(Relativistic Heavy Ion Collider) QCD Matter in the Laboratory Peter A. Steinberg Brookhaven National Laboratory Physics Colloquium, George Washington University

Phases of Normal Matter Electromagnetic interactions determine phase structure of normal matter solidliquidgas

Two Faces of the Strong Force “Fundamental” Quarks are held together by exchanging colored gluons V~1/r at short distance V~kr at long distances We say that quarks and gluons are confined in hadrons – mesons & baryons “Residual” Hadrons are made of confined quarks and gluons with net zero color Hadrons interact by exchanging other hadrons A substantially weaker, non-confining force proton pion

We have strong interaction analogues of the familiar phases Nuclei behave like a liquid Nucleons are like molecules Quark Gluon Plasma “Ionize” nucleons with heat “Compress” them with density New state of matter! Phases of QCD Matter Quark-Gluon Plasma Hadron Gas Solid

Where to study QCD matter Neutron stars Lattice QCD RHIC Big Bang Only one chance… Who wants to wait?…

Lattice QCD calculations Non-Abelian gauge theory Perturbative QCD (pQCD) only applicable at large momentum transfer Teraflop-scale computers simulate equilibrium QCD Predict phase transition: (F. Karsch, hep-lat/ ) hadrons  quark/gluon

Heavy-Ion Collisions Colliding Nuclei Hard Collisions Parton Cascade Hadron Gas & Freeze-out 1234

Goals of the Field QGP: Does QCD become simpler at high T, high density? What is the phase diagram of QCD? Do the lattice predictions agree with nature? Strongly-interacting systems Can we understand the evolution of a system of two colliding nuclei? Can we understand the early phase of a system by observing the late phase?

Some QGP Diagnostics Global Observables Is initial state dense enough? Particle Multiplicities Energy Density Collective Behavior Is QGP a thermalized state? Elliptic Flow Hard Probes Formed early, probe medium Energy loss of jets WhyWhat

Legacy from the SPS Program Measurements suggest collisions at CERN have: Tantalizing evidence for a phase transition to deconfined quark matter in Pb+Pb CERN Press release in Feb. 2000! Interpretation is not uncontroversial Expectations Measurements We need an independent check!… Transition volume

The RHIC Complex 1 2 3

RHIC Capabilities & Run I Nucleus-nucleus (AA) collisions up to  s NN = 200 GeV Polarized proton-proton (pp) collisions up to  s NN = 450 GeV Performance Au + Au RHIC Design  s nn 130 GeV 200 GeV L [cm -2 s -1 ] ~ 2 x x Interaction rates ~ 100 Hz 1400 Hz

RHIC Experiments Constraints on design High multiplicity events High rate needed Low-cost required Choices made Two large, flexible (& expensive!) experiments Two small, optimized (& inexpensive!) experiments

What have we learned from RHIC so far? Colliding Nuclei Hard Collisions Parton CascadeHadron Gas & Freeze-out 1234 Initial Conditions Initial State Dynamics Properties of Final State Probes of Initial State But let’s start with something sensitive to all of these…

Charged Multiplicity at Midrapidity N ch integrates over the history of the collision Measures entropy Sensitive to phase transitions Rapidity distribution separates produced from colliding particles Produced particles dominate near y=0 dN/d  p p  Mid-rapidity

Geometry of Nuclear Collisions Nuclei are extended R Au ~ 6.4 fm ( m) Impact parameter (b) determines N part – 1 or more collisions N coll – binary collisions Proton-nucleus: N coll = N part + 1 Nucleus-Nucleus N coll  N part 4/3 Participants Spectators pp collisions pA collisions b b N coll N part Useful quantities to compare Au+Au to N+N collisions!

Measuring Centrality Cannot directly measure the impact parameter! but can we distinguish peripheral collisions from central collisions? “Spectators” Zero-degree Calorimeter “Spectators” Paddle Counter Can look at spectators with zero-degree calorimeters, and participants via monotonic relationship with produced particles

RHIC Experiments I: PHOBOS   Large acceptance to count charged particles Small acceptance, high-resolution spectrometer Focus is on simple silicon technology, timely results

Charged Particle Multiplicity Au+Au collisions produce more particles per NN collision than proton-proton collisions at same  s proton-antiproton collisions

Energy Density Q: Does energy density (considered by lattice calculations) relate to particle density? PHENIX finds a constant amount of transverse energy(E T ) per particle and a total E T : A: Energy density comes from producing more particles! (iff R~1.18A 1/3 &   ~ 1fm/c) Bjorken Estimate R  503±2 GeV (130 GeV)  (200 GeV) = 4.6 x 1.15 = 5.3 GeV/fm 3

Initial Conditions How do we characterize the state that gives rise to such a large initial energy density? Initial parton densities 1

QCD Structure of the proton Protons (and all matter) has a quark/gluon (parton) substructure Can probe this with high-energy electrons Scale invariance found in the 1960’s – evidence for quarks (& a Nobel Prize) At low x, QCD scale can be seen – evidence for gluons Q2Q2 xP electron proton

Initial conditions: QCD at very low x Structure functions rise rapidly at low-x More rapid for gluons than quarks 30! 1.5 valence sea

Parton Saturation Gluon distribution rises rapidly at low-x Gluons of x~1/(2mR) overlap in transverse plane with size 1/Q At “saturation” scale Q s 2 gluon recombination occurs In RHIC Au+Au collisions, saturation occurs at a higher Q s 2 (thus higher x) Saturation describes HERA data! Scale depends on volume

Particle Density vs. Centrality Saturation models are consistent with data for N part >200 Centrality is a way of changing system size (A eff ) Is this picture unique?… UA5 (pp) EKRT KN

Soft & Hard Particle Production Soft processes (p T < 1 GeV) Scales with N part (2 for pp!) Color exchange leads to excited nucleons that decay Hard processes (p T > 1 GeV) pQCD can calculate jet cross sections Scales with N coll (1 for pp!) Might expect “2-component model” to extrapolate pp  Au+Au

Two Component Model Two-component model Is an equally good description in the presented centrality range UA5 KN 2C

Consequences of Parton Saturation  Does successful description imply large gluon density?… Saturated initial state gives predictions about final state.  N(hadrons) = c  N(gluons) (parton-hadron duality)  Describes energy, rapidity, centrality dependence of charged particle distributions Kharzeev & Levin, nucl-th/

Dynamics of Initial State Saturated gluon densities Parton Cascade If parton cascade thermalizes: Initial state geometry (almond) Final state angular distribution Hydrodynamic Limit: v 2 ~  Mild rescattering v 2 ~  dN/dy 2

Observation of Asymmetry preliminary b Side view Front view Emission pattern

Hydrodynamic calculations agree with flow in central events – suggests that gluon densities are sufficient SPS AGS Results on Elliptic Flow

Description of Final State We wish to characterize evolving hadron gas Chemical freeze-out (no more flavor production) Particle yields and ratios (hadro-chemistry) Thermal freeze-out (free streaming) Measure Temperature and Size Saturated gluon densities Hydrodynamic Flow Hadron Gas & Freeze-out 4

RHIC Experiments II: STAR 2000 particles/event Measures momentum, charge, mass Excellent for characterizing final state (inclusively or event-by- event)

Final State: Chemical Equilibrium M. Kaneta, STAR Collaboration Thermal model lets us put data on QCD phase diagram RHIC energies appear close to T c !! Data from different energies has simple trend  E h  /  N h  ~ 1 GeV Simply probing properties of hadronization itself? Particle Ratios

Final State: Temperature & Size T  = 190 MeV T  = 300 MeV T p = 565 MeV mid-rapidity Temperature shows effect of radial expansion (  ~.6) Bose-Einstein correlations give source size via HBT Dynamics of final state

Probing the Early Stages Saturated gluon densities Hydrodynamic Flow Expanding Final State pQCD jets can be used to study medium Partons interact strongly with other partons Weakly with colorless bound hadrons QGP Hadron gas Hard Probes 2

RHIC Experiments III: PHENIX High rate, high granularity and high resolution Emphasis on hard probes High-p T hadrons e &  (charm & Drell-Yan) Direct  and high p T  o

“Jet-Quenching” Fragmentation functions allow prediction of hadron spectra from pQCD cross sections Quenching leads to a suppression of expected leading hadron spectrum While jet production can be calculated in pQCD, it is hard to see jets like at LEP Soft production fills in the gaps

PHENIX results for 130 GeV Scaled pp collisions (from UA1) up to Au+Au by N coll Same spectrum as UA1 for peripheral events Central events are suppressed by ½ Identified pions are even lower Evidence for Jet Quenching

Nuclear Modifications at high p T Cronin Effect Multiple scattering in nucleus modifies p T spectra Nuclear Shadowing Low-x gluons “see” entire nucleus (cf. saturation) Reduces effective gluon flux ‘EMC’ effect “shadowing” p T (GeV)

Persistence of Quenching Calculations incorporate known nuclear effects Shadowing and the Cronin effect do not have an appreciable effect on the spectrum So far, only energy loss can explain the PHENIX observation Scaled pp Shadowing + Cronin Energy Loss

Status of RHIC Physics Program RHIC appears to be creating a hot,dense and expanding state of deconfined QCD matter All results so far consistent with this interpretation 1.Energy Density – exceeds lattice QCD expectations 2.Initial conditions – saturated gluon distributions 3.Initial state – hydrodynamic flow 4.Final state – rapidly expanding, thermalized state 5.Hard Probes – jet quenching  deconfined medium Run II (200 GeV) is nearing completion All experiments with new capabilities, larger data sets Polarized pp for spin program starting in December

RHIC Experiments IV: BRAHMS The other “small” experiment at RHIC Only experiment to study hadron production at forward angles High resolution spectrometer & good particle ID Hopes to understand proton stopping TPC display