University of Bucharest An overview of the experimental results obtained with BRAHMS experimental set-up Alexandru JIPA Atomic and Nuclear Physics Chair, Faculty of Physics, University of Bucharest, ROMANIA 3 rd Winter School on RHIC, 8-11.XII.2003, Budapest, Hungary

University of Bucharest Summary 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 - 3 rd Winter School on RHIC, 8-11.XII.2003, Budapest

University of Bucharest Questions 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 - 3 rd Winter School on RHIC, 8-11.XII.2003, Budapest

University of Bucharest  K  p  n  d, Heavy ion collisions

University of Bucharest Two independent rings ~3.8 km in circumference  Run 1: June - September 2000 First Physics Run two energies  S NN = 56 and 130 GeV  Run 2: July January 2002  S NN = 200 GeV (maximal design energy) p+p (reference data)  Run 3: December May 2003  S NN = 200 GeV  S NN = 200 GeV RHIC experiments

University of Bucharest The BRAHMS experiment Setup for Au+Au data in , 2003 Added Cherenkov + 2. TOF in MRS The BRAHMS experiment Setup for Au+Au data in , 2003 Added Cherenkov + 2. TOF in MRS

University of Bucharest Charged particle  - 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 /  /  R 2 dN/d  ~ 4.5 GeV/fm 3 Rapidity density uniform over +-2 units of pseudo-rapidity.  s NN = 200 GeV Ref PRL 88, (2002) Centralities 0-10,10-20,..

University of Bucharest Inverse 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.

University of Bucharest Colective transverse flow EXP HIJING UrQMD BRAHMS PRELIMINARY

University of Bucharest Hydrodynamics-based parameterization Blast-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, and the particle density distribution is independent of the radial position.

University of Bucharest Fitting 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% centrality BRAHMS PRELIMINARY

University of Bucharest Fitting 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

University of Bucharest Inverse 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.

University of Bucharest 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):

University of Bucharest 0-10%, 40-60%

University of Bucharest Coulomb 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.

University of Bucharest Chemical 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/

University of Bucharest Chemical 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.

University of Bucharest Baryonic 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.

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

University of Bucharest 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  - distributions d-Au  s NN = 200 GeV

University of Bucharest 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.

University of Bucharest Rapidity 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 p T 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 (Mar )

University of Bucharest Thermal Interpretation The baryon chemical potential is given by p-bar/p = exp(- 2  B /T) By simple quark counting in quark recombination K - /K + = exp(2m s /T)exp(-2m q /T) = exp(2m s /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. (PRC ): Based on SPS results and assuming T=170 MeV

University of Bucharest Is there a common‘Temperature’ if 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:

University of Bucharest 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 -

University of Bucharest Longitudinal Bulk Properties BRAHMS Preliminary Pion: Power law fit A(pt/p0+1) -n Kaon: m T single exponential fit         D. Ouerdane (NBI) Thesis

University of Bucharest Longitudinal 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/(4m 2 )) P.Carrruthers and M.Duong-van PRD8,859(1973)

University of Bucharest Net-Baryon Densities Earlier 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. Preliminary The 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.

University of Bucharest Energy systematic of Rapidity loss 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/E initial ~ 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~0  y y y y

University of Bucharest High p t Suppression & Jet Quenching q q hadrons leading particle leading particle Schematic view of jet production  Particles with high p t ’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 p t 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

University of Bucharest Systematizing Our Expectations no effect  Describe in terms of scaled ratio R AA = 1 for “baseline expectations” > 1 “Cronin” enhancements (as in proton-nucleus) < 1 (at high p T ) “anomalous” suppression

University of Bucharest Charged Hadron Spectra  Reference spectrum p+pbar spectra (UA1) R AA = Yield(AA) N COLL (AA)  Yield(NN) Scaled N+N reference Nuclear Modification Factor R AA <1  Suppression relative to scaled NN reference  Data do not show suppression  Enhancement (R AA >1) due to initial state multiple scattering (“Cronin effect”) Known in p+A collisions

University of Bucharest High p t Suppression in Au+Au  Central Collisions R AA < 1 at high p t At Mid-Rapidity (  =0 )  Peripheral Collisions R AA ~ 1 at high p t  Clear suppression effect Consistent with Jet Quenching  No suppression (as expected)  Consistent with observations by PHENIX and STAR  BRAHMS can also measure at more forward rapidities

University of Bucharest Suppression at large   R AA exhibits same trends  R CP : 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 ) R CP = Yield(0-10%) / N COLL (0-10%) Yield(40-60%) / N COLL (40-60%) The traversed medium which causes the suppression is extended in the longitudinal direction

University of Bucharest 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

University of Bucharest 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

University of Bucharest High pt Suppression – Hydro-Jet Model Calculations Hirano & Nara (nucl-th/ )  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 momentum spectrum 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

University of Bucharest Information from high-pt quenching an bulk properties Both  Au-Au suppression (I. Vitev and M. Gyulassy, hep-ph/ )  d-Au enhancement (I. Vitev, nucl-th/ ) understood in an approach that combines multiple scattering with absorption in a dense partonic medium  Our high p T probes have been calibrated dN g /dy ~ 1100 e > 100 e 0

University of Bucharest State 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:

University of Bucharest Gluon 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.

University of Bucharest BRAHMS can reach very small values of x in the Au gluon distributions: A is d and B is Au. Energy and momentum conservation x L = x a - x b =(M T /√s)sinh y k a + k b = k x a x b = M T 2 /s A solution to this system is: x a = (M T /√s) e y x b = (M T /√s) e -y where y is the rapidity of the (x L,, k) system

University of Bucharest Two extreme Model predictions I. Vitev nucl-th/ v2 D. Kharzeev hep-ph/ CGC at y=0 Y=0 Y=3 Y=-3 Very high energy As y grows

University of Bucharest p-p & d-Au distributions. 2.9 <  <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.

University of Bucharest d-Au Nuclear Modification factor at  ~3.2 BRAHMS preliminary R dAu 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 N coll =7.2 Error is systematic. PRL (2003) The high rapidity d-Au do also show a significant suppression. This is consistent 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.

University of Bucharest 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-p t particles relative to elementary pp collisions is observed in central Au-Au collisions, but neither in peripheral, nor in the control d-Au experiment.

University of Bucharest Conclusions and Outlook The 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-p t 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.

University of Bucharest 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 Collaboration