University of Bucharest An overview of the experimental results obtained with BRAHMS experimental...

University of Bucharest

An overview of the experimental An overview of the experimental results obtained with BRAHMS results obtained with BRAHMS

experimental set-upexperimental set-up

Alexandru JIPA

Atomic and Nuclear Physics Chair, Faculty of Physics,

University of Bucharest, ROMANIA

3rd Winter School on RHIC, 8-11.XII.2003, Budapest, Hungary


SummarySummary

• The importance of the heavy ions collisions • BRAHMS experimental set-up: structure, opportunities and

goals• Global information: charged particle multiplicities and

rapidities; estimation of the energy density• Transverse dynamics: temperatures and radial flow• Longitudinal dynamics• Antiparticle to particle ratios: Coulomb momentum, chemical

potentials, entropy per barion• New aspects:

– - High-pt suppression – was new matter formed and observed?

– - Does Gluon Saturation manifest itself at RHIC energies? • Final remarks

Al.Jipa - 3rd Winter School on RHIC, 8-11.XII.2003, Budapest


Questions of InterestQuestions of Interest What are the New States of Matter at High Density and Temperature?

The theory of how protons and neutrons form the atomic nuclei of the chemical elements is well developed. At higher densities, neutrons and protons may dissolve into an undifferentiated media of quarks and gluons, which can be probed in heavy-ion accelerators. Densities beyond nuclear densities occur and can be probed in neutron stars, and still higher densities and temperatures existed in the early universe.

What has RHIC , and in particular BRAHMS done in its first 3 runs?

Al.Jipa - 3rd Winter School on RHIC, 8-11.XII.2003, Budapest


Kpnd,

Heavy ion collisions


Two independent rings ~3.8 km in circumference

Run 1: June - September 2000

First Physics Run

Au+Au @ two energies

SNN = 56 and 130 GeV

Run 2: July 2001- January 2002

Au+Au @ SNN = 200 GeV

(maximal design energy)

p+p (reference data)

Run 3: December 2002- May 2003

d+Au @ SNN = 200 GeV

p+p @ SNN = 200 GeV

RHIC experimentsRHIC experiments


The BRAHMS experimentThe BRAHMS experimentSetup for AuSetup for Au++Au data in 2001Au data in 2001-2002,-2002,

2003 Added Cherenkov + 2. TOF in MRS2003 Added Cherenkov + 2. TOF in MRS


Charged particle Charged particle - distributions - distributions• dN/d resembles Bjorken boost

invariant assumption. Due to this as well as easier to deal with Hydro calculations have typically been done with assumption.

• Part of the shape is effected by the use of rather the y.

• Most of Brahms data were collected for central collisions

• Energy densities seen in meson production can be estimated by Bjorken’s formulae:

• E= 1.5 <pt>//R2 dN/d ~ 4.5 GeV/fm3

• Rapidity density uniform over +-2 units of pseudo-rapidity.

sNN =200 GeV

Ref Ref PRL PRL 88, 202301(2002)88, 202301(2002) Centralities 0-10,10-20,.. Centralities 0-10,10-20,..


Inverse slope vs. Mass and centralityInverse slope vs. Mass and centrality

The dependence of the effective temperature on both mass and collision centrality is an indication of radial expansion.

Experimental temperatures are greater than the temperatures obtained from simulated data with HIJING and UrQMD codes.


Colective transverse flowColective transverse flow

EXP

HIJING

UrQMD

BRAHMS PRELIMINARY

BRAHMS PRELIMINARY


Hydrodynamics-based parameterizationHydrodynamics-based parameterizationBlast-wave modelBlast-wave model

Considering a hydrodynamically behaving boosted source, a parameterization is fitted simultaneously to all the particle spectra to determine the magnitude of the radial flow. It is assumed that:

all particles decouple kinematically on a freeze-out hypersurface at the same freeze-out temperature Tfo,the particles collectively expand with a velocity profile increasing linearly with the radial position in the source, andthe particle density distribution is independent of the radial position.


Fitting the Transverse Mass SpectraFitting the Transverse Mass Spectra

For 0-10% and 40-60% centrality, the first 3 n- contour levels are shown.

From the peripheral to the central data, the single particle spectra are fit simultaneously for all pions, kaons, and protons.

0-10% centralityBRAHMS PRELIMINARY


Fitting the Transverse Mass SpectraFitting the Transverse Mass Spectra 40-60% centrality

For 0-10% and 40-60% centrality, the first 3 n- contour levels are shown.

From the peripheral to the central data, the single particle spectra are fit simultaneously for all pions, kaons, and protons.

BRAHMS PRELIMINARY


Inverse slope vs. EnergyInverse slope vs. Energy

BRAHMS preliminary results for 10% most central events in comparison with the results from other experiments at lower energies (AGS, SPS).[1] The BRAHMS extracted Tfo and beta have statistical errors only.

[1]. N. Xu, M. Kaneta – Nucl. Phys. A698 (2002) 306c

At RHIC energies, the collective flow velocity parameter is larger than that from collisions at AGS/SPS energies.

The temperature parameters, compared to results from lower energy collisions, seem to be lower .


Coulomb interaction study Coulomb interaction study Coulomb interaction is investigated through the produced charged pions ratio in Au-Au collisions obtained with BRAHMS experimental set-up.

Coulomb momentum (“kick”) is:

The pion ratio can be described by the relationship:

Where:

Freeze-out radius is: – geometrical (initial) radius of the fireball – transverse flow velocity – freeze-out time

Rgeom

Results obtained at lower energies (AGS si SPS):


0-10%, 40-60%0-10%, 40-60%


Coulomb momentum at BRAHMSCoulomb momentum at BRAHMS

The Coulomb effects in pion spectra are sensitive to the degree of stopping and the distribution of positive charge, as well as at the flow velocity of the participant region.

The values reflect a reduced Coulomb effect because of higher flow velocities of the nuclear matter from participant region.


Chemical potential vs. EnergyChemical potential vs. Energy

The energy dependence of the chemical potential was shown to be parametrized as:

P. Braun-Munzingen, K. Redlich, J. Stachel - nucl-th/0304013


Chemical freeze-out temperature vs. energyChemical freeze-out temperature vs. energy

The energy dependence of the chemical temperature can be parametrized as:

The chemical freeze-out temperature seems to saturate close to the critical temperature of 170 MeV extracted from lattice QCD calculation.


Baryonic chemical potentialBaryonic chemical potential

The chemical potential increases from midrapidity to forward rapidities, because at y=0, the net-baryon density is much reduced than what was observed at forward rapidities.


Strange chemical potentialStrange chemical potential

The small value obtained for 200 GeV may suggest that the we are close to the full chemical equilibrium for strange particles.


Charged particle multiplicities for the centrality ranges of 0-30% and 30-60%.

The square points and circular points from SiMA and TMA detectors, respectively, while the triangles are from the BBC detectors.

Charged particle Charged particle - distributions - distributions

d-Au sNN =200 GeV


Charged particle multiplicities for the centrality range 0-30% and 30-60%.

The shaded regions indicate the total (statistical and systematic) uncertainties.

The dotted and dashed curves are the results of HIJING and Saturation Model predictions.

Model calculations based on perturbative QCD (shadowing and jet-quenching mechanisms) lead to excellent agreement with experimental results.

Model calculations based on the saturation picture of non-perturbative QCD do not reproduce the centrality or pseudorapidity dependence of the measurements.


Rapidity dependent ratiosRapidity dependent ratios

•At y=0 (20% central) pbar/p = 0.75 ±0.04 K-/K+ = 0.95 ±0.05 p-/p+ = 1.01 ±0.04•Highest pbar/p ratio indicating a nearly transparent system with very few net baryons.•Ratios ~identical over +-1 unit around mid-rapidity.•Only weak centrality and pT dependence (not shown here) •No Hyperon feed down correction applied: less then 5% correction to ratio’s.•Dynamical (cascade, string) models do NOT describes rapidity dependent ratios and yields correctly

PRL 90 102301 (Mar. 2003)


Thermal InterpretationThermal Interpretation•The baryon chemical potential is given by p-bar/p = exp(- 2B/T) •By simple quark counting in quark recombination K-/K+

= exp(2ms/T)exp(-2mq/T)

= exp(2ms/T)(pbar/p)1/3

= (pbar/p)1/3

by assuming local (in y) strangeness conservation •K-/K+=(p-bar/p)a a = 0.24±0.02 for BRAHMS a = 0.20±0.01 for SPS•Good agreement with the statistical-thermal model prediction by Becattini et. al. (PRC64 2001): Based on SPS results and assuming T=170 MeV


Is there a common‘Temperature’ ifIs there a common‘Temperature’ ifall particle are considered?all particle are considered?Apparently:

Assume all distributions described by one temperature T and one ( baryon) chemical potential : One ratio (e.g., p / p ) determines / T :

A second ratio (e.g., K / ) provides T Then predict all other hadronic yields and ratios:

pdedn E 3/)(~ Tμ

TìTì

Tì/2

/)(

/)(

eee

pp

E

E


This exercise in “hadro-chemistry”

Applies to final-state (ordinary) hadrons at end of reaction.

Does not (necessarily)indicate

QGP formation

Deconfinement

New state of matter

The exploration of the freeze-out phase diagram shows a smooth continuation with RHIC results and of trends seen

at lower energies

in p-p, even e+e-


Longitudinal Bulk PropertiesLongitudinal Bulk Properties

BR

AH

MS

Pre

limin

ary

Pion: Power law fit

A(pt/p0+1)-n

Kaon: mT single exponential fit

D. Ouerdane (NBI) Thesis


Longitudinal Meson DistributionsLongitudinal Meson Distributions

No wide “plateau” observed in rapidity for identified mesons. Close to a Gaussian shape ((+) =2.35 ~ (k+) =2.39)

Total yield in agreement with published dN/d measurements from multiplicity sub-system.

The RMS of distributions from low energy to RHIC is strickingly close to prediction of Landau Hydro model

2 = 0.5 ln(s/(4m2)) P.Carrruthers and M.Duong-van PRD8,859(1973)


Net-Baryon DensitiesNet-Baryon DensitiesEarlier saw that p-bar/p ~0.75 near mid-rapidity.The system has very few –net-baryons ie. baryon number that must be conserved in the reaction transported to mid-rapidity.

PreliminaryThe shape of the net-proton distribution measured at RHIC is different rofm what is observed at lower energies.

At RHIC the mid-rapidity region is almost net-proton free. Pair baryon production dominates at RHIC.

The # net-baryons at y~0 is ~10+-2, compared with #produced pions of 900.

The rapidity loss

y = (yb-y) dN/dy / dN/dy

Represents the energy transfer from incident beam.


Energy systematic of Rapidity lossEnergy systematic of Rapidity lossand Net-Proton and Net-Proton

These data showing the ‘increase’ in y for AA, while pp is approximately constant.

The estimated value at RHIC is consistent with a continuous increase of y.

E/Einitial ~ e- y

This implies that ~85 % of the initial energy is stopped and emerges as internal energy, produced particles and at end of reactions in longitudinal and transverse momentum distributions.

Net-protons at y~0 continuously decrease with energy.

pp

Net protons at y~0Net protons at y~0

yy


High High pptt Suppression & Jet Suppression & Jet QuenchingQuenching

q

q

hadronsleadingparticle

leading particle

Schematic view of jet production Particles with high pt’s (above ~2GeV/c) are primarily produced in hard scattering processes early in the collision Probe of the dense and hot stage

Experimentally depletion of the high pt region in hadron spectra

In A-A, partons traverse the medium

p+p experiments Hard scattered partons fragment into jets of hadrons

If QGP partons will lose a large part of their energy (induced gluon radiation) Suppression of jet production Jet Quenching


Systematizing Our ExpectationsSystematizing Our Expectations

no effect

Describe in terms of scaled ratio RAA

= 1 for “baseline expectations”> 1 “Cronin” enhancements (as in proton-nucleus)< 1 (at high pT) “anomalous” suppression


Charged Hadron SpectraCharged Hadron Spectra Reference spectrum

p+pbar spectra (UA1)

RAA =Yield(AA)

NCOLL(AA) Yield(NN)

Scaled N+N reference

Nuclear Modification Factor

RAA<1 Suppression relative to scaled NN reference

Data do not show suppression Enhancement (RAA>1)

due to initial state multiple scattering (“Cronin effect”) Known in p+A collisions


High pHigh ptt Suppression in Au+Au Suppression in Au+Au

Central Collisions RAA < 1 at high pt

At Mid-Rapidity (=0)

Peripheral Collisions RAA ~ 1 at high pt

Clear suppression effectConsistent with Jet Quenching

No suppression (as expected)

Consistent with observations by PHENIX and STAR

BRAHMS can also measure at more forward rapidities


Suppression at large Suppression at large

RAA exhibits same trends

RCP : No need for p+p reference

but large syst. errors (no available p+p reference)

It shows also the suppression at both =0 and 2.2 Effect seems to be similar

Forward Rapidity (=2.2)

RCP =Yield(0-10%)/NCOLL(0-10%)

Yield(40-60%)/NCOLL(40-60%)

The traversed medium whichcauses the suppression is extended in the longitudinal direction


Is this a new Result ?Is this a new Result ?

ISR 31 GeV

SPS 17 GeV

Yes- all previous nucleus-nucleus measurements see enhancement, not suppression.

Effect at RHIC is qualitatively new physics made accessible by RHIC’s ability to produce

(copious) perturbative probes New states of matter?

Au-Au 200


Is this Unique to Heavy Ion ?Is this Unique to Heavy Ion ?

Enhancement in d+Au Typical behaviour of Cronin effect

Absence of suppression in d-Au

Supports the Jet Quenching interpretation for central Au+Au collisions

Excludes alternative interpretation in terms of initial state parton saturation effects

at RHIC energies -- YES! a crucial control measurement via d-Au collisions


High pt Suppression – Hydro-Jet High pt Suppression – Hydro-Jet Model CalculationsModel Calculations

Hirano & Nara (nucl-th/0307087)

Use full 3-D hydro simulations to study the density effects on parton energy loss

Hydro description of the soft Part of the produced matter

Hard part use a pQDC model (PYTHIA)

Generation of momentumspectrum jets

Good agreement with BRAHMS data at both =0 and 2.2

Similar effect at =0 and 2.2. Due to comparable time evolution of the parton density at =0 and 2.2 in hydro. Indirect evidence of the presence of hot thermalized

matter in the region -2.2 < < 2.2


Information from high-pt quenchingInformation from high-pt quenchingan bulk propertiesan bulk properties

Both Au-Au suppression (I. Vitev and M. Gyulassy, hep-ph/0208108)

d-Au enhancement (I. Vitev, nucl-th/0302002 )

understood in an approach that combines multiple scattering with absorption in a dense partonic mediumOur high pT probes have been calibrated dNg/dy ~ 1100 e > 100 e0


State of MatterState of Matter

•The systematics of the flow pattern can be tested for various equations of state (EOS)•At RHIC, the QGP EOS for P(T) is preferred:


Gluon SaturationGluon Saturation

Results just obtained from d-Au measurement near y=0 have shown that final state effects are dominant.

New regimes of partonic physics are expected to appear as x->0. Gluon structure functions are rising; in d-A the #gluons with be very large and the effect from the Parton Distribution Functions will saturate.

To reach small x regions one needs high energies.

Physics near the fragmentation region of the nucleon in p-A collisions offer similar window: go as forward as possible and use the highest A you can work with. Higher rapidities are equivalent to higher energies.


BRAHMS can reach very small values of x in the Au gluon distributions:

A is d and B is Au.

Energy and momentum conservation

xL = xa - xb =(MT/√s)sinh y

ka + kb = k

xaxb = MT2/s

A solution to this system is:

xa = (MT/√s) ey

xb = (MT/√s) e-y

where y is the rapidity of the (xL,, k) system


Two extreme Model predictionsTwo extreme Model predictionsI. Vitev nucl-th/0302002 v2

D. Kharzeev hep-ph/0307037

CGC at y=0Y=0

Y=3

Y=-3

Very high energy

As y grows


p-p & d-Au distributions . 2.9 <p-p & d-Au distributions . 2.9 <<3.3<3.3

BRAHMS preliminary

This distribution was obtaine from different magnetic field settings.

Geometric acceptance and tracking efficiency corrections have been applied

Pythia describes the pp data well.


d-Au Nuclear Modification factor at d-Au Nuclear Modification factor at ~3.2 ~3.2

BRAHMS preliminary

RdAu compares the yield of negative particles produced in dAu to the scaled number of particles with same sign in p-p

For d-Au min.bias data Ncoll=7.2

Error is systematic.

PRL 91 072305 (2003)

The high rapidity d-Au do also show a significant suppression. This isconsistent with the schematic prediction of gluon saturation, albeit it does not prove it. It is certainly significant below the pQCD calculation of Vitev including the Cronin effect.


Final Remarks Final Remarks

• RHIC has obtained a wealth of new and detailed information on relativistic heavy ion reactions.

• From these experimental data we now know that the stage is set to explore and quantify very hot and dense matter.

• The Net-baryon density is very small dN/dy~10, and the corresponding baryon chemical potential B ~29 MeV.

• The system exhibit a large transverse and longitudinal expansion with the azimuthally asymmetries being large, reflecting the initial partonic distributions. The system has reached a hydro dynamical limit, which can be used to explore the Equation of State of the hot dense matter.

• Suppression of high-pt particles relative to elementary pp collisions is observed in central Au-Au collisions, but neither in peripheral, nor in the control d-Au experiment.


Conclusions and OutlookConclusions and OutlookThe heavy ion data from RHIC are consistent with formation

of a hot dense system that• exhibits hydrodynamic behaviour with rapid transverse

and longitudinal expansion.• Absorbs high-pt probes corresponding to a large gluon

density in the initially formed system• Is an almost Baryon-free system

Much remains to be done before one can claim the discovery and characterization of Quark Gluon Plasma done. Examples• suppression pattern of J/Y’s sensitive to the screening in de -confined phase.•Properties of thermal photons from the initial hot phase.In addition it may be that we can also start probing the gluon saturation at forward rapidities.


Other romanian physicists participating in BRAHMS: Dr. Dan Argintaru, Dr. Florin Constantin, Dr. Daniel Felea, Ciprian Mitu, Mihai Potlog, Silvia

Ochesanu, Costin Caramarcu

The BRAHMS CollaborationThe BRAHMS Collaboration

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