Results on High p T Particle Production from

Results on High p. T Particle Production from the PHENIX-Experiment at RHIC Nucleus-Nucleus Collisions 2003, Moscow Klaus Reygers University of Münster for the PHENIX Collaboration Klaus Reygers, University of Münster

PHENIX-Setup Relevant for this talk: Detectors in the central spectrometer arms (pseudorapidity |h| < 0. 35) Charged Particle Tracking: ● Drift-Chambers (DC) ● Pad-Chambers (PC) Identification of charged hadrons: ● Tine-of-Flight (TOF) with start signal from the Beam-Counters (BBC) p 0 via p 0 : ● Lead scintillator calorimeter (Pb. Sc) ● Lead glass calorimeter (Pb. Gl) Klaus Reygers, University of Münster

Centrality Selection in Au+Au Measurement of charged particles with the BBC (3. 0 < |h| < 3. 9) Spectators Measurement of unbound spectator neutrons with the ZDC (~18 m away from vertex) Participants Centrality characterized by Npart: Number of nucleons which suffered at least one inelastic nucleon-nucleon collision Ncoll: Number of inelastic nucleon-nucleon collisions Npart and Ncoll from Glauber calculations Klaus Reygers, University of Münster

p+p-Collision at s = 200 Ge. V PHENIX Event-Display by J. Mitchell, BNL Klaus Reygers, University of Münster

Au+Au-Collision at s. NN = 200 Ge. V PHENIX Event-Display by J. Mitchell, BNL Klaus Reygers, University of Münster

High p. T Particle Production High p. T particles in p+p and A+A Result from parton-parton scatterings with high momentum transfer Q 2 (hard processes) p+p: Hot and dense matter created in A+A might affect High p. T particle yields Ratio of particle species Jet-like angular correlations Fragmentation of partons to hadrons in the QCD vacuum A+A: Hard parton-parton scatterings in the early stage of the collision Thus, high p. T partons are probes of the hot and dense nuclear matter formed in later stages Initial hard parton-parton scatterings Klaus Reygers, University of Münster

p 0 -Production in p+p at s = 200 Ge. V hep-ex/0304038 Good agreement with NLO p. QCD Constrains D(Gluon p) fragmentation function Result needed as reference for interpretation of Au+Au-Spectra Klaus Reygers, University of Münster

Nuclear Modification Factor average number of inelastic nucleon-nucleon collisions “no effect” Hard processes: Yield scales with Ncoll (reason: small cross sections, incoherent superpositions) In the absence of nuclear effects: RAA = 1 at high p. T Klaus Reygers, University of Münster

Initial State Effects: Effects which lead to RAA 1 at high p. T but which are not related to properties of the hot and dense nuclear matter Cronin-Effect observed in fixed target experiments: Suspects: Initial state multiple soft scatterings (Cronin Effect): increases RAA Modification of the nucleon structure functions in nuclei (Shadowing): decreases RAA Gluon saturation (Color Glass Condensate): decreases RAA (? ) Klaus Reygers, University of Münster

p 0 -Production in Au+Au at s. NN = 200 Ge. V → Ncoll-scaling works in peripheral Au+Au, but strong suppression in central Au+Au Klaus Reygers, University of Münster

RAA for Neutral Pions in Central Collisions Expectation from Cronin-Effect: RAA > 1 WA 98 (CERN SPS) ISR data Observed: Factor 4 -5 suppression in central Au+Au-collisions at s. NN = 130 and 200 Ge. V PHENIX at RHIC nucl-ex/0304022 Klaus Reygers, University of Münster

Jet-Quenching? Comparison with model calculations with and without parton energy loss: Au+Au at s. NN = 200 Ge. V without parton energy loss Levai Wang with parton energy loss Wang Pion-suppression reproduced by models with parton energy loss p. T-dependence not well described Vitev Levai Other explanations not ruled out at this stage Klaus Reygers, University of Münster

p/p – Ratio Au+Au at s. NN = 200 Ge. V Expectation for particle production from jetfragmentation: p/p less than 0. 25 at high p. T Observed: Strong centrality dependence Central Au+Au: p/p 1 nucl-ex/0305036 Klaus Reygers, University of Münster

Proton-Scaling Au+Au at s. NN = 200 Ge. V Proton/anti-proton-yield scales with Ncoll in the range 2 Ge. V < p. T < 3 Ge. V Possible explanations Collective expansion (hydrodynamic flow) Jet-Fragmentation within quark-gluon plasma Klaus Reygers, University of Münster

The Control Experiment: d+Au, Au+Au at s. NN = 200 Ge. V Two possible explanations for suppression in central Au+Au: Initial state effects Final state effects minimal in d+Au No suppression in d+Au Paper will be submitted to preprint server today (June 17 th, 2003)! Initial state effects ruled out as explanation for suppression in Au+Au Klaus Reygers, University of Münster

p 0's in d+Au: Data vs. Theory Data well reproduced by NLO p. QCD calculation, plus Phenomenological model of Cronin-Effect, plus Shadowing Levai et al. , nucl-th/0306019 Kopeliovich et al. , Phys. Rev. Lett. 88, 232303 (2002) Klaus Reygers, University of Münster

Summary Strong suppression of high p. T particle production in central Au+Au collisions at s. NN = 130 Ge. V and 200 Ge. V with respect to Ncoll-scaled p+p-reference No suppression seen in s. NN = 200 Ge. V d+Au Suppression of particle production in Au+Au is caused by properties of the created hot and dense medium Klaus Reygers, University of Münster

Klaus Reygers, University of Münster

Klaus Reygers, University of Münster

Centrality Dependence of the Suppression Au+Au at s. NN = 200 Ge. V Consider integrated p 0 yield above p. T = 4 Ge. V/c Smooth increase of suppression with centrality Neither Ncoll- nor Npart-scaling nucl-ex/0304022 Klaus Reygers, University of Münster