Particle Production, Correlations and Jet Quenching at RHIC

QCD@Work 2003 - International Workshop on QCD, Conversano, Italy, 14–18 June 2003 Particle Production, Correlations and Jet Quenching at RHIC John W. Harris∗ Physics Department, Yale University, P.O. Box 208124, 272 Whitney Avenue, New Haven CT, U.S.A. 06520-8124 Results from RHIC depict formation of a dense system at an energy density greater than 5 GeV/fm3, well above that where hadrons exist. Statistical models that describe RHIC data indicate a temperature and baryo-chemical potential at the deconfinement phase transition boundary predicted by lattice QCD calculations. The anisotropic collective flow observed in collisions at RHIC provides direct evidence of strong pressure gradients in the highly interacting dense medium. Suppression of hadrons at large transverse momentum, and quenching of di-jets are observed in central Au + Au collisions and provide evidence for large energy loss of partons traversing the dense matter created at RHIC. 1 Introduction (high transverse momentum) probes to determine its response. Calculations of Quantum Chromodynamics (QCD) on the lattice predict a deconfinement phase transition from hadrons to a Quark-Gluon Plasma (QGP) and an accompanying chiral phase transition [1,2]. A sharp 2 Creating High Density Matter deconfinement transition occurs at a temperature of approximately 175 MeV in 2-flavor QCD and at about Many aspects of the matter created at RHIC, such 20 MeV lower temperature for 3-flavors in the chi- as its bulk properties and collective effects, can be ral limit at zero baryo-chemical potential. There has understood by studying the particles most abundantly been recent success in implementation of techniques produced in these collisions, i.e. those with transverse to calculate on the lattice at a small but finite baryon momentum less than 2 GeV/c. density [3]. Understanding the nature of these phase transitions is expected to have implications for nuclear physics, astrophysics, cosmology and particle physics. 2.1 Particle Production The Relativistic Heavy Ion Collider (RHIC) was con- structed to create the QGP in the laboratory and A large number of particles is produced in a cen- study its properties. RHIC accelerates and collides tral collision at RHIC. The measured charged hadron ions from protons to the heaviest nuclei over a range multiplicity density at midrapidity is dnch/dη |η=0= of energies, up to 250 GeV for protons and 100 A-GeV ± syst 700 27( √) [5] for the 3% most central collisions of for Au nuclei. Since 2000 in the three production runs Au+Auat sNN = 200 GeV. This corresponds to for physics, RHIC√ has collided Au + Au at center-of- a hadron multiplicity density dntotal/dη |η=0= 1050 s mass energies√ NN = 19.6, 130, and√ 200 GeV, d + and a total hadron multiplicity in the most central Au at sNN =200GeV,andp+pat s = 200 GeV. events of ∼ 7000 [5,6]. In terms of number of quarks, RHIC has also begun to collide polarized protons for consider the case where all observed hadrons in the studies of the spin content of the proton. RHIC and final state are mesons, each containing a quark and its four experiments (BRAHMS, PHENIX, PHOBOS, anti-quark. Thus, 7,000 hadrons correspond to 14,000 and STAR) are described comprehensively in [4]. quarks and anti-quarks. A lower limit for the number This presentation will describe the experimental of produced quarks and anti-quarks can be obtained search for the QGP at RHIC and will focus on particle by subtracting off the valence quarks that originally production, correlations, and large transverse momen- enter the collision in the incident nuclei. The number tum phenomena. It will be divided into two parts: 1) of valence quarks in a head-on collision of two Au nu- creating and establishing the properties of the high clei is 2 (Au nuclei) x 197 nucleons per Au x 3 quarks ∼ density matter at RHIC through the study of soft per nucleon 1200. Thus, more than 90% of the more (low transverse momentum) particle production and than 14,000 quarks and anti-quarks in the final state correlations, and 2) probing the matter with hard are produced in the collision. In addition, there are a large number of gluons present that have not been ∗[email protected] considered in this simple estimate. 2 QCD@Work 2003 - International Workshop on QCD, Conversano, Italy, 14–18 June 2003 Λ Λ Ξ Ξ Ω Ω π- π+ - + - π- π- * - φ - Λ - Ξ - Ω π- - + - π- φ - π- Ω - p/p / / / / K/K K/ p/ K0/h /h /h /h / p/p K/K K/ /K p/ /h Ratios 1 × 10 × 50 sNN =130 GeV -1 10 STAR PHENIX PHOBOS sNN =200 GeV BRAHMS -2 10 Model re-fit with all data Model prediction for µ µ T = 176 MeV, b = 41 MeV T = 177 MeV, b = 29 MeV Figure 1. Particle ratios (defined at top) measured in RHIC experiments (denoted by various symbols and defined in √the legend) along with statistical-thermal model predictions (horizontal dashes) [11] for central collisions of Au + Au at sNN = 130 (left) and 200 GeV (right). 2.2 Energy Density values of the particle ratiosp ¯/p, Λ¯/Λ, Ξ¯/ΞandΩ¯/Ω at mid-rapidity are reproduced using the measured Large energy densities are reached in a central collision value of K+/K− and the coalescence formulae [10], at RHIC. Measurements of the transverse energy per indicating that the particle formation mechanism in- unit pseudorapidity dE /dη [7] or the total particle T volves coalescence of quarks with little or insignificant multiplicity density and mean transverse momentum rescattering after the particle ratios are established. per particle [8] can be used to estimate the energy density produced in a Bjorken longitudinal expansion scenario [9]. 2.4 Temperature and Baryo-Chemical Poten- √ tial In Au + Au collisions at sNN = 130 GeV there is longitudinal boost invariance in a region around By comparing the ratios of the many different types mid-rapidity at RHIC [8]. Here the Bjorken scenario of particles measured in RHIC collisions, properties of applies and the energy density can be estimated by the system at chemical freeze-out can be established. 1 dET = 2 × , with dE /dy the transverse en- Ratios of the numbers of anti-particles to particles are Bj τoπR dy T ergy per unit rapidity, R the transverse radius of the sensitive to the net baryon number density. Likewise, system, and τo the formation time. Assuming a max- measurement of the total number of produced parti- imum value for the formation time τo =1fm/c,the cles and ratios of non-identical particles provide infor- minimum energy density for central collisions of Au + mation on the temperature of the system when the √ 3 Au at sNN = 130 GeV is 4.6 GeV/fm for the 2% particles are formed. The results of a thermal model 3 most central Au + Au collisions [7] and 4.3 GeV/fm comparison [11] to the measured particle√ ratios are dis- s for the 5% most central Au + Au collisions [8]. This played in Fig. 1 for Au + Au at NN = 130 and 200 3 is a 60% increase over the value 2.9 GeV/fm mea- GeV (left and right panels). The√ best global fits for √ s suredforPb+Pbat sNN = 17.2 GeV at the CERN central collisions of Au + Au at NN = 130 and 200 µ SPS. Note that the value for τo is expected to be sig- GeV are (T, B) = (176 MeV, 41 MeV) and (177 MeV, nificantly smaller at RHIC than at the SPS, thus the 29 MeV), respectively. These values are in good agree- energy density derived for RHIC from this formula is ment with other thermal model analyses and are at the a minimum and may be significantly (∼ ten times) deconfinement phase boundary. This is seen in Fig. 2, µ larger. which is a compilation [12] of (T, B) points derived from thermal model analyses of experimental results 2.3 Hadronization and Quark Coalescence at various energies. The data and thermal model sys- tematics in conjunction with the lattice QCD calcula- Measurements of the ratios of various types of particles tions (see Fig. 2) indicate that at chemical freezeout provide insight into aspects of the hadronization pro- the system at RHIC is at the phase boundary. cess. A particle formation mechanism involving the coalescence of quarks [10], predicts that these ratios 2.5 Collective Effects - Radial Flow should have specific relationships that are determined strictly from combining the various types of quarks The transverse momentum spectra of hadrons provide that make up the observed particles. The measured information about the system at the time of thermal QCD@Work 2003 - International Workshop on QCD, Conversano, Italy, 14–18 June 2003 3 ) 2 PHENIX 2 10 π STAR early universe GeV PHOBOS . ( T BRAHMS . p d Hydro quark-gluon T Kolb, Rapp, p 0 plasma 10 K / 10 PRC 67 044903 (2003) Dense Hadronic Medium ] 3 ) dN/dy nπ=0.5 /fm π 2 250 3 2 / 10 MeV n =0.38 /fm =2.5 n b 0 1 [ ( p /100 200 LQCD 4 Central Au+Au @ 200 GeV RHIC SPS 10 Bag Model RHIC data preliminary 150 0 1 2 3 temperature T pT (GeV/c) AGS 100 Figure 3. Transverse momentum spectra of charged pi- SIS -3 √ nb=0.12 fm ons, kaons and anti-protons measured in sNN = 200 GeV 50 Dilute Hadronic Medium Au + Au central collisions. Shown as the curves are the 3 nπ=0.34 /fm atomic 3 results of a hydrodynamics calculation [13]. nb=0.038 /fm =1/3 n0 nuclei neutron stars 0.2 0.4 0.6 0.8 1 1.2 1.4 µ [ ] ing medium.

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