Heavy-Ion Physics XXIII Physics in Collision Raimond Snellings Zeuthen, Germany June 26-28, 2003

Outline Brief introduction to Heavy-Ion Physics CERN SPS: a new state of matter BNL Relativistic Heavy Ion Collider BRAHMS, PHOBOS, PHENIX and STAR (a few selected) RHIC results from year 1-3 Summary

“Large” as compared to mean-free path of produced particles. Collisions of “Large” nuclei convert beam energy to temperatures above 200 MeV or 1,500,000,000,000 K ~100,000 times higher temperature than the center of our sun. “Large” as compared to mean-free path of produced particles.

QCD Phase Diagram Phase diagram of nuclear matter We normally think of 4 phases: Plasma Gas Liquid Solid Phase diagram of nuclear matter Phase diagram of water

QCD on the Lattice F. Karsch, hep-lat/0106019 Z. Fodor and S.D. Katz, hep-lat/01060002

Schematic Space-Time Diagram of a Heavy Ion Collision

Schematic Time Evolution p K L g e space time f jet J/Y Freeze-out g e Hadronization  Expansion ----------- QGP? Thermalization? Hard Scattering Au

CERN SPS: A New State of Matter? NA50 Are hadronic scenarios ruled out? Co-mover absorption? canonical suppression? J/Y suppression indication of deconfinement? Strangeness enhancement Melting of the r

SPS, NA49: Indications of a Phase Transition at ≈ 30 GeV ?

3.83 km circumference Two independent rings 120 bunches/ring 106 ns crossing time Capable of colliding ~any nuclear species on ~any other species Energy: 200 GeV for Au-Au (per N-N collision) 500 GeV for p-p Luminosity Au-Au: 2 x 1026 cm-2 s-1 p-p : 2 x 1032 cm-2 s-1 (polarized) ` A New Era for Heavy Ion Physics: The Relativistic Heavy Ion Collider at BNL

Hadron PID over broad rapidity acceptance Two conventional beam line spectrometers Magnets, Tracking Chambers, TOF, RICH

Charged Hadrons in Central Spectrometer Nearly 4p coverage multiplicity counters Silicon Multiplicity Rings Magnetic field, Silicon Pad Detectors, TOF

Electrons, Muons, Photons and Hadrons Measurement Capabilities Focus on Rare Probes: J/y, high-pT Two central spectrometers with tracking and electron/photon PID Two forward muon spectrometers

Online Level 3 Trigger Display Hadronic Observables over a Large Acceptance Event-by-Event Capabilities Solenoidal magnetic field Large coverage Time-Projection Chamber Silicon Tracking, RICH, EMC, TOF

Heavy-ion Physics at RHIC RHIC different from previous (fixed target) heavy ion facilities ECM increased by order-of-magnitude Accessible x (parton momentum fraction) decreases by ~ same factor Study pp, pA to AA Comprehensive set of detectors All final state particles measured with overlap between the detectors Study QCD at high density with probes generated in the medium If QGP produced at RHIC most likely to live longer than at the SPS and therefore easier to observe and study its properties

Event Characterization Cannot directly measure the impact parameter b! but we can distinguish peripheral collisions from central collisions! b Ncoll Npart Impact Parameter (b) 5% Central STAR

Soft Physics Particle Yields Spectra shapes Elliptic Flow

Particle distributions (PHOBOS) dNch/dh h 19.6 GeV 130 GeV 200 GeV PHOBOS Preliminary Central Peripheral central collisions at 130 GeV: 4200 charged particles ! mid rapidity plateau

Energy Density (Bjorken estimate) to 503±2 GeV (130 GeV) PRL 87 (2001) preliminary

Particle spectra at RHIC Superimposed on the thermal (~Boltzmann) distributions: Collective velocity fields from Momentum spectra ~ ‘Test’ by investigating description for different mass particles: Excellent description of particle production (P. Kolb and U. Heinz, hep-ph/0204061)

Particle spectra at the SPS Rather well described by Hydro motivated fit

Particle ratios: chemical potentials and freeze-out temperature Assume distributions described by one temperature T and one ( baryon) chemical potential m One ratio (e.g.,p / p ) determines m / T A second ratio (e.g., K / p ) provides T  m Then predict all other hadronic yields and ratios:

Where is RHIC on the phase diagram?

Three Forms of Collective Motion Only type of transverse flow in central collision (b=0) is transverse flow. Integrates pressure history over complete expansion phase x y Elliptic flow, caused by anisotropic initial overlap region (b > 0). More weight towards early stage of expansion. x y Directed flow, sensitive to earliest collision stage (pre-equilibrium, b > 0) z x

What makes elliptic flow an unique probe? y x coordinate space Non central collisions coordinate space configuration anisotropic (almond shape). However, initial momentum distribution isotropic (spherically symmetric). Only interactions among constituents (mean free path small) generate a pressure gradient which transforms the initial coordinate space anisotropy into the observed momentum space anisotropy Multiple interactions lead to thermalization -> limiting behavior hydrodynamic flow py px Momentum space

Elliptic Flow at the SPS (NA49 and CERES) Clearly deviates from ideal hydrodynamic model calculations

Integrated Elliptic Flow Hydrodynamic limit STAR PHOBOS Compilation and Figure from M. Kaneta First time in Heavy-Ion Collisions a system created which at low pt is in quantitative agreement with hydrodynamic model predictions for v2 up to mid-central collisions

Differential Elliptic Flow Hydro calculation: P. Huovinen et. al. Typical pt dependence Heavy particles more sensitive to velocity distribution (less effected by thermal smearing) therefore put better constrained on EOS

Soft Physics Energy density estimate well above critical Lattice values Particle yields are well described in a thermal model Spectra shapes are consistent with thermal boosted distributions Elliptic flow reaches hydrodynamical model predictions First time in heavy-ion collisions Observables consistent with strong early partonic interactions and approaching early local equilibrium However, size measurements (HBT) are not completely understood yet

Hard probes and the produced medium

Thermally-shaped Soft Production Hard probes p+p->p0 + X hep-ex/0305013 S.S. Adler et al. At RHIC energies different mechanisms are responsible for different regions of particle production. Rare process (Hard Scattering or “Jets”), a calibrated probe Hard Scattering Thermally-shaped Soft Production “Well Calibrated”

Hard Probes and the Produced Medium Hard scatterings in nucleon collisions produce jets of particles. In the presence of a color-deconfined medium, the partons strongly interact losing much of their energy “Jet Quenching” hadrons q leading particle leading particle schematic view of jet production

p+p jet+jet (STAR@RHIC) Jets at RHIC p+p jet+jet (STAR@RHIC) Au+Au X (STAR@RHIC) find this in this

Find partonic energy loss with leading hadrons Energy loss  softening of fragmentation  suppression of leading hadron yield Binary collision scaling p+p reference

Measurements of jet suppression BRAHMS preliminary nucl-ex/0305015 nucl-ex/0304022 Binary scaling Participant scaling Relative to UA1 p+p

Elliptic Flow at higher-pt M. Gyulassy, I. Vitev and X.N. Wang STAR preliminary R.S, A.M. Poskanzer, S.A. Voloshin, STAR note, nucl-ex/9904003

Back to back “jets” at the SPS (CERES) Centrality 24-30% Centrality 11-15% Cronin Effect: Multiple Collisions broaden high PT spectrum SPS. CERES: Away side jet broadening, no disappearance

Disappearance of back to back “jets” near side away side peripheral central PRL 90, 082302 (2003) In central Au+Au collisions the away-side “jet” disappears !!

High-pt phenomena: Initial state or final state effect? nucl-ex/0305015 Final state Initial state pT>5 GeV/c: well described by KLM saturation model (up to 60% central) and pQCD+jet quenching

Theory expectations for d+Au Inclusive spectra RAB If Au+Au suppression is final state 1.1-1.5 1 If Au+Au suppression is initial state (KLM saturation: 0.75) ~2-4 GeV/c pT High pT hadron pairs broadening? pQCD: no suppression, small broadening due to Cronin effect saturation models: suppression due to mono-jet contribution? /2  suppression?  (radians) All effects strongest in central d+Au collisions

Comparison of Au+Au to d+Au (PHOBOS and BRAHMS) central Au+Au PHOBOS d+Au: nucl-ex/0306025

Comparison of Au+Au to d+Au (PHENIX and STAR) Dramatically different behavior of Au+Au observables compared to d+Au observables. Jet Suppression is clearly a final state effect.

Back to back “jets” in d+Au Central Au+Au ? d+Au “PHENIX Preliminary” results, consistent with STAR data in submitted paper

Summary High-pt probes are a new unique tool at RHIC to understand heavy-ion collisions New phenomena have been found: Suppression of the inclusive yields (“jet quenching”) Large elliptic flow Disappearance of the away-side “jet” Pointing at very dense (≈ 30x nuclear densities) and strongly interacting matter Low-pt (bulk) and high-pt observables consistent with expectations from a QGP (but not as proof, still more work to be done. RHIC program just started)

Thanks Many figures on the slides are “borrowed” from: W. Zajc, P. Steinberg, N. Xu, P. Jacobs, F. Laue, P. Kolb, U. Heinz, T. Hemmick, G. Roland, I. Bearden, M. van Leeuwen and many others

Time Evolution in a Hydro Calculation Calculation: P. Kolb, J. Sollfrank and U.Heinz Elliptic Flow reduces spatial anisotropy -> shuts itself off

Structure Functions