Recent Results from the Graal and LEGS Beams Carlo Schaerf Citation: AIP Conf. Proc. 1056, 404 (2008); doi: 10.1063/1.3013072 View online: http://dx.doi.org/10.1063/1.3013072 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1056&Issue=1 Published by the American Institute of Physics. Additional information on AIP Conf. Proc. Journal Homepage: http://proceedings.aip.org/ Journal Information: http://proceedings.aip.org/about/about_the_proceedings Top downloads: http://proceedings.aip.org/dbt/most_downloaded.jsp?KEY=APCPCS Information for Authors: http://proceedings.aip.org/authors/information_for_authors Downloaded 14 Nov 2012 to 193.206.209.24. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions
Transcript
Recent Results from the Graal and LEGS BeamsCarlo Schaerf Citation: AIP Conf. Proc. 1056, 404 (2008); doi: 10.1063/1.3013072 View online: http://dx.doi.org/10.1063/1.3013072 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1056&Issue=1 Published by the American Institute of Physics. Additional information on AIP Conf. Proc.Journal Homepage: http://proceedings.aip.org/ Journal Information: http://proceedings.aip.org/about/about_the_proceedings Top downloads: http://proceedings.aip.org/dbt/most_downloaded.jsp?KEY=APCPCS Information for Authors: http://proceedings.aip.org/authors/information_for_authors
Downloaded 14 Nov 2012 to 193.206.209.24. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions
Presented by Carlo Schaerf for the Graal ^ and LSC collaborations ^
Universita degli Studi di Roma "Tor Vergata" INFN - Sezione di Roma "Tor Vergata"
Abstract. The polarized and tagged Graal gamma-ray beam is obtained by backward Compton scattering of laser light on the high-energy electrons circulating in the ESRF storage ring. This technique, first developed for the Ladon beam on the storage ring Adone at LNF [1], provides gamma-ray beams with linear or circular polarizations close to one and well known. The Graal beam covers the energy region between 600 and 1500 MeV thus allowing the study of baryon resonances up to an energy of 1916 MeV, in large part by precision measurements of beam polarization asymmetries in meson photoproduction on the nucleons.
The differential cross section for the photoproduction of mesons with linearly polarized y-rays on unpolarized nucleons is:
^ ^ ^ = ^ [ l + P . A ( M ) - ( 2 ^ ) ] (1)
where —"J'̂ is the cross section for unpolarized y-ray beam (function of k, the y-ray energy, and 6 the polar angle of the meson in the CMS), (j) the azimuthal angle between the polarization and reaction planes, Pyik) the polarization of the y-ray beam as function of k, and A{k, 6) the beam polarization asymmetry (function of k and 6).
The Graal detector [2] has been designed to have cylindrical symmetry around the beam axis and we have collected data with two orthogonal orientations of the polarization plane exploiting the feature of Ladon beams that the polarization of the y-ray beam is in the same direction of that of the laser and therefore can be easily rotated with optical half-wave plates. Graal has produced high quality asymmetries for the reactions:
in Hydrogen
Y + p ^ Jt^ + p , Y + p ^ rj + p , y - | - / 5 — > ; r + - | - «
^ V. Bellini, J. P. Bocquet, L. Casano, M. Castoldi, A. D'Angelo, J. P Didelez, R. Di Salvo, A. Fantini, D. Franco, G. Gervino, F Ghio, G. Giardina, B. Girolami, A. Giusa, M. Guidal, E. Hourany, R. Kunne, A. Lapik, P. Levi Sandri, A. Lleres, F Mammoliti, G. Mandaglio, M. Manganaro, D. Moricciani, A. Mushkarenkov, V. Nedorezov, L. Nicoletti, C. Randieri, D. Rebreyend, N. Rudnev, G. Russo, C. Schaerf, M.-L. Sperduto, M.-C. Sutera, and A. Turinge, V. Vegna. ^ K. Ardashev, C. Bade, M. Blecher, A. Caracappa, A. D'Angelo, R. Di Salvo, A. Fantini, C. Gibson, H. Gluckler, K. Hicks, S. Hoblit, T. Kageya, M. Khandaker, O.C. Kistner, S. Kizigul, S. Kucuker, A. Lehmann, F Lincoln, R. Lindgren, M. Lowry, M. Lucas, J. Mahon, L. Miceli, D. Moricciani, M. Pap, B. Preedom, A.M. Sandorfi, H. Seyfarth, C. Schaerf, H. Stroher, C.E. Thornl, C.S. Whisnant, K. Wang, X. Wei.
Downloaded 14 Nov 2012 to 193.206.209.24. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions
A <E,>-0.702S2 0eV 0 <E,>-0,70JCeV
0.75
0,5
0.25
0
r 0 A
/N <E,>=0.799625 0eV
0.7&
O.b
0.2&
0
---_
1 ,
«w«*V
% 0
%
. . 1 . . .
a •:£,> =0,835835 QBV O <E^>=0,895GeV
0.75
0.5
0 .25
0
:
L CcrtcQp
0 100 A <£,>=0.99119SG<!V O <E;>^0.99a Gov
0.5
0
• %
r ^ S
^ : o'i dP*o
10
(
0.5
:
) s
:,
100
<£>•• 1.085 GeV
\
6
0 .5
n
A O
<E,>= 1.281 Gov <E,>= 1.261 CeV
a
o ^
0 100 A <£,> = l.>J7GeV O <E,> = 1.J'.4 0eV
mo
100 <E,>-1.4247CcV <E,> = :.425GeV
: -: r _ ^ : ,
0
4 "«
«=g o°* o i 0 ^ * '«f
, , , 1 , , ,
100 100
FIGURE 1. Beam polarization asymmetries 2 for jr" photoproduction on free (open circles) and quasi-free proton (triangles). The results are almost identical with some smoothing of the quasi-free curves near maxima or minima probably due to Fermi motion of the target nucleon.
Y + P
Y + P
in Deuterium
^ K+ + A , V -Y + p -i- Jt -\
p ^ K+ + lP (2)
Y + d ^ n + p
Y + p{n) -^ 7] + p(n) .
Y + p{n) -> JT*̂ + p{n)
n(p) -^ 7] + n{p)
- n(p) —̂ JT" + n{p) (3)
A Deuterium target has been used to study the reactions on the neutron. To compare the experimental results obtained by us on a neutron bound in the deuteron with the available theoretical results for a free neutron, we have compared the asymmetries obtained for the free proton in Hydrogen with those obtained for the bound proton in Deuterium. This comparison is indicated in figures 1 and 2 for JT" and r\ photoproduction, respectively. Figures 3 and 4 present a comparison of the results for the quasi-free neutron with those for the quasi-free proton for JT" and r\, respectively.
In Fig. 6 we have indicated the tantalizing enhancement of the 77-n yield compared to that for 77-p above the peak of the S11 resonance. This anomalous peak at an invariant mass around 1.68 GeV, first discovered at Graal [5] and successively confirmed by the Bonn cross section measurements [6], has been interpreted in various ways [7]. As
405
Downloaded 14 Nov 2012 to 193.206.209.24. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions
FIGURE 2. Beam polarization asymmetries 2 for 77 photoproduction on free (full dots) and quasi-free proton (open squares). The different results between 900 and 1100 MeV coincide with the minimum of the cross section and have been explained with the Fermi motion of the target nucleon. The energy values outside and inside parenthesis indicate the mean value of the bin for quasi-free and free protons respectively. Dotted and solid lines are the predictions of Maid2001 [3] for the free and quasi-free protons respectively while the dashed lines are those for the reggeized model of ref. [4].
function of y-ray energy its width can be explained completely as due to the Fermi motion of the neutron in the deuteron while as function of the 17-n invariant mass its width (~ 40 MeV FWHM) is due entirely to the experimental resolution of our apparatus. No lower limit of the width can be derived from these experiments.
The production of strange particles in the reactions
y + ;? ^ /<:+ + A , and y + p ^ K^ + iP (4)
allows the measurement of double-polarization beam-recoil observables by the asymmetries in the weak decay of the strange baryons. Fig. 7 compares our preliminary results [8] for the angular distributions of the beam-recoil observables Ox and O^ with the theoretical Coupled Channel calculations of Bonn [9] and the Regge Plus Resonance calculations of Ghent [10]. Beam-recoil double-polarization asymmetries for the same reaction have been measured at CLAS with a circularly polarized y-ray beam producing the Cx and Q observables. The consistency of the Graal and CLAS results can be tested comparing the angular distributions of a combination of the observables measured by each experiment. We must have:
(1 + r̂ - I? - ol - o\) 1/2 (f -rCl^C\fl^<\ (5)
406
Downloaded 14 Nov 2012 to 193.206.209.24. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions
1
0 .75
A < E , > - 0 . 7 0 2 5 Z GeV O <E,> =0.702605 Gov
0.5 fr
0 .25
0 I
0 100
A < L > = 0 , 9 9 1 1 9 5 G c V O < E ; > ^ 0 , 9 9 1 1 8 e e V
0.5
Q
-015
0 100
A < E ^ = 1,2B1 GeV Q <E,> = 1,2611 GeV
0,5 -
O A I
T
100
1
0 .76 '-
0.5 ^
0 .25 '-
0 -
A <E,>=0.7S9625CeV O <E,>=0.79933G4V
a <E ,> = Q.a9683&CeV O <E,>=0 .89547 GeV
AAdSo
1
0.5
0
- 0 . 5
1
0,75
0.5
0.25
0
A < E , > - l , 0 a + 4 G s V O <E,> = 1,0B47GeV
0 100 A <E,>=1 .34J7GeV O <£ ,>=1 .J437GeV
* S o *
1
0.75
0.5
0.25
0
0 100 A < £ , > = ! .1745 GeV
^ O < g > ^ 1 . 1 7 4 S G e V
- a 0.5 - ®
8
0 r
-0.5 -
o}
0 IQQ A < E , > - 1 . 4 2 + 7 GeV O <E^>^1 .424a G$V
FIGURE 3. Sample of beam polarization asymmetries 2 for jr" photoproduction on quasi-free neutron (open circles) and quasi-free proton (empty triangles). The striking differences between the two nucleons is due to isospin T=l of the jr".
I
075
0.5
0.26
0 1
E 07=
0.6
0,25
0 1
E
0.75
0-6
0.25
(H >^0-746C;tV
i * tT*t = j
(I'J^O.SfMOcV
i ' • *?T ,
(R Msci-'sri^v
V ' \__\_i i • • • i f I 1
' T
(E^M.99 GeV <E,)=1J (E,)=l.lGcy
1 * oSi ;
..' »..
t ? . j j - ..t .•.'*.:
..°..i (B,)=l <U,)=
t t h ««t * • ;
<R|.>= 1.435 GeV
...i
FIGURE 4. Beam polarization asymmetries 2 for rj photoproduction on quasi-free neutron (full triangles) and quasi-free proton (open squares). The main differences between the two nucleons are at forward angles and at energies above 1 GeV where the proton has a peak while the neutron exhibits a more symmetric behavior around 90°.
407
Downloaded 14 Nov 2012 to 193.206.209.24. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions
FIGURE 5. The same data of Fig. 4 for rj on quasi-free neutron (full triangles). The mean energies for each bin are indicated. The solid and dashed lines illustrate the predictions for neutrons of Maid2001 and the reggeized model of ref. [4].
where the first quantity has been obtained with the Graal results and the last with those of CLAS. As indicated in Fig. 8 both conditions are reasonably satisfied.
The LEGS Spin Collaboration (LSC) has successfully operated a frozen-spin polarized HD target on the polarized and tagged Ladon beam installed on the NSLS storage ring at BNL. The HD molecule allows the independent polarization of its p and d nuclei. Figure 9 indicates the p and d polarizations during its 2004 and 2005 runs.
The LEGS tagged beam provides y-ray with an energy between 180 and 421 MeV in the region of the A33 pion-nucleon resonance. Therefore the LSC collaboration has focused on the measurement of the E double polarization observable using a mainly circularly polarized beam on a longitudinally polarized target to verify the Drell-Hearn-Gerassimov sum rule on the nucleons. Fig. 10 provides a sample of the results obtained for the Jt^ and Jt^ yields as a function of the missing energy in the reconstructed kinematics when the spins of the target and the beam are oriented parallel or antiparallel. The angular distribution of the difference between the parallel and antiparallel cross sections are shown in Fig. 11. The LSC are compared with those of Mainz.
REFERENCES
1. L. Federici, et al., // Nuovo Cimento 59B, 247 (1980). 2. O. Bartalmi et al., Eur. Phys. J. A 26, 399 (2005). 3. W.-T. Chiang et al., Nucl. Phys. A 700, 429 (2002). 4. W.-T. Chiang et al., Phys. Rev. C 68, 045202 (2003).
408
Downloaded 14 Nov 2012 to 193.206.209.24. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions
" • profon-A neutron
f i
J ^ Egamma(GeV) Lgamma(GeV)
^ ttli • ^ f *u'rt 1 120.<l5,r<150
f lift : :rn w r% ̂
; • - . . ' » \
n l" j
- 1
''
li If «.
"T"5D".""<"-iy,̂ "'<:"T8a.""
11 • -proton •••
^
' • ^ ' %
FIGURE 6. The yield for rj photoproduction on the neutron (triangles) and the proton (squares) at different angles. The four picture on the left are plotted as a function of the y-ray energy assuming a neutron at rest while those on the right are plotted as function of the invariant mass of the rj -n system calculated from the measured kinematical variables of the two particles.
L5
O 1.0
60 no ISO 60 120 ISO
60 no ISO 60 no iso 60 no ISO 60 no iso
9™
FIGURE 7. The beam-recoil double-polarization observables Ox and O^ are compared with the theoretical calculations of Bonn (solid lines) and Ghent (dotted lines) as indicated in the text.
409
Downloaded 14 Nov 2012 to 193.206.209.24. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions
GRAAL vs CLAS data
+
+
9
1^
2.0
1.5
1.0
0.5
0.0
7.5 P
7.0 p
0.5 \-
0.0
1.5
1.0
0.5
0.0
7.5 P
1
0.5 h
980 MeV
1122 MeV (1132 MeV)
Oh
+^Vj%4 '-4-+-i-i-r + ̂ 4
1272 MeV
+-t-H;t
1421 MeV (1433 MeV)
•4-*4U'''-^-+-^-H-r^-h-J
1027 MeV (1032 MeV)
m d-t-
1171 MeV
-ft
1321 MeV (1332 MeV)
1466MeV
1074 MeV
>-f- f
1222 MeV (1232 MeV)
-h ,*
1372 MeV
^-i^-f-f
60 120 180
• GRAAL data
(l + f-I.^-0^2.02f
D CLAS data
(I^+C2 + C2f^
60 120 180 60 120 180
FIGURE 8. Angular distribution of the quantities: (1 + T ̂ - 1 : ^ - O^ - O^) ̂ /̂ = (p2 ^ ^2 ^ ^2^ 1/2 < ^ For a comparison between the Graal and CLAS results. The combination of Graal data is indicated by full points while that of CLAS by empty squares. The Graal energies are indicated above and those of CLAS below in parenthesis.
Fall'2004: Sprmg'2005 _
10 15 20 25 30 Time (days)
10 15 20 25 30 35 Time (days)
FIGURE 9. Polarizations of H (full dots) and D (gray diamonds-vector and full squares-tensor) nuclei in HD during the two data collection periods. Mid-way through each, the H polarization was flipped using a RF transition.
410
Downloaded 14 Nov 2012 to 193.206.209.24. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions
Figure 10. Differences between 2-body kinematics and the measured energy for jr^ (top panels) and jr^ (bottom panels), in the cases of parallel (left panels) and anti-parallel (right panels) beam and target spin alignments. The simulated energy differences are shown as the solid curves.
-100 -50 0 50 100 -100 -50 0 50 100 E missing (MeV) E missing (MeV)
0 45 9 0 135 0 45 9 0 135 1 8 0 I • ' I • ' I • ' I ' • I ' • I • ' I ' •
^
• g - 4 0
^\
i I \
X ̂•^
EY=304MeV f Ey= 320 MeV
Figure 11. Angular dependence of the [da{P) - da{A)] spin-difference cross section for polarized H at beam energies near the A peak. The full data from Fair04 and Spring'05 are shown as solid circles. Unpolarized limits (solid squares) at 0^ and 180^ are the mean of SAID [8] and MAID [9]. Open diamonds are results from Mainz [11] at 310 MeV (left) and at 330 MeV (right). Predictions from SAID and MAID are shown as dotted and dashed curves, respectively. The solid curves in the top panels are a Legendre fit to the new data.
0 46 90 135 0 45 90 135 180
5. C. Schaerf et al., Proceedings ofNSTAR2005, World Scientific, Tallahasse (Fl), 2005, page 176; V. Kuznetsov et al., ibidem page 336.
6. I. Jaegle et al., Phys. Rev. Lett. 100,252002 (2008). 7. For example see: M. V. Polyakov and A. Rathke, Eur. Phys. J. A 18,691 (2003). 8. A. Lleres etal. to be published. 9. A. V. Anisovich et al., Eur. Phys. J. A 25, 427 (2005); A. V. Sarantsev et al., Eur. Phys. J. A 25,441
(2005). 10. T. Corthals, J. Ryckerbush and T. Van Cauteren, Phys. Rev. C 73, 045207 (2006). 11. J. Ahrens et al.,Eur. Phys. J. All,323 (2004).
411
Downloaded 14 Nov 2012 to 193.206.209.24. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions
