlig, F.; Wagner, A.; Walus, W.
NQMA-EXP-13 Dielectron production in C+C and pp collisions 130HADES Collaboration
Author: HADES CollaborationKeywords: nuclear matter; dielectrons; medium modifications
NQMA-EXP-14 Pion and kaon production in A+A collisions at low SPS energies 133NA49 CollaborationAuthors: Alt, C.; Blume, C.; Dinkelaker, P.; Flierl, D.; Friese, V.; Gazdzicki, M.; Hoehne, C.; Kniege,S.; Kraus, I.; Kliemant, M.; Lungwitz, B.; Meurer, C.; Mitrovski, M.; Renfordt, R.; Richard, A.;Sandoval, A.; Schuster, T.; Stock, R.; Strabel, C.; Strobele, H.
NQMA-EXP-00
116
Probing the Coulomb field in relativistic collisionsP.Napolitani1,2,* for the CHARMS collaboration
1GSI, Darmstadt, Germany; 2IPN, Orsay, France
Two main questions determine the recent directions of the research in nuclear dynamics. One is the role of the Cou-lomb field in diluted nuclear matter: in cosmological ob-jects (e.g. neutron stars, supernovae) theoretical studies relate it to the formation of crystalline structures by frus-tration effects [1]. The Coulomb field can be probed ex-perimentally at the nuclear scale, where it rules the mechanism of disintegration in multifragmentation reac-tions. Another question is the research of the conditions (density and excitation) that determine the appearing of a bimodal behaviour of the heaviest-residue distribution, which can be related to the order parameter of a liquid-gas phase transition [2].
Fig. 1. Normalized experimental velocity spectra, measured for a series of isotopes having N = Z + 1 in the systems 136Xe+1H (left) and 136Xe+Ti (right) at 1 A GeV.
Among the inclusive measurements, the FRS operated in inverse kinematics provides unique experimental informa-tion about the reaction kinematics. The high resolution and absence of energy thresholds in the detection of ejectiles allow for extracting the full longitudinal-velocity spectrum of each nuclide formed as a reaction fragment [3]. More-over, the possibility of employing liquid-hydrogen and heavy targets allows for studying the largest variation of excitation energy at a given incident energy in relativistic collisions. In Fig. 1, a series of longitudinal-velocity spectra recently measured for the 136Xe+1H, Ti systems [4], is
shown for twenty isotopes having N=Z+1. The velocities are shown in the reference frame of the projectile. For both reactions, each spectrum can be decomposed in a double-humped shape and a wide Gaussian-like compo-nent. The widths and the relative weights of these com-ponents vary as a function of the isotope. Two wide humps are mostly evident in the lightest isotopes pro-duced in 136Xe+1H. Also the system 136Xe+Ti manifests the presence of a two-humped component for isotopes lighter than 15N. As the two humps are largely spaced, the system 136Xe+1H shows clearly the superposition of the Gaussian-like and the two-humped contribution. The high resolution of the spectra even shows that the central Gaussian-like component is shifted in forward directions with respect to the two-humped component.
Multifragmentation reactions are characterised by the simultaneous disintegration of the nuclear system into several fragments. The concept of simultaneity is related to the Coulomb field: the nucleus disassembles so rapidly (10-22-10-21s) that the fragments can exchange mutual interactions while they are accelerated by the Coulomb field of the system. For this reason, the shape of the ve-locity distributions is directly associated to the multiplic-ity and size of fragments. Two-humped shapes reflect an asymmetric split: we observe one light fragment repelled by the heavy partner at velocities that even exceed fission velocities [4]. The central component is associated with more complex emission mechanisms such as a volume emission of several light fragments. The forward shift of the central shape with respect to the two-humped compo-nent is rather surprising. The central component is related to more violent collisions resulting in volume emission. If we associate more violent collisions only to larger fric-tional effect, we would expect a shift of the central com-ponent in the opposite direction that we observe. The forward peaking of the emission was also measured in ion-ion collisions at the FRS and related to “blast” effects induced on the spectator by the fire-ball [5]. A similar ‘highly unusual feature’ was observed in proton-induced collisions at high-energy and interpreted as the effect of shock waves [6]. In our experiment such a feature was observed for the first time in proton-induced collisions in the 1 A GeV incident-energy range. * Present addr.: GANIL, Blvd. H. Becquerel, Caen, France
References [1] Horowitz et al., Phys. Rev. C69 (2004) 045804 [2] Ph. Chomaz et al., Phys. Rev. E64 (2001) 046114 [3] P. Napolitani et al., Phys. Rev. C70 (2004) 054607 [4] P. Napolitani, PhD Thesis, Univ. Paris XI, Sept. 2004 [5] M.V. Ricciardi et al., Phys. Rev. Lett. 90 (2003) 212302 [6] N.T. Porile et al., Phys. Rev. Lett. 43 (1979) 918
NQMA-EXP-01
117
Investigation of isotopic composition of heavy residues produced in the fragmentation of 124Xe and 136Xe beams
D.Henzlovaa,*for the CHARMS collaboration aGSI, Germany
The isotopic compositions of the residues produced in the heavy-ion collisions attain an increasing interest since they provide information on the nuclear equation of state. The range of the isotopes presently available for similar investi-gations is, however, constrained by the mass resolution of the majority of the large-acceptance experimental devices. This corresponds to the mass range generally not exceeding A ~ 20. With the use of a high-resolution magnetic spec-trometer, such as the Fragment Separator (FRS) at GSI, mass identification may be extended over the whole mass range up to the mass of the heavy projectile.
Two dedicated experiments on the investigation of the isotopic composition of the residues produced in the frag-mentation of 124Xe and 136Xe beams in a lead target at 1 A GeV have been performed at the FRS. The main focus was to provide fully isotopically identified residues in the whole mass range to study the dependence of the final isotopic composition on the N/Z of the projectile. The experiments have been performed using the standard FRS set-up with several settings of magnetic fields to cover fragments with different N/Z. More details on the experimental technique may be found in [1].
Figure 1: The identification pattern for the reaction 136Xe+Pb at 1 A GeV, in logarithmic color scale. Yields of different isotopes are not normalized to the beam doses.
Figure 1 displays the residues identified in the charge range 25<Z<56 in the fragmentation of 136Xe. A clear sepa-ration between the single isotopes is observed together with a broad range of isotopes measured for each element.
Figure 2 shows the mean N-over-Z (<N>/Z) ratio deduced from the measured isotopic distributions. The <N>/Z of the residues close to the projectile decreases rather steeply with charge as a consequence of the dominating neutron evapora-tion. For lower charges (higher excitation energies) the <N>/Z changes more moderately and surprisingly the dif-
ference between the final <N>/Z is preserved over the whole charge range. It has been shown [2] that the isotopic compo-sition of the final residues after the long sequential evapora-tion gradually approaches the region of equilibrium prob-abilities of proton and neutron emission, known as the evaporation-attractor line (EAL). For high excitation ener-gies, the final isotopic composition should therefore no longer depend on the N/Z of the initial system. The differ-ence between the <N>/Z of residues from 136Xe and 124Xe projectiles preserved in the whole nuclear-charge range may be interpreted as an indication for the break-up of a highly excited nucleus. In the break-up process part of the initial excitation energy is consumed in the formation of smaller fragments, reducing thus the excitation energy available for further evaporation.
Figure 2: Comparison of the <N>/Z of the fragmentation residues from 136Xe and 124Xe at 1 A GeV.
Thanks to the full isotopic identification available with the FRS, the connection between the <N>/Z of the final resi-dues and the N/Z of the fragmenting projectile is followed over the full nuclear-charge range. Provided the N/Z evolu-tion in the dynamical stage of the reaction is known, this is particularly well suited to study the sensitivity of the final isotopic composition to the conditions at the freeze-out of the break-up stage as proposed in [3].
* on leave from NPI, Rez, Czech Republic
References [1] T. Enqvist et al., Nucl. Phys. A 658, 47 (1999) [2] R.J. Charity, Phys. Rev. C 58 (1998) 1073 [3] K.-H. Schmidt et al., Nucl. Phys. A 710 (2002) 157
NQMA-EXP-02
118
Spectroscopy of η-mesic nuclei using recoilless (d,3He) reaction
Hans GeisselA, Albrecht GillitzerB, Kenji GomikawaC, Ryugo S. HayanoC, Satoru HirenzakiD, Kenta
ItahashiE, Masahiko IwasakiE, Paul KienleF,G, Karl LindbergH, Yuri LitvinovA, Gottfried
MunzenbergA, Haruhiko OutaE, Miki ShindoC, Ken SuzukiF, Per-Erik TegnerH, Agnieszka
TrzcinskaI, Helmut WeickA, Toshimitsu YamazakiJ, and Irina ZartovaH
AGesellschaft fur Schwerionenforschung, Darmstadt, Germany; BInstitut fur Kernphysik, Forschungszentrum Julich;CDepartment of Physics, University of Tokyo; DDepartment of Physics, Nara Women’s University; EAdvanced
Meson Science Laboratory, RIKEN; FPhysik-Department, Technische Universitat Munchen; GStefan Meyer Institutfur subatomare physik of the Austrian Academy of Sciences; HDepartment of Physics, Stockholm University; IHeavy
Ion Laboratory, Warsaw University ; JRI Beam Science Laboratory, RIKEN
Deeply bound pionic atoms were for the first time pro-duced and discovered in 1996 at GSI by a novel techniqueof using the 208Pb(d,3He) reaction at Td=600MeV [1].In contrast to the conventional method to produce exoticatoms, the pion is directly produced in the nuclear reactionand substitutes one of the valence neutrons in the targetnucleus. Fulfilling the zero-momentum transfer conditionadditionally delivers two unique features, namely the en-hancement of the cross section to form such deeply boundstates, and the selective population of the substitutionalstate, both of which are important for a precision determi-nation of the binding energy and the width of the states.We performed subsequently a series of experiments for aprecise measurement of the deeply bound pionic 1s state,from which we determined the s-wave pion-nucleus opticalpotential [2, 3, 4]. With their uniqueness and precision,the data on deeply bound pion-nucleus states initiated anexploration of their sensitivity to the partial restoration ofchiral symmetry in the nuclear medium [4, 5].
We extended the program to the spectroscopy of evenmore exotic systems, namely η meson-nucleus boundstates. Since the η meson is a neutral particle, the systemis bound solely by the strong interaction. Such a systemshould not be seen as an exotic atom but as a new kindof exotic nucleus. We use the 12C(d,3He) reaction withTd=3.5GeV provided from SIS. The experimental feasi-bility and the impact on chiral models are discussed inRefs. [6, 7] (Fig.1). A serious problem is the very highrate of protons from the break-up of the incident deuteronsreaching up to 108/s. We have developed a new type ofCherenkov counter (TORCH) [8], which is only sensitiveto the slow 3He nuclei (β � 0.87) while being totally blindto the fast background protons (β � 0.94) even at a rateof 108/s. This indispensable background suppression ca-pability of the experiment is realized by making use of thedifferent total reflection conditions of the photons at theradiator surface which are created at different Cherenkovemission angles for 3He and protons selected by the Frag-ment Separator (FRS). To achieve this, the proper refrac-tive index of the Cherenkov radiator has to be chosen.Simultaneously a good position resolution is ensured bymeasuring the photon intensity distribution as a functionof the x coordinate with PMTs (Fig.2). TORCH detectorsare placed at the central and final focal plane of the FRSin order to do Time-of-Flight measurements.
Although ready in the beginning of 2004, the TORCHdetector was further improved to have better rejection ca-pability for delta-electrons produced from background pro-
tons by increasing the quality of the radiator.
0
0.2
0.4
0.6
0.8
1
1.2
-40 -20 0 20 40 60 -20 0 20 40 60 -20 0 20 40 60
(0p3/2)-1 php
(0s 1/2)-1 sηp
E ex - E 0 [MeV ]
[nb/
(sr
MeV
)]d2 s
dE d
W
E ex - E 0 [MeV ] E ex - E 0 [MeV ]
(0p3/2)-1 php
(0s 1/2)-1 sηp
(0p3/2)-1 php
(0s 1/2)-1 sηp
(a) T -ρ approximation (b) C hiral doublet model (C =0.2) (c) C hiral unitary
Figure 1: The theoretical spectra of 12C(d,3He)11B⊗η re-action at Td=3.5 GeV calculated by (a) the t-ρ approxi-mation, (b) the chiral doublet model with C=0.2, and (c)the chiral unitary approach. From Ref. [7]
Figure 2: View inside the TORCH detector.
References
[1] T. Yamazaki et al. Z. Phys. A 355 (1996) 219[2] H. Gilg, K. Itahashi et al. Phys. Rev. C 62 (2000)
025201-025202.[3] H. Geissel et al. Phys. Rev. Lett. 88 (2002) 122301.[4] K. Suzuki et al., Phys. Rev. Lett. 94 (2004) 072302.[5] E.E. Kolomeitsev, N. Kaiser and W. Weise, Phys.
Rev. Lett. 90 (2003) 092501.[6] R.S. Hayano, S. Hirenzaki and A. Gillitzer, Eur. Phys.
J. A6 (1999) 99.[7] H. Nagahiro, D. Jido and S. Hirenzaki, Phys. Pev. C.
68 (2003) 035205.[8] T. Yoneyama, Master thesis, Tokyo Institute of Tech-
nology.
NQMA-EXP-03
119
Au+Au around the Balance Energy
J. Lukasik and W. Trautmann for the INDRA-ALADIN Collaboration
Gesellschaft fur Schwerionenforschung mbH, D-64291 Darmstadt, Germany
The study of the directed flow for the Au + Au reac-tions, measured with the 4π detector INDRA at the GSIfacility [1], has been continued and revealed new obser-vations [2]. The excitation function of the directed flowhas been found to increase monotonically with incidentenergy, contrary to [3], and to change its sign between 50and 60 AMeV. The observed anti-flow has been linked tothe strong Coulomb effects in Au + Au system.
Values for the balance energy in Au + Au, at which thedirected flow vanishes, have previously been determinedby extrapolating from higher energies [4, 5, 6], and also bysearching for the minimum of flow [3]. The extrapolationsyielded values between 47 and 56 AMeV. The parabolicexcitation function of flow reported in [3] showed a mini-mum at Ebal = 42± 4 MeV per nucleon. Contrary to thismeasurement, the present data yield a monotonic excita-tion function with the zero-crossing energy at 54± 4 MeVfor Z=2 particles (solid symbols in Fig. 1).
0 20 40 60 80 100-0.4
-0.2
0
0.2
12≤>0, ZCMAzim. Corr., y6≤>0, ZCMAzim. Corr., y
Azim. Corr., no cuts, no cutsz=pωQ-vect.,
mid
ry
| ∂/
1 v ∂
(MeV/nucleon)beamE
Figure 1: Excitation functions of the mid-rapidity slopes of the v1
parameter for Z = 2 particles and impact parameters 0-4 fm. The
symbols correspond to the indicated methods and conditions used to
define the reaction plane. The lines represent parabolic fits.
The parabolic excitation function of the flow parame-ter could be qualitatively reproduced, provided the sameupper limits Z ≤ 12 as in [3] or Z ≤ 6 were used in theanalysis (Fig. 1, open symbols).
The origin of the observed anti-flow in the present datais illustrated in Fig. 2. For peripheral collisions at highincident energy (top panels) the deflections of the projec-tile and target, as represented by the three-dimensional Q-vector, are small. The Coulomb repulsion from the heavyresidues leads to the apparent depression of helium yieldsnear the entrance-channel rapidities and to maximum in-tensities at lower absolute rapidity, as discussed in [7]. Thestronger deflection of mid-rapidity particles is evident fromthe rapid rise of v1 with ycm which, at ycm ≈ 0.2, starts
to be modified by the effect of the spectators. The re-sulting pattern appears similar to the two-component flowobtained in QMD calculations [8].
-0.1
0
0.1
8-10 fm
150 AMeV
-0.1
0
0.1
-0.1
0
0.1
8-10 fm
40 AMeV
-0.1
0
0.1
-0.2 0 0.2
-0.1
0
0.1
-0.2 0 0.20-2 fm
40 AMeV
-0.1
0
0.1
/mx
p
1v
cmy
Figure 2: Contour plots (in linear scale) of the in-plane component
of the transverse velocity (px/m) versus the center-of-mass rapidity
ycm (left column) and the coefficient v1 as a function of ycm (right
column) for Z = 2 particles and three selected cases of incident
energy and impact parameter as indicated. The arrows represent
the directions of the three-dimensional Q-vector.
At 40 AMeV, and the same peripheral impact-parameters (middle panels), the structure of v1 as a func-tion of ycm is qualitatively the same as at 150 AMeV butcompressed into a smaller range of absolute rapidity. Incentral collisions, at this energy, the distributions are evenmore compact (bottom left). This concentration of massand charge, apparently, causes the Z = 2 particles to bepreferentially deflected to the opposite side. The result-ing flow, evaluated around mid-rapidity, is negative (anti-flow). Clearly, Q-vectors that do not contain the momentaof the heavier fragments causing this deflection or shadow-ing will have different directions, and the flow measuredrelative to them may appear positive, leading to parabolicexcitation functions.
The commonly invoked picture of the attractive meanfield globally balancing the repulsive effect of the collisionsaround the balance energy seems to be too simple here toexplain the observed inversion of flow. The main differ-ence, as compared to lighter reaction systems for whichthis picture may hold, is the enlarged Coulomb field whichnot only has a strong impact on the entrance and exitchannel trajectories [8] but also manifests itself in largerecoil effects (larger Coulomb rings).
References
[1] J. Lukasik, W. Trautmann et al., GSI Scientific Report 2003.
[2] J. Lukasik et al., nucl-ex/0410030, Phys. Lett. B, In Press.
[3] D.J. Magestro et al., Phys. Rev. C 61 (2000) 021602.
[4] W.M. Zhang et al., Phys. Rev. C 42 (1990) 491.
[5] M.D. Partlan et al., Phys. Rev. Lett. 75 (1995) 2100.
[6] P. Crochet et al., Nucl. Phys. A 624 (1997) 755.
[7] J. Lukasik et al., Phys. Lett. B 566 (2003) 76.
[8] S. Soff et al., Phys. Rev. C 51 (1995) 3320.
NQMA-EXP-04
120
Isotopic Scaling and the Symmetry Energy in Spectator Fragmentation
A. Le Fevrea, A. Botvinaa,l, W. Trautmanna, and the ALADIN-INDRA Collaborationa,b,c,d,e,f,g,h,i,j,k
The growing interest in isospin effects in nuclear reac-tions is motivated by an increasing awareness of the im-portance of the symmetry term in the nuclear equation ofstate, in particular for astrophysical applications. Super-nova simulations or neutron star models require inputs forthe nuclear equation of state at extreme values of densityand asymmetry [1, 2, 3]. The demonstration in the lab-oratory of the effects of the symmetry term at abnormaldensities is, therefore, an essential first step within a pro-gram aiming at gaining such information experimentally[4, 5].
Isotopic scaling, or “isoscaling”, a phenomenon commonto many different types of heavy ion reactions [6, 7, 8, 9].is observed by comparing product yields from otherwiseidentical reactions with isotopically different projectiles ortargets. It is constituted by an exponential dependence ofthe measured yield ratios R21(N, Z) on the neutron num-ber N and proton number Z of the considered product,following the relation:
The data were obtained with INDRA at GSI, for the col-lisions of 12C at 300 and 600 A.MeV on enriched targets of112Sn and 124Sn [10]. Selected light isotopes (Z ≤ 5) wereidentified over the range of polar angles 45◦ ≤ θlab ≤ 176◦.For impact-parameter selection, the charged-particle mul-tiplicity MC measured with the full detector was used. Toreduce preequilibrium contributions, upper limits were setfor hydrogen and helium isotopes. The ratios of the frag-ment yields measured for the two reactions obey the lawof isoscaling, as illustrated in Fig. 1. It shows the scaledisotopic ratios S(N) = R21(N, Z)/exp(βZ) (the indices 1and 2 denote the neutron poor and neutron rich system,respectively). Their slope parameters change considerablywith impact parameter, extending from α = 0.62 to valuesas low as α = 0.25 for the most central event group at600 MeV per nucleon (Fig. 2, top).
= 300 MeV (left panel)and 600 MeV (rightpanel) for 3 intervals ofreduced impact parame-ters b/bmax (0-0.4-0.6-1)and with offset factors ofmultiples of three. Thedashed lines are expo-nential fits according toEq. (1).
In the grand-canonical approximation, assuming thatthe temperature T is about the same, the scaling param-eters α and β are proportional to the differences of the
(middle) and resultingγapp (bottom) for E/A =300 MeV (full symbols) and600 MeV (open symbols),as a function of the central-ity parameter 1 − b/bmax.The temperatures for the112Sn and 124Sn targets aredistinguished by full anddashed lines, respectively.
neutron and proton chemical potentials for the two sys-tems, α = ∆µn/T and β = ∆µp/T . As shown e.g. in[6], their connection with the symmetry term coefficientfollows the relation
αT = ∆µn = µn,2 − µn,1 ≈ 4γ(Z2
1
A21
−Z2
2
A22
) (2)
where Zi and Ai are the charges and mass numbers of thetwo systems. Hence, with the knowledge of the tempera-ture and the isotopic compositions, the coefficient γ of thesymmetry term can be determined from isoscaling.
Temperature estimates were obtained from the yields of3,4He and 6,7Li isotopes, and the deduced THeLi containsa correction factor 1.2 for the effects of sequential decay.The temperatures are quite similar for the two target casesand increase with centrality from about 6 MeV to 9 MeV(Fig 2, middle). This rise does not compensate for the de-crease of the isoscaling parameter α with centrality, whichimplies a decreasing symmetry term coefficient γ (Eq. (2)).Deduced symmetry term coefficients are near γ = 25 MeVfor peripheral and γ < 15 MeV for central collisions.
References
[1] J.M. Lattimer et al., Phys. Rev. Lett. 66, 2701 (1991).
[2] J.M. Lattimer and M. Prakash, Phys. Rep. 333, 121(2000).
[3] A.S. Botvina and I.N. Mishustin, Phys. Lett. B 584,233 (2004).
[4] Bao-An Li, Phys. Rev. Lett. 88, 192701 (2002).
[5] V. Greco et al, Phys. Lett. B 562, 215 (2003).
[6] A.S. Botvina, O.V. Lozhkin, and W. Trautmann,Phys. Rev. C 65, 044610 (2002).
[7] M.B. Tsang et al., Phys. Rev. Lett. 86, 5023 (2001).
[8] G. A. Souliotis et al., Phys. Rev. C 68, 024605 (2003).
[9] W. A. Friedman, Phys. Rev. C 69, 031601(R) (2004).
[10] A. Le Fevre et al., nucl-ex/0409026.
NQMA-EXP-05
121
Gross Properties of Multifragment Decay at Relativistic Energies
C. Sfienti and the ALADiN2000 Collaboration
P. Adrich, T. Aumann, C.O. Bacri, T. Barczyk, R. Bassini,S. Bianchin, C. Boiano, A.S. Botvina, A. Boudard,J. Brzychczyk, A. Chbihi, J. Cibor, B. Czech, M. De Napoli, J.-E. Ducret, H. Emling, J. Frankland, M. Hellstrom,
D. Henzlova, K. Kezzar, G. Imme, I. Iori, H. Johansson, A. Lafriakh, A. Le Fevre, E. Le Gentil, Y. Leifels,W.G. Lynch, J. Luhning, J. Lukasik, U. Lynen, Z. Majka, M. Mocko, W.F.J. Muller, A. Mykulyak, H. Orth,
A.N. Otte, R. Palit, P, Paw lwoski A. Pullia, G. Raciti, E. Rapisarda, H. Sann, C. Schwarz, C. Sfienti, H. Simon,A. Sokolov, K. Summerer, W. Trautmann, M.B. Tsang, G. Verde, C. Volant, M. Wallace, H. Weick, J. Wiechula,
A. Wieloch and B. Zwieglinski
A systematic study of isospin effects in the breakup ofprojectile spectators at relativistic energies has been per-formed with the ALADiN spectrometer. Four differentbeams, 107Sn, 124Sn, 124La and 197Au, all with an incidentenergy of 600 AMeV, have been used. The two proton-rich secondary beams 107Sn and 124La were produced atthe FRS by fragmentation of a primary 142Nd beam. Themeasurement of the charge and the momentum vector of allprojectile fragments with Z ≥ 2 has been performed withhigh efficiency and high resolution with the TP-MUSIC IVdetector [1].In order to investigate to which extent the isotopic compo-sition of the excited spectator affects the gross propertiesof the multifragmentation pattern, charge partitions andmultiplicity distributions have been analyzed and the re-sults have been compared with the SMM prediction [2]. In
Figure 1: Experimental Rise and Fall of multifragmenta-tion correlating the mean multiplicity of intermediate-massfragments and Zbound.
Fig. 1, the obtained correlation between the mean multi-plicity of intermediate-mass fragments, < MIMF >, andthe variable Zbound for the 107Sn, 124Sn and 124La systemsis shown. The global universality of the Rise and Fallbehavior [3] is preserved, but already some distinct differ-ences can be observed. At small excitation energies (largeZbound values) the curves end, as expected, approximatelyat the charge of the original projectiles. However, the slopeof the curve is steeper in the case of the 124Sn. This ef-fect can be understood by considering that in the case ofthe neutron-rich system, heavy residues with low excita-
tion energy will predominantly emit neutrons, a channelthat is suppressed in the case of the two neutron-poor nu-clei. In these latter cases, peripheral collisions are morespread out towards smaller values of Zbound, thus leadingto a slower rise of < MIMF >. This effect, as well as thecorresponding difference in the Zbound distribution, is ingood agreement with the SMM predictions.
Figure 2: Prediction of the Statistical MultifragmentationModel for the mean IMF multiplicity as a function of theexcitation energy for excited 124La, 124Sn and 107Sn nuclei.
Towards more central collisions, we observe a lower max-imum in < MIMF > for the lighter Sn isotope, whereasthe two A=124 systems exhibit the same value for themaximum mean multiplicities. This mass effect, repro-duced by SMM calculations (Fig. 2 ) is unexpected sincethe definition of IMF (3 ≤ Z ≤ 20) is based on the charge.Therefore, the partition space should be primarily a func-tion of the charge of the source. In the rise, up to 6 or 7MeV per nucleon, the number of fragments is just given bythe excitation energy. The 107Sn bends over more quicklytowards vaporization: less neutrons are available and wemay have more alpha-type fragments (like 8Be, 12C) thatdecay easily into alpha particles. Note that the absolutemultiplicities are overpredicted because the calculationsare performed for sources of fixed mass and not for theensemble of spectator systems produced in the collision.Very specific isotopic effects which support the idea ofoverall equilibrium at breakup have also been extractedfrom the mean N/Z of medium-size fragments [2].
References
[1] C. Sfienti et al., GSI Scientific Report 2003, p.232.
[2] C. Sfienti et al., nucl-ex/0410044.
[3] A. Schuttauf et al., Nucl. Phys. A 607 (1996) 457.
NQMA-EXP-06
122
Flow and stopping in Ni+Ni and Pb+Pb at 0.4, 0.8 and 1.16AGeV
Z.G.Xiao,1,4, A. Andronic1, O.N. Hartmann1, N. Herrmann2, K.D. Hildenbrand1, B. Hong3,
Y.J. Kim1,3, M. Kirejczyk1, P. Koczon1, Y. Leifels1, W. Reisdorf1, A. Schuttauf1, and Z. Tyminski1
This report present a systematic study on transverseflow and nuclear stopping in the central symmetric col-lisions of Ni+Ni and Pb+Pb at 0.4, 0.8 and 1.16AGeV.The transverse flow is extracted from the dependence ofthe mean kinetic energy on the mass of light charged par-ticles(LCP) at midrapidity. The degree of stopping is ob-tained from their rapidity spectra.
Figure 1 shows⟨Ekin
⟩of the LCP at midrapidity as a
function of the mass in Ni+Ni and Pb+Pb at three beamenergies. The “slope” of the linear fit,κ, expresses the flowvelocity βr via κ = 1/
√1− β2
r − 1, and the “intercept”,Eth, reflects the temperature of the fireball approximatelyby Eth = 3T/2. Both κ and Eth increase with the beam
energy. The linear extrapolation of⟨Ekin
⟩to zero mass,
i.e. the freeze-out temperature shows no dependence onthe system size. The slope, however, exhibits a larger valuein the Pb+Pb than in Ni+Ni, indicating that heavier sys-tem expands with a higher velocity.
Figure 2 presents the rapidity spectra of p, d and t. Thedifferential yield, dN/dy(0) is scaled to a constant systemsize of 100 nucleons by multiplying with (100/Asys), whereAsys is the system mass. The curves are the predictionsof the blast model, where βr and T are taken from the⟨Ekin
⟩analysis as shown in fig. 1. The area of the curve
is normalized to the total measured yield for each particleindividually. It is shown that the scaled yield of protonis much higher in Ni+Ni than in Pb+Pb, while that ofdeuteron exhibits smaller difference, for tritons, however,the scaled yield becomes higher in Pb+Pb than in Ni+Ni.Heavier particles are more favored in heavier system. Thisis consistent with our earlier results[1], where the chargeresolved cluster with Z > 2 is found to follow this law.
Another globe feature is that the measured distributionis broader than the prediction of the blast model and hencecontradicts the picture of isotropic expansion of a fullystopped fireball. Considering the rapidity distribution ofthe light particles is insensitive to the form of the βr profile[2], we could conclude that full stopping is not achieved inthe collision. Figure 3 depicts the ratio of the RMS valueof the dN/dy(0) spectra predicted by the blast model overthe RMS of the measured dN/dy(0) distribution for the hy-drogen isotopes. For p, the difference betwen two reactionsystems is small. For d and t, however, the ratio, whichreflects the degree of stopping, is much lower in Ni+Nithan in Pb+Pb, indicating higher partial transparency inlighter system. The same conclusion is drawn from theobservation of “vartl” defined in [3].
References
[1] W. Reisdorf et al., Phys. Lett. B595,118(2004).
[2] W. Reisdorf et al., Nucl. Phys. A612,493(1997).
[3] W. Reisdorf et al., Phys. Rev. Lett. 92,232301(2004).
100
150
200
250
300
0.5 1 1.5 2 2.5 3 3.5 4
0.4 (Z=1) Z=2
0.8 (Z=1)
1.16 (Z=1)
Mass (GeV/c2)
<Eki
n> (M
eV)
Figure 1: Dependence of⟨Ekin
⟩on the mass of LCP for
Ni+Ni(solid) and Pb+Pb(open) at three beam energies
10
20
-1 0 1
0.4GeV/u
p Ni+NiPb+Pb10
20
-1 0 1
0.8GeV/u
10
20
-1 0 1
1.16GeV/u
2
4
6
8
-1 0 1
dN/d
y(0) (×
100/
A sys)
d
2
4
6
8
-1 0 1
2
4
6
8
-1 0 1
1
2
3
4
-1 0 1
t
1
2
3
4
-1 0 1
y(0)
1
2
3
4
-1 0 1
Figure 2: Reduced dN/dy(0) distribution of p,d and t forNi+Ni(solid) and Pb+Pb(open). The lines are the predic-tion of blast model.
0.5
0.6
0.7
0.8
0.9
1
0.5 1 1.5
Ebeam(GeV/u)
σ bm/σ d
p
d
t
Figure 3: (a) Ratio of the RMS of dN/dy(0) spectra ex-tracted from the blast model over the RMS of the mea-sured dN/dy(0) spectra.
NQMA-EXP-07
123
Strangeness production with π− beam at 1.15 GeV/c
M.L. Benabderrahmane1, E. Cordier1, N. Herrmann1, Y. Leifels2, A. Mangiarotti1,
M. Merschmeyer1, and the FOPI Collaboration1Physikalisches Institut, Universitat Heidelberg, Germany; 2GSI, Darmstadt, Germany
First measurements with a π− beam of 1.15 GeV/cat GSI were performed with the FOPI detector. In Au-gust 2004 five different targets: Carbon, Aluminum, Cop-per, Tin and Lead, were irradiated with an intensity ofup to 2 · 104 pions per spill. The aim is to investi-gate the strangeness production and possible in-mediummodifications of the production cross section at normalnuclear matter density (ρ = ρ0 =0.17 fm−3). Theo-retical calculations [1] predict a dramatic change of thethresholds and cross sections for the elementary reactionπ− + N → K0 + Λ, Σ, when in-medium effects are takeninto account.
0
2000
4000
6000
8000
10000
0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75
Minv(GeV/c2)
Coun
ts
-500
-250
0
250
500
750
1000
1250
1500
1750
0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75
Minv(GeV/c2)
Coun
ts
N ≈ 28⋅103 k0
M = 503 ± 23 MeV/c2
S/B ≈ 0.1
Figure 1: Invariant mass distribution for (π+π−) pairs mea-
sured with the carbon target.
K0S and Λ particles can be reconstructed in the FOPI
detector acceptance by combining (π+π−) pairs detectedin the central drift chamber (CDC), and (pπ−) pairs in theforward wall (p) and CDC (π−), respectively.
As a first preliminary result, we present the inclusiveproduction cross sections of K0
S meson. Figure 1 shows theinvariant mass spectrum of (π+π−) pairs for the Carbontarget. In the top panel the red curve represents the theinvariant mass fromm all (π+π−) pairs found in an event,and in blue the combinatorial background from a mixedevent analysis is given. In the bottom panel the signaldistribution after background subtraction is shown. K0
S
mesons are clearly visible at a signal-to-background ratioof 1:10.
The inclusive cross section for the reaction: π− + X →
K0S + Y is calculated via: NK0
s/(Nπ− ·Ntarg · ε). Nπ− is
the number of incident pions, Ntarg is the number of nu-clei per cm−2 in the target, and ε is the efficiency for thedetection of the K0
S. The contributions to the efficiency ε
are: branching ratio, geometrical acceptance, tracking effi-ciency and angular distribution. To estimate the efficiencyε, we used MC - simulations based on IQMD [2]. For thereaction π− +12 C → K0
S + Y we find an efficiency of 41%for the reconstruction of (π+π−) pairs originating from K0
S
decays. This number is used to estimate the inclusive crosssection for all targets. In Table 1 the preliminary results ofreconstructed K0
S, the number of recorded events and the
inclusive cross section for each target assuming the sameefficiency is given.
Table 1: Statistics and preliminary results for the inclusivecross section for the various targets.
To compare the cross section of (π− + A ) reactions tothat of the elementary reaction (π− + p), we have to scalethe cross section to account for the proton number of thetarget nuclei. To resolve the scaling parameter we show inFig. 2 the inclusive cross section as function of the chargenumber Z of the nucleus in a logarithmic representation.The error bars are statistical only. The data were fited bya power law, which has the form: σinc = a ·Zb. The resultof the fit is shown in the figure 2. Within errors the param-eter ”b” is close to 2/3, indicating that the cross sectionscales with the surface (σ ∝ Z2/3). The second parameter”a” gives the averaged scaled inclusive cross section andamounts to about 1mb. This has to be compared to thetheoretical predictions, e.g. a cross section of 1.3mb thatis expected when disregarding all medium effects [1].
0
5
10
15
20
25
30
6 7 8 9 10 20 30 40 50 60 70 80 90
a = 0.98±0.13 (mb)
b = 0.71±0.04
Z
σ (m
b)
0.4705 / 3P1 0.9893P2 0.7125
Figure 2: Inclusive cross section of the K0S as function of
the target charge Z
A more reliable view of the in-medium reduction of thecross section will be obtained from an individual efficiencycorrection for the various target that is currently beingevaluated. Further constraints of the reaction kinematicscan be obtained from the analysis of the K0
S − Λ coinci-dences of which about 1000 are contained in the presentlyavailable data samples for the carbon and lead targets.
References
[1] K.Tsushima et al. Phys.Rev.C62(2000)064904
[2] C.Hartnack et al. Eur.Phys.J.,A1:151-169,1998
NQMA-EXP-08
124
Neutral strange particle production in Ni+Ni collisions at 1.93 AGeV
X. Lopez1, M. Merschmeyer2, and the FOPI Collaboration1LPC Clermont-Ferrand, France; 2Physikalisches Institut, Universitat Heidelberg, Germany
Strangeness production in heavy ion collisions is an in-tense field of experimental and theoretical researches sinceit is expected to provide insight into possible changes ofhadron properties in a hot and dense nuclear medium [1].The SIS energy range is best suited to investigate the in-medium properties of strange particles because the latterare produced below threshold or close-to-threshold. Indi-cations for in-medium modifications of charged kaon prop-erties at SIS energies have been already observed exper-imentally [2]. We present here recent experimental re-sults [3] on the production of K0
S and Λ in Ni+Ni collisionsat 1.93 AGeV measured with the FOPI detector. The datawere taken in a high statistics experiment, with about 110millions of central (σgeo = 760 mb) events recorded.
The neutral strange particles are identified by recon-struction of the invariant mass of their charged decay prod-ucts measured in the FOPI Central Drift Chamber. K0
S
and Λ are reconstructed via K0S → π+π− (branching ratio
68.6 %) and Λ → pπ− (branching ratio 63.9 %), respec-tively1. Conditions on track quality and on geometricaland kinematic properties are applied in order to suppressthe combinatorial background.
invariant mass [ GeV/c2 ]
N
KSK0
mass :σ :
S/B :signif. :
counts :
0.503 GeV/c2
16.0 MeV/c2
1.75 (in ±2⋅σ)115.9 (in ±2⋅σ)21106 (in ±2⋅σ)
0500
1000150020002500
300035004000
0.4 0.5 0.6 0.7 0.8
invariant mass [ GeV/c2 ]
N
Λmass :
σ :S/B :
signif. :counts :
1.116 GeV/c2
4.3 MeV/c2
2.58 (in ±2⋅σ)173.3 (in ±2⋅σ)41671 (in ±2⋅σ)
01000
2000300040005000
600070008000
1.05 1.1 1.15 1.2 1.25 1.3
Figure 1: Invariant mass spectra of π+π− (top) and pπ−
(bottom) pairs. The solid and dashed histograms show thecombinatorics and mixed-events background respectively.
Figure 1 shows the invariant mass spectra of π+π− andpπ− pairs fulfilling the reconstruction requirements forcentral (bgeo < 3.3 fm, σgeo = 350 mb) reactions. The com-binatorial background (dashed histograms) is estimated byusing the event-mixing technique. In the present analysis,a total of ∼ 34 · 106 events were analyzed. After back-ground subtraction, more than 2 ·104 K0
S and 4 ·104 Λ are
1Λ from Σ
0→ Λγ are also included in the Λ signal.
reconstructed with signal-to-background ratios of 1.8 and2.6, respectively. The widths of the peaks of 16.0 MeV/c2
(K0S) and 4.3 MeV/c2 (Λ) are due to the detector resolu-
Figure 2: Transverse mass spectra of K0S (left) and Λ
(right) for several rapidity bins (y0 = y/ycm − 1).
Figure 2 shows the transverse mass spectra (mt =√p2
t + m20) of K0
S and Λ particles for several rapidity binsindicated on the figure. The spectra are corrected for thedetector efficiency by means of a complete simulation ofthe FOPI detector using the GEANT package. In orderto compile all spectra in one plot they are multiplied by10n starting from the lowermost spectrum (n = 0) to theuppermost spectrum (n = 6). If particles are emitted bya thermal Boltzmann-like source, their behaviour is thendescribed by the following simple function:
1
m2t
d2N
d(mt −m0)dy0= A · exp
−(mt −m0)
TB
(1)
within a narrow window of rapidity dy0. Both the inverseslope parameter (TB) and the integration constant (A) arefunctions of rapidity. Each spectrum is adjusted with thisfunction and the results are displayed by the solid lines.This allows to extract the rapidity distributions of the in-verse slope parameter TB and the rapidity density distri-butions dN/dy0 by integrating the fitting function frommt −m0 = 0 to ∞.
For a thermalized system, the inverse slope parametercorresponds to the particle temperature at freeze-out. Foran isotropically emitting source, the Boltzmann tempera-ture TB has a simple dependence on rapidity:
TB =T0
cosh y(2)
where T0 is the effective temperature at mid-rapidity.In the following figures the circles indicate measured
data points for K0S and Λ while the squares denote points
NQMA-EXP-09
125
reflected with respect to mid-rapidity. The results includeonly statistical errors. The stars correspond to previouslymeasured distributions of K+ [4] and protons [5] in thesame reaction. The upper plot in Fig. 3 shows the TB
rapidity distribution for the K0S . A good agreement is
found between K0 and K+. An effective temperature of111±1 MeV is extracted from the fit using equation 2. Thelower plot of the figure depicts the corresponding resultsfor the Λ with an effective temperature of 125±1 MeV.The comparison to protons shows that both measurementsroughly agree around mid-rapidity but differ significantlyat projectile and target rapidities. This difference can beattributed to the presence of cold spectator matter.
y0
TB [
GeV
]
Best et al. (K+)measuredreflected
Teff: 111 ± 1 MeV
K0KS
0.050.060.070.080.090.1
0.110.120.130.140.15
-1 -0.5 0 0.5 1
y0
TB [
GeV
]
Hong et al. (protons, 420mb)measuredreflected
Teff: 125 ± 1 MeV
Λ
0.050.060.070.080.090.1
0.110.120.130.140.15
-1 -0.5 0 0.5 1
Figure 3: Inverse slope parameter distributions of K0S (up-
per plot) and Λ (lower plot). The stars correspond to pre-viously measured K+ [4] and protons [5].
The rapidity density distributions are presented inFig. 4. The distributions for K0
S and Λ particles are cor-rected for the corresponding branching ratios. In additionthe original yields of K0
S are multiplied by a factor 2 in or-der to take into account K0
L. The upper plot shows a com-parison between K0 and K+ distributions which are foundvery similar in the overlap regions around target/projectilerapidities. The total K0 production yield per collision of0.074±0.005 is obtained from a Gaussian fit to the data.
The dN/dy0 distribution of Λ is displayed in the lowerpanel of Fig. 4 and is compared to the one of protons(down-scaled in order to have the same integral as theΛ data). One observes a pronounced longitudinal spreadof the proton distribution which could be an indicationfor a large degree of transparency in the Ni+Ni system.Unlike protons, Λ are produced in the fireball and scatterwith the surrounding nuclear medium leading to a morenarrow distribution. The Λ production yield per collisionis 0.120±0.004.
The curves in both plots denote the Siemens-Rasmussendistributions [6] for two different sets of temperature T andradial expansion velocity βr. The dashed (solid) line corre-
y0
dN
/dy0
Best et al. (K+)
measuredreflected
tot. yield: 0.074 ± 0.005
K0
00.010.020.030.040.050.060.070.080.090.1
-1 -0.5 0 0.5 1
y0
dN
/dy0
Hong et al. (protons, 420mb)(scaled by 1/362.)measuredreflected
tot. yield: 0.120 ± 0.004
Λ
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
-1 -0.5 0 0.5 1
Figure 4: Rapidity density distributions of K0 (upper plot)and Λ (lower plot). The stars correspond to previouslymeasured K+ [4] and protons [5].
sponds to T =92 (100) MeV and βr = 0.32 (0.23). Whilekaons are insensitive to a change of the two parametersand fully agree with the Siemens-Rasmussen distribution,a difference is visible for Λ. This confirms that despite thefact that their yield distribution is significantly differentfrom that of protons, even Λ are not fully thermalized.
The large statistics of reconstructed K0S and Λ allows for
more detailed studies on e.g. the centrality dependence ofthe production yield and flow [3, 7] as it has been alreadyinvestigated at the AGS [8]. This should help to improveour understanding of production mechanisms for strangeparticles close to threshold.
References
[1] For reviews, see e.g. W. Cassing and E. Bratkovskaya,Phys. Rep. 308 (1999) 65; C.M. Ko, V. Koch, G. Li,Ann. Rev. Nucl. Part. Sci. 47 (1997) 1, and ref.therein.
[2] N. Herrmann, K. Wisniewski, Acta Phys. Polon. B35 (2004) 1091; P. Senger, Prog. Part. Nucl. Phys. 53(2004) 1, and ref. therein.
The two body final state Λ - hyperon + deuteron repre-sents one of the possible decay channels of the kaonic clus-ter state ppnK− proposed in [1]. Heavy ion collisions atbeam energies close to the strangeness production thresh-old could offer an alternate way to produce these exoticstates that have been observed so far only in kaon inducedreactions [2, 3].
The FOPI data set of 108 recorded events of Ni+Ni col-lisions at an incident energy of 1.93 AGeV was analyzed.About 65k Λ - baryons were found under selection criteriathat give a signal - to - background ratio of about 5:1. Theevents containing those Λ were inspected for the existenceof Λ-d correlations. The deuterons were identified by theirspecific energy loss in the central drift chamber and theirmomentum, their purity is about 70%.
Signal=-460.253
Signif=-1.99296
S/B=-0.00855595
Minv(GeV)
iv42_alls_-
Minv(GeV)
0
2000
4000
6000
8000
10000
12000
3 3.2 3.4 3.6 3.8
-200
-150
-100
-50
0
50
100
150
3 3.2 3.4 3.6 3.8
Figure 1: Distribution of invariant mass for Λ-d pairs. On theleft side the correlated pairs within one event (red) are com-pared to mixed pairs from different events (blue). The distri-butions are normalized in the range indicated by the horizontalarrow. The right panel shows the difference of both distribu-tions. The color code and the layout is kept for all followingfigures.
φ(dΛ)-φRP
iv42_alls_-
φ(dΛ)-φRP
0
500
1000
1500
2000
2500
-100 0 100
-400
-200
0
200
400
600
800
-100 0 100
Figure 2: Distribution of azimuthal angle of Λ-d pairs withrespect to the reaction plane (RP).
The reconstructed invariant mass distribution of Λ-dpairs is compared in Fig. 1 to the mixed event distributionwhere Λ candidates are combined with deuterons measuredin different events of the same charged particle multiplic-ity (registered in FOPI’s forward wall). The shape of theinvariant mass distribution is not well reproduced by theevent mixing. The main reasons for this mismatch wereidentified as: 1) Deuterons and Λ hyperons are emittedpreferentially into the reaction plane. Therefore also thepairs show this preferred direction (Fig. 2). Hence, theevents were rotated in a way that the reaction planes are
aligned. 2) The difference of the azimuthal angles of agiven pair is shown in Fig. 3. At small difference anglesa deficit is found with respect to the mixed event sample.This is caused by the particle reconstruction efficiency thatis lower for intercepting and close-by tracks, an effect thatis not present when tracks from different events are com-bined. The distortions can be avoided by eliminating alltrack pairs from the analysis, where the deuteron trackcrosses any of the daughter tracks of the Λ hyperon.
∆φd-Λ
iv42_alls_-
∆φd-Λ
0
200
400
600
800
1000
1200
1400
1600
1800
-100 0 100
-250
-200
-150
-100
-50
0
50
100
150
-100 0 100
Figure 3: Distribution of difference of azimuthal angle.
The final result after removing the artificial correlationsand additionally requiring a pair rapidity of y < 0.65 isshown in Fig. 4 in comparison to MC simulations wherethe same procedures and selection criteria were employedfor data and simulations. It was verified that a resonanceintroduced into IQMD Ni+Ni events can be reconstructedcorrectly (middle row) while no resonance like structure isgenerated due to the method (lower row). The remainingexcess in the data has a mean mass of 3.16 GeV, a widthof Γ ≈ 100 MeV and a statistical significance of 4.7. Towhich extent this finding is compatible with [2] is subjectto further investigations.
Figure 4: Distributions of invariant mass of Λ-d pairs in data(top), signal -MC (middle) and background -MC (bottom).
References
[1] Y.Akaishi, T. Yamazaki, Phys. Rev. C 65 (2002) 044005.[2] M. Iwasaki et al., submitted to PLB (nucl-ex/0310018).[3] T. Suzuki et al., Phys.Lett .B 597 (2004) 263.
NQMA-EXP-10
127
Search for K Nuclear Clusters in Nuclear Collisions with FOPI
L.Fabbietti 1, N. Herrmann 2, P.Kienle 1, 3 , K. Suzuki 1, T. Yamazaki 4 and J. Zmeskal 3 1 Physik Department, Technische Universität München,Germany ,2 Physikalisches Institut der Universität Heidelberg, Germany, 3Stefan Meyer Institut für subatomare Physik, Wien, Austria, 4 Heavy-Ion Nuclear Physics Laboratory, RIKEN, Japan
We propose to search for K nuclear clusters (ppK−, ppnK−, pppK−, pppnK− and ppnnK−) as residues in nuclear collisions. These kaonic nuclei have recently been predicted by [1] to be narrow discrete bound states with large binding energies of about 100 MeV, so that their main decay channels to Σπ are energetically forbidden. Due to the strong K−-p attraction these K nuclei are expected to be enormously condensed systems with an average nucleon density as much as 0.5 fm-3 and thus are called K clusters. Very recently, experimental evidences [2] for T=0 ppnK− and T=1 pnnK− clusters have been obtained at KEK from 4He(stopped-K−,n) and 4He(stopped-K−,p) reactions, respectively. The observed K binding energies are substantially larger than the predicted ones.
We propose to produce and identify K clusters in nuclear collisions by making use of the excellent capability of the FOPI [3]detector at GSI in identifying both K0 mesons and Λ hyperon. Using 3.5 – 4.5 GeV protons on a d target the Λ*(=Λ(1405)= pK−) particle serves as a doorway to form ppK− clusters as follows:
p+d → [Λ*p]+K0+p → ppK-+K0+p. In this reaction a missing-mass (MM) spectrum of ppK− can be constructed using p and K0 and according to the decay pattern:
ppK- → Σ +,- + π +,- + p or Λ + p, an invariant mass (IM) spectrum can also be built. Detailed simulation studies have been carried out for the reaction p+d @ 4.5GeV to check the feasibility of the measurement of the ppK− signal. The formation cross section for the Λ* at this energy was estimated [4] to be of about 20 µb, the probability that it combines with a proton to form a ppK− cluster depends on the relative momentum of the two and is about 1% (Prel<300 MeV/c).
Fig 1: Charged particle multiplicity distribution for the
signal and the background in p+d collisions. The multiplicity
shown refers to the particles emitted in the FOPI acceptance. The formation probability for the cluster is hence about 0.2 µb and it has to be compared with the total reaction cross-section of 78 mb. All the p+d output channels have been simulated and an online multiplicity trigger has been investigated using the FOPI plastic wall and the Barrel. Fig 1 shows tha charged particles distribution in the FOPI geometrical acceptance, requiring at least 4 charged particles in the acceptance the background is reduced of a factor 8000 respect to the signal.
Fig 2: (upper) Correlation plot of the IM versus MM and
(lower) projection along the IM axis. The red line in the lower histogramm represents the signal, the black line the sum of the signal and background.
Applying this trigger condition and assuming for the
ppK− a mass of 2279 MeV/c2 and a width of 20 MeV/c2, the combination of the MM and IM analysis, together with energy and momentum conservation condition, delivers the plot shown in fig. 2. The projection on the IM axis shows that a signal to background ratio of 1/3 is achieved. This hypothesys will be verified by the scheduled beam-time at the end of 2005.
References:
[1] Y. Akaishi and T. Yamazaki, Phys. Rev. C 65 (2002) 044005.
[2] M. Iwasaki et al.. submitted to Phys. Lett. B (ArXiv-nucl-ex/0310018).
[3] J.L. Ritman et al., FOPI Collaboration, Nucl. Phys. Proc. Suppl. 44 (1995) 708.
[4] J.Ritmann, private comunication.
NQMA-EXP-11
128
Azimuthal angular emission patterns of K+ and K−
from Au+Au at 1.5 AGeV
M. P loskona, E.Grossef,g, A.Forsterb, P.Koczona, B.Kohlmeyerd, S.Langb, L.Naumannf, H.Oeschlerb,W.Scheinastf, A.Schmahb, T.Schuckc, E.Schwaba, P.Sengera, Y.Shinc, H.Strobelec, C.Sturmb,
Relativistic heavy-ion collisions provide an opportunityto study modifications of particles properties in the hotand dense nuclear medium. Especially strange mesonsand their features are expected to be influenced by thein-medium meson-baryon potential. Theoretical calcula-tions show that the superposition of the scalar and vectorconstituents leads to a small repulsive K+N and a strongattractive K−N potential.
Additionally, the interaction of kaons and antikaons withnuclear matter is different. The strangeness conservationlaw inhibits the absorption probability of K+ as they con-tain an s-quark. K−, however, interacting with a nucleonare easy to be converted into a Λ or Σ and a pion. More-over, the reverse process is the dominant production mech-anism of K− (strangeness-exchange) at SIS energies [1].
The azimuthal angular emission patterns of kaons areexpected to be sensitive observables for their in-mediumproperties [2]. Previous observations have confirmed theinfluence of the repulsive force on K+ emerging from thehot and dense phase of the collision. An enhanced out-of-plane emission of K+ was observed in Au+Au reactions at1.0 AGeV and 1.5 AGeV, and also in Ni+Ni at 1.93 AGeV[3, 4]. Recent measurements presented here confirm theseresults (see figure 1).
-180 -90 0 90 1800
0.1
[cor
rect
ed]
φddN N1
[Deg]φ
Au+Au 1.5 AGeV+K
0.01± = 0.04 1v 0.01± = -0.06 2v
-180 -90 0 90 1800
0.1
[cor
rect
ed]
φddN N1
[Deg]φ
Au+Au 1.5 AGeV-K
0.01± = 0.02 1v 0.02± = -0.06 2v
Figure 1: Azimuthal angular emission patterns of K+
(left) and K− (right) measured close to mid-rapidity(0.25 < y/ybeam < 0.6) with centrality selection b > 6.4fmand transverse momenta 0.2 < pt < 0.8GeV/c.
Up to now the emission pattern of K− was studied onlyin Ni+Ni collisions at 1.93 AGeV and it has been found tobe very different than the one of K+ [4]. The elliptic flowof K− revealed a slight in-plane emission. In contrast tothat, recent measurement of the emission pattern of K−
from non-central Au+Au at 1.5 AGeV collisions performedwith the KaoS spectrometer [5] close to mid-rapidity showsan enhanced out-of-plane emission (fig. 1, right).
The predictions concerning K−, however, suggest an al-most isotropic emission pattern as a consequence of theattractive K−N potential balancing large rescattering andabsorption probability of K− in the spectating fragments[6]. On the other hand, the observed in-plane flow of K−
from Ni+Ni at 1.93 AGeV collisions has been explainedwith a scenario where the strangeness-exchange reaction(πY ↔ K−N , with Y = Λ,Σ) plays a crucial role in de-laying the antikaon emission from the fireball, and thusminimizing their interaction with the spectators [1, 7].
-180 -90 0 90 1800
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φddN N1 [Deg]φ
Au+Au 1.5 AGeV-K t low p
0.02± = 0.05 1v 0.04± = 0.03 2v
-180 -90 0 90 1800
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[cor
rect
ed]
φddN N1
[Deg]φ
Au+Au 1.5 AGeV-K t high p
0.02± = 0.00 1v 0.03± = -0.15 2v
Figure 2: Azimuthal angular emission pattern of K−
mesons with low (0.2 < pt < 0.5GeV/c - left) and high(0.5 < pt < 0.8GeV/c - right) transverse momentum mea-sured close to mid-rapidity (0.25 < y/ybeam < 0.6) withnon-central events (b > 6.4fm).
Analysis of the azimuthal emission pattern as a func-tion of the transverse momentum has revealed that K+
show the out-of-plane emission independent of the pt, butthis is not the case for K−. The antikaons registered withpt < 0.5 GeV/c are preferentially emitted in the reactionplane and the particles with pt > 0.5 GeV/c show strongout-of-plane enhancement (see fig. 2). The emission pat-terns of K− can be explained in terms of two competingphenomena: one of them is indeed the influence of theattractive K−N potential, however, the second one origi-nates from the strangeness-exchange process. If one relatesthe transverse momentum of the particle to its emissiontime it becomes clear that the low momenta K− are mostlyinfluenced by the strangeness-exchange mechanism beingretarded and thus emitted preferentially in-plane (no spec-tator shadowing). On the other hand high pt K
− emergeearly from the fireball with the spectators shadowing theirflight path, causing an out-of-plane emission. It is verymuch required to verify this picture with detailed trans-port model calculations.
References
[1] A. Forster, F. Uhlig et al., PRL 91 (2003) 152301.[2] G.Q.Li et al., Phys. Lett. B 381 (1996)[3] Y. Shin et al., PRL 81 (1998) 1576.[4] F. Uhlig, A. Forster et al., nucl-ex/0411021[5] P. Senger et al., NIM A327 (1993) 393.[6] Z.S. Wang et al., Eur. Phys. J. A 5 (1999) 275[7] C. Hartnack et al. PRL 90 (2003) 102302.
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129
Dielectron production in C+C and pp collisions
The HADES collaboration
1 Introduction
In 2004 the HADES collaboration concentrated on com-pleting the first experimental proposal S262 focused ondielectron production in C+C and pp collisions. Two dedi-cated production runs using carbon and proton beams of1 AGeV and 2.2 AGeV kinetic energy, respectively, weresuccessfully conducted. Based on experiences gained in afirst commissioning run using the liquid hydrogen targetbuilt at Orsay in late 2003, effective data taking in the pro-ton production run was possible. Since 2004 the spectrom-eter is fully equipped with all detector sub-systems withthe exception of two outermost drift chambers (MDCIV).The main goal of the pp production run is to experimen-tally determine the detection efficiency of electron pairs.To this end, η production in both a hadronic and a leptonicdecay channel is studied. We like to note that the reac-tion pp → ppη can be determined unambiguously usingthe missing mass technique. Detailed information on thereconstruction efficiency for electron and positron tracks,an important ingredient in the analysis of dielectron pro-duction in heavy ion collisions, can be deduced.
In the second run in August 2004, dielectron produc-tion was measured in the C+C collisions at 1 AGeV. Inthis measurement, we took data for a system in which asubstantial excess of electron pairs above the pion Dalitzregion was found by the DLS collaboration [2]. The analy-sis of these data will permit a substantial assessment of theresults published by the DLS collaboration. In the mean-time, we continued construction of the last two missingdrift chambers (MDCIV) in IPN Orsay. Both chamberswill be completed by end of 2005. Moreover, substan-tial progress was achieved in the design of the new innertime-of-flight system, which is based on single cell RPCdetector elements. This system will replace the prelimi-nary TOFINO detector which does not provide sufficientgranularity and is achieving moderate time resolution only.
2 Dielectron production in C+C collisions
In our previous status report [1] we presented preliminaryresults on dielectron production from C+C at 2 AGeV ob-tained in a commissioning run in 2001. Here, we showpreliminary results obtained from the analysis of the high-statistics experiment performed in the end of 2002. Theanalysis procedure was similar to the one used for the firstexperiment and is described in [1, 3, 4]. The experimen-tal set-up, however, was more advanced in the 2002 runas compared to the one realized in 2001. All six sectorswere fully equipped with RICH, inner tracking chambers(MDCI/II) and time-of-flight/PreShower (META) detec-tors. In two sectors two outer tracking planes (MDCIIIand MDCIV) were installed, for two additional sectors thethird tracking plane was equipped. However, in the first in-stance, the data analysis concentrated on a symmetric sixsector configuration not making use of detector informa-tion provided by the outer drift chambers. This allowed a
direct comparison to the results obtained in the 2001 com-missioning run. The two sectors with complete trackingsystem were utilized for tracking studies focusing on thepurity and efficiency of track reconstruction. This allowedto better estimate tracking errors of the low-resolution con-figuration.
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Figure 1: Top: Invariant-mass distributions of unlike-signpairs (black symbols), combinatorial background (CB, solidhistogram) and signal pairs (full red symbols) normalized tothe number of LVL1 events from C+C collisions at 2 AGeV.Bottom: Signal (red points) after CB substraction The greyshaded area displays variations due to the pair cuts used in theanalysis and the green shaded area depicts systematic bias (seetext for more details). Blue symbols represent simulated signalbased on events from the UrQMD transport code analyzed andnormalized as the experimental data.
In the C+C experiments data acquisition is triggered bya first level trigger (LVL1) based on the hit multiplicityin the time-of-flight wall with a condition requiring 4 or
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more charged particles in the acceptance. In the produc-tion run in 2002 the second level trigger (LVL2) was suc-cessfully used for the first time. It on-line selects eventswith at least one electron candidate inside the HADES ac-ceptance by requiring correlated electron hits in the RICHand META detectors. With this trigger setting a pair ef-ficiency of 92% was realized reducing at the same timethe accepted event rate by a factor of 12. The recordedstatistics of pairs could thus be increased by almost oneorder of magnitude as compared to the commissioning runin 2001. The LVL2 trigger bias was studied carefully byexamination of electron distributions in events taken withand without positive LVL2 trigger. The later events wererecorded applying a proper scale down to LVL1 triggeredevents. Besides overall scaling no significant effect wasfound [5].
For the analysis a total statistics of 6 × 107 LVL2, cor-responding to more than 6 × 108 LVL1 events, was used.In the first stage of the analysis single electron tracks werereconstructed requiring charged tracks matched to a ringin the RICH detector as well as to a hit in the META de-tectors. The candidates are further cleaned by requiringa velocity β ' 1. From the identified single electron (andpositron) tracks unlike and like-sign pair distributions werederived. Only those combinations were accepted, whichcontain separated tracks in all detectors, i.e. with no com-mon hit in either the RICH, the inner MDC or the METAdetector. Furthermore, only tracks with an opening angleθe+e− > 90 and with a good χ2 from the track fitting in theinner MDCs were accepted as a valid pair. The χ2 condi-tion was found to represent a powerful tool to reject closepairs originating from external photon conversion and π0-Dalitz decays which account for most of the combinatorialbackground. Such pairs to a large extent do not appearas separated tracks in the inner drift chambers but ratherconstitute single track candidates. From detailed Monte-Carlo studies we have deduced that this cut rejects morethan 90% of close pairs with opening angles θe+e− < 3◦,still achieving an efficiency of 95% for single electron trackreconstruction.
The upper panel in Fig. 1 shows the unlike-sign invariantmass distributions together with the corresponding combi-natorial background. The combinatorial background NCB
was calculated from like-sign pairs N++, N−− using thesum NCB = N+++N−−. We checked via Monte-Carlo cal-culations that this prescription describes our combinatorialbackground reasonably well. The signal pair distributions(red symbols) were obtained subtracting the combinato-rial background from the unlike-sign distribution and bynormalizing it to the number of accepted LVL1 triggers.The total pair statistics, after subtraction of combinatorialbackground and analysis cuts described above, amounts to∼ 16k. The data are neither corrected for the reconstruc-tion efficiencies nor spectrometer geometrical acceptance.
The lower panel in Fig. 1 shows a comparison of thereconstructed dielectron invariant-mass distribution withsimulations. In the simulation we used 1× 108 events ob-tained from the UrQMD transport model (Version 1.3)which were transported through the spectrometer usingGEANT and digitized applying realistic detector response
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Figure 2: Invariant-mass distributions of unlike-sign pairs(black symbols), combinatorial background (solid histogram)and signal pairs (full red symbols) from C+C collisions at1 AGeV.
functions. The data obtained in that way was processedthrough exactly the same analysis procedure as it was usedfor the experimental data. Finally, the simulation eventswere normalized to the number of UrQMD events pass-ing the LVL1 trigger condition used for the real data. Wewould like to point out that the simulated spectra containcontributions from π0 and η decay only. It is apparent thatin the low-mass region (dominated by π0 Dalitz pairs) thesimulation overestimates the data but in the higher-massregion the simulation and data agree rather well. Thepresented results are fully consistent with our first anal-ysis presented in the previous report [1]. The systematicerrors of the comparison were studied in detail and areshown in Fig. 1 by two colored bands. The grey shadedarea shows the variation of the experimental dielectron sig-nal due to the effect of various selection criteria imposedon pair tracks. It was obtained by switching off one cutin the experimental data analysis but keeping the othersactive. As it can be seen, variations of the signal yield arerather small. However, a systematic difference betweenthe simulation and the experimental data was found inthe reaction of the pair yields to the analysis cuts at lowinvariant masses. Its magnitude (green shaded area) wasobtained by comparing the relative reduction of the unlikesign and like sign pair yields due to the action of all cutsin the data and the simulation. In a perfect case thesereductions should be identical. However, we observe a sys-tematic difference (bias) which leads to smaller reductionof unlike sign and larger reduction for like sign pairs, andas a consequence larger signal yield, in the simulation. Thereason for this difference is presently under investigation.
In order to complete the investigations of the C+C col-lision system another production run at 1 AGeV was per-formed in the end of 2004. The experimental set-up withfull coverage in the outer MDCIII and four chambers in the
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MDCIV layers were achieved as compared to the 2 AGeVrun. Using the same trigger setting as for the previous run9×107 LVL2 events, corresponding to almost 2×109 LVL1events were accumulated. An online, and hence very pre-liminary, dielectron analysis was performed in parallel tothe data taking using on-line detector calibration. Fig. 2shows the invariant mass distributions of unlike-sign, com-binatorial background and signal pairs normalized to thenumber of LVL1 triggers. A statistics equivalent to 5.4ksignal pairs was obtained from the analysis of 75% of thecollected statistics. It is expected that better detector cal-ibration and analysis of all available data will further im-prove the pair statistics. Nevertheless, one may alreadyconclude that the data quality is sufficient to determinea spectrum with experimental uncertainties substantiallysmaller than the current discrepancies between theoreti-cal and experimental (DLS) results. Data analysis is inprogress and the final spectra are expected in this year.
3 pp collisions at 2.2 A GeV
The main goal of the pp experiment was to collect datafor pp elastic scattering to be used for tracking and align-ment studies, and to identify exclusively reaction channelsof the type pp → pp(π0, η). The reaction channels witha single meson were selected as a means to verify the di-electron reconstruction efficiency needed for an absolutenormalization of the C+C data. A proton beam with anintensity of 2 × 107 protons/s and a 5 cm long liquid hy-drogen target was used in this experiment. Typical ratiosof trigger rates for the full and empty target of 10 : 1 wereachieved during data taking. We collected around 4× 108
events with two main LVL1 trigger settings: (1) MUL ≥ 2in opposite META sectors for pp elastic scattering and (2)MUL ≥ 4 in the META for exclusive meson identification.In the latter case we also used a LVL2 trigger setting asfor C+C collisions to enhance events with electron pairs.
The prerequisite for the identification of a reaction chan-nel with a single neutral meson is missing mass resolution.In order to achieve it, precise MDC calibration and de-tector alignment is mandatory. For this purpose severalalignment procedures are being pursued. One is based onstraight track reconstruction from runs without magneticfield and the second one uses the advantage of the pp elas-tic scattering kinematics. Fig. 3 shows the current state ofthe art of our tracking precision (status end of 2004). Inthe upper panel we present the angular correlation betweentwo tracks using the relation tan(θ1) × tan(θ2) = 1/γ2
CM
reconstructed for the trigger type (1). The prominentpeak indicating elastic pp events is centered around theexpected 1/γ2
CM = 0.46. The lower figure shows the to-tal center of mass energy reconstructed from two tracksfulfilling the elastic scattering condition imposed on thepolar (tan(θ1) × tan(θ2) ∼ 1/γ2
CM and the azimuthal an-gles (coplanarity). The width of the peak is equivalent toa mass resolution of 4% and is still a factor 2 lower thanthe one expected assuming ideal detector calibration andgeometry. However, it is already sufficient for a single neu-tral meson channel identification and therefore processingof all collected data has been started.
Figure 3: Top: Polar angular correlation of two tracks re-constructed in pp collisions at 2.2 GeV . A prominent peakcentered at tan(θ1)× tan(θ2) = 0.46 indicates elastic scatteringevents. Bottom: Invariant mass of two tracks fulfilling elas-tic scattering kinematics. The total center of mass energy isEcm = 2.767 GeV.
The collaboration gratefully acknowledges the sup-port by BMBF and GSI, GA CR 202/00/1668 andGA AS CR IAA1048304 (Czech Republic), KBN5P03B 140 20 (Poland), INFN (Italy), CNRS/IN2P3(France), MCYT FPA2000-2041- C02-02 and XUGAPGID T02PXIC20605PN (Spain), INTAS 03-51-3208(EU).
References
[1] Hades collaboration, GSI annual report 2003[2] R. J. Porter et al. [DLS Collaboration], Phys. Rev.
Lett. 79 (1997) 1229[3] P. Tlusty, J.Otwinowski for HADES collaboration,
Proceedings of XLII International meeting on NuclearPhysics, Bormio 2004, Vol 120.
[4] P. Salabura et al., Nucl. Phys. A 749(2005) 150c[5] A. Toia, PHD thesis, University of Giessen, 2005
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Pion and kaon production in A+A collisions at low SPS energies
C. Alt1, C. Blume1, P. Dinkelaker1, D. Flierl1, V. Friese2, M. Gazdzicki1, C. Hoehne2, S. Kniege1,
I. Kraus2, M. Kliemant1, B. Lungwitz1, C. Meurer2, M. Mitrovski1, R. Renfordt1, A. Richard1,
A. Sandoval2, T. Schuster1, R. Stock1, C. Strabel1, H. Strobele1, and the NA49 Collaboration1Fachbereich Physik der Universitat Frankfurt 2 GSI Darmstadt
The results from the NA49 energy scan programme atthe SPS have revealed an anomalous behavior of the en-ergy dependence of the 〈K+〉/〈π+〉 ratio in central Pb+Pbcollisions [1] which is absent in p+p interactions. Here wereport the results on charged kaon and pion production incentral collisions at 20 and 30 A·GeV as well as their sys-tem size dependence at 40 A·GeV. The former data extendthe energy range of the NA49 data to a region well belowthe observed structure. The latter studies the onset of theanomalous behavior by comparing particle ratios obtainedin p+p, C+C, Si+Si, and minimum bias Pb+Pb collisions.
In order to identify kaons, an analysis of the energyloss dE/dx in the NA49 time projection chambers is per-formed. Raw K+ and K− yields were extracted from fitsof the distributions of dE/dx and tof (where available)in narrow bins of momentum and transverse momentum.The yields at mid-rapidity are obtained using the com-bined dE/dx and tof information (tof + dE/dx analysis).
To obtain the π− yields, all negatively charged particleswere binned in rapidity (calculated assuming the π–mass)and pT . The contamination from K−, p and e− from theinteraction vertex as well as non–vertex hadrons originat-ing from strange particle decays and secondary interac-tions was subtracted.
The resulting K± and π− yields were corrected for geo-metrical acceptance, losses due to in–flight decays, ineffi-ciencies of the tracking algorithms and quality cuts. Thedetails on the correction procedure can be found in [1].
Spectra of transverse mass mT =√
p2T + m2 (m is the
rest mass of the particle) for K+, K− (tof +dE/dx analy-sis) and π− mesons produced near mid-rapidity in centralPb+Pb collisions (0-7.2%) at 20 and 30 A·GeV (Fig. 1 left)and in C+C, Si+Si, and Pb+Pb with different centralityselections (Fig. 1 right) are fitted by Eq. 1 in the range 0.2GeV < mT −m < 0.7 GeV:
dn
mT dmT dy= C · exp
(−
mT
T
). (1)
The five centrality selected event samples were obtainedfrom minimum bias Pb+Pb collisions on the basis of theenergy deposited by the projectile spectator nucleonsin the forward calorimeter [2]. The resulting fractionsof the total inelastic cross–section are 0-5%, 5-12.5%,12.5-23.5%, 23.5-33.5%, and 33.5-43.5%.
The rapidity distributions dn/dy plotted in Fig. 2 wereobtained by summing the measured mT spectra and usingthe fitted exponential function (Eq. 1) to extrapolate tofull mT . For most bins the necessary correction is small(∼= 5%). The rapidity spectra were parameterized by thesum of two Gaussian distributions placed symmetricallywith respect to mid-rapidity. The results of the fits are in-
-m (GeV)Tm0 0.5 1 1.5
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Figure 1: left: Transverse mass spectra of π−, K+ andK− mesons produced at mid-rapidity (|y| < 0.1 for kaonsin the tof + dE/dx analysis, and 0 < y < 0.2 for pions)for central Pb+Pb collisions at 20 and 30 A·GeV.right: 40 A·GeV transverse mass spectra of π−, K+, andK− at midrapidity (−0.5 < y < 0.5) for C+C, Si+Si, andminimum bias Pb+Pb collisions.
dicated by the full lines in Fig. 2. The mean multiplicitieswere obtained by integration of the fitted curves.
Figure 3 shows the system size dependence of the〈π−〉/〈NW 〉, 〈K+〉/〈NW 〉, 〈K−〉/〈NW 〉, 〈K+〉/〈K−〉,〈K+〉/〈π+〉, and 〈K−〉/〈π−〉 ratios. 〈NW 〉 is the meannumber of wounded nucleons calculated within a Glaubermodel. For p+p interactions no measurements of these ra-tios exist at this energy, therefore an interpolation of mea-surements at other energies is used [3]. To ease the com-parison to the isospin symmetric light nuclei, the π yieldin isospin averaged nucleon-nucleon interactions (N+N)was estimated as 〈π−〉(NN) = 0.5 · (〈π+〉(pp) + 〈π−〉(pp))and the ratios which use this yield are also shown in thefigures.The 〈π−〉/〈NW 〉 ratio shows a fast rise for the smaller
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y-2 0 2
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Figure 2: Top: Rapidity distributions of π−, K+ and K−
mesons produced in central Pb+Pb collisions at 20 and30 A·GeV. For kaons squares and circles indicate the re-sults of tof and dE/dx only analyzes, respectively. Theclosed symbols indicate measured points, open points arereflected with respect to mid-rapidity. The lines indicatedouble Gaussian fits to the spectra. The plotted errors,which are mostly smaller than the symbol size, are statis-tical only, the systematic errors are ±5%.Bottom: rapidity spectra for Si+Si (squares) and the fivecentrality bins of minimum bias Pb+Pb collisions (circles)at 40 A·GeV beam energy. The triangles illustrate themid-rapidity TOF measurements. The solid lines repre-sent the fits described in the text.
systems and decreases slowly from there on to centralPb+Pb collisions. The 〈K+〉/〈NW 〉 ratio rises fast withsystem size for the small systems. The value for centralSi+Si reactions is slightly higher than the measurementfor peripheral Pb+Pb collisions. The dependence oncentrality seen in the minimum bias Pb+Pb data, on theother hand, is weak. A qualitatively similar behaviour isobserved for the 〈K−〉/〈NW 〉 ratio.The 〈K+〉/〈K−〉 ratio rises at small numbers of woundednucleons with the system size and remains almost constantfrom peripheral to central Pb+Pb collisions. A similarbehaviour is observed by the E802 Collaboration at AGSenergies [4].The 〈K+〉/〈π+〉 ratio is constructed by scaling the 〈π−〉
yield with the π+/π− ratio, as measured at mid-rapiditywith the time-of-flight detectors. For the isospin sym-metric systems C+C and Si+Si the ratio π+/π− is unity.Again, a steep rise of the 〈K+〉/〈π+〉 ratio is observed forthe small systems with the ratio in Si+Si collisions beinghigher than in peripheral Pb+Pb collisions. However, thecentrality dependence of this ratio in minimum bias colli-sions is more pronounced than the one of the 〈K+〉/〈NW 〉
ratio. The system size dependence of the 〈K−〉/〈π−〉
ratio resembles the one of the 〈K+〉/〈π+〉 ratio. Theobserved 〈K−〉/〈π−〉 ratio in Si+Si collisions is close tothe measurement in central Pb+Pb collisions.
The new results on π and K production at 20 and 30A·GeV will now be discussed together with published mea-surements at lower (AGS) and higher (SPS, RHIC) ener-gies. The dependence of basic hadron production prop-
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Figure 3: System size dependence of various ratios at40 A·GeV. The p+p (full stars) and N+N (open stars)data points result from an interpolation of measurementsat other energies (see text). C+C (triangles), Si+Si(squares), and minimum bias Pb+Pb (circles) are dis-played at their corresponding 〈NW 〉.
erties on the collision energy, expressed by Fermi’s mea-sure F [5] (F ≡ (
√sNN−2mN)3/4/
√sNN
1/4, where√
sNN
is the c.m.s. energy per nucleon–nucleon pair and mN therest mass of the nucleon) is plotted in Fig. 4 for centralPb+Pb (Au+Au) collisions and p+p(p) interactions (seereferences in [6, 7]).
The top panel of the figure shows that the total numberof pions (〈π〉 = 1.5 ·(〈π〉+〈π〉)) produced per wounded nu-cleon increases with energy as expected in both reactions.However, the rate of increase in nucleus–nucleus collisionsbecomes larger within the SPS energy range and then staysconstant up to the RHIC domain. Below 40 A·GeV theratio 〈π〉/〈NW 〉 in A+A collisions is lower than in p+pinteractions (pion suppression), while at higher energies〈π〉/〈NW 〉 is larger in A+A collisions than in p+p(p) in-teractions (pion enhancement). The transition from pionsuppression to pion enhancement (the “kink”) is demon-strated more clearly in the insert of Fig. 4, where thedifference between 〈π〉/〈NW 〉 for A+A collisions and thestraight line parameterization of the p+p(p) data is plot-ted as a function of F up to the highest SPS energy.
The energy dependence of the ratio 〈K+〉/〈π+〉 of themean multiplicities of K+ and π+ mesons produced in
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300
A+A:NA49AGSRHICp+p
Figure 4: Energy dependence of the mean pion multiplic-ity per wounded nucleon, the 〈K+〉/〈π+〉 ratio and theinverse slope parameter T of the transverse mass spectraof K+ mesons measured in central Pb+Pb (Au+Au) col-lisions (solid symbols) compared to results from p+p(p)reactions (open dots). The changes in the SPS energyrange (solid squares) suggest the onset of the deconfine-ment phase transition.
central Pb+Pb (Au+Au) collisions is plotted in the mid-dle panel of Fig. 4. Following a fast rise from threshold,the ratio passes through a maximum (the “horn”) in theSPS range and then seems to settle to a lower plateauvalue at higher energies. Kaons are the lightest strangehadrons and 〈K+〉 counts for about half of all the anti-strange quarks produced in the collisions at AGS and SPSenergies, the other half being contained in K0 (isospinsymmetry). Thus, the relative strangeness content of theproduced matter passes through a sharp maximum at theSPS in nucleus–nucleus collisions. This feature is not vis-ible in p+p(p) reactions.
The energy dependence of the inverse slope parameterT of transverse mass spectra of K+ mesons producedin central Pb+Pb (Au+Au) collisions is presented inthe bottom panel of Fig. 4. The plateau at the SPSenergies (the “step”) is preceeded by a steep rise of T
measured at the AGS and followed by a further increaseindicated by the RHIC data. Although the scatter of datapoints is large, T appears to increase smoothly in p+p(p)interactions [7].
The system size dependence of the yields of negativelycharged pions and charged kaons are presented for C+C,
Si+Si, and minimum bias Pb+Pb collisions at 40 A·GeVbeam energy. The nearly constant 〈K+〉/〈K−〉 ratio fromC+C interactions up to central Pb+Pb collisions indicatesthat there are no significant changes in the respective pro-duction mechanisms of K+ and K−. The slow decrease ofthe 〈π−〉/〈NW 〉 ratio for centrality selected Pb+Pb colli-sions could be an effect of the increased pion absorptionprobability in the extended hot and dense nuclear matterof the larger systems.The results on the 〈K〉/〈NW 〉 and the 〈K〉/〈π〉 ratios sug-gest that the mean number of wounded nucleons 〈NW 〉 isnot the appropriate scaling parameter for the system sizedependence of the yields of charged kaons [8, 9].New results on charged pion and kaon production in cen-tral Pb+Pb collisions at 20 and 30 A·GeV are presentedand compared to measurements at lower and higher ener-gies. A change of energy dependence is observed around 30A· for the yields of pions and kaons as well as for the shapeof the transverse mass spectra. Of the available model ex-planations [10, 11, 12, 13] a reaction scenario with the on-set of deconfinement best reproduces the measurements.
References
[1] S V Afanasiev et al., (NA49 Collaboration), Phys.Rev. C66 (2002) 054902.
[2] S. Afanasiev et al. (NA49 Collab.), Nucl. Instrum.Meth. A430, 210 (1999).
[3] A M Rossi et al., Nuclear Physics B84 (1975) 269.
[4] Ahle et al., (E802), Phys. Rev. C60 (1999) 044904.
[5] E. Fermi, Prog. Theor. Phys. 5, 570 (1950).
[6] M. Gazdzicki and D. Rohrich, Z. Phys. C65, 215(1995); C71, 55 (1996) and references therein.
[7] M. Kliemant, B. Lungwitz and M. Gazdzicki, Phys.Rev. C 69, 044903 (2004) [arXiv:hep-ex/0308002].
[8] C Hohne for the NA49 Collaboration, Nucl. Phys.A715 (2003) 474c.
[9] C Alt et al., (NA49 Collaboration), nucl-ex/0406031.
[10] M. Gazdzicki, M. I. Gorenstein, F. Grassi, Y. Hama,T. Kodama and O. . J. Socolowski, Braz. J. Phys. 34,322 (2004) [arXiv:hep-ph/0309192].
[11] S. A. Bass et al., Prog. Part. Nucl. Phys. 41, 225(1998) [arXiv:nucl-th/9803035],
[12] H. Sorge, H. Stocker and W. Greiner, Nucl. Phys. A498 (1989) 567C.
[13] W. Cassing, E. L. Bratkovskaya and S. Juchem, Nucl.Phys. A 674, 249 (2000) [arXiv:nucl-th/0001024].
