Results from RHIC Measurements of High Density Matter Thomas S. Ullrich Brookhaven Nation Laboratory...

transcript

Results from RHIC Measurements of High

Density Matter

Thomas S. UllrichBrookhaven Nation Laboratory and Yale University

January 7, 2003

• Introduction

• Soft Physics

• Hard Physics

2Thomas Ullrich, BNL

(QCD) Phase Diagram of Nuclear Matter

• T >> QCD: weak coupling deconfined phase (Quark Gluon Plasma)• T << QCD: strong coupling confinement

phase transition at T~ QCD?

e.g. two massless flavors (Rajagopal and Wilczek, hep-ph/-0011333)

Lattice QCD at Finite Temperature

• Coincident transitions: deconfinement and chiral symmetry restoration • Recently extended to B> 0, order still unclear (2nd, crossover ?)

F. Karsch, hep-ph/0103314

Critical energy density:4)26( CC T

TC ~ 175 MeVC ~ 1 GeV/fm3

Ideal gas (Stefan-

Boltzmann limit)

The Phase Transition in the Laboratory

Chemical freezeout (Tch Tc) : inelastic scattering stops

Kinetic freeze-out (Tfo Tch): elastic scattering stops

e.m. probes (ll)

hard (high-pT) probes

soft physics regime

RHIC @ Brookhaven National Laboratory

Long Island

Relativistic

Collider

• 2 concentric rings of 1740 superconducting magnets• 3.8 km circumference• counter-rotating beams of ions from p to Au

PHENIX

PHOBOSBRAHMS

• 2000 run: • Au+Au @ sNN=130 GeV

• 2001 run: • Au+Au @ sNN=200 GeV (80 mb-1)• polarized p+p @ s=200 GeV (P ~15%, ~1 pb-1)

Geometry of Heavy Ion Collisions

Number of participants (Npart): number of incoming nucleons (participants) in the overlap regionNumber of binary collisions (Nbin): number of equivalent inelastic nucleon-nucleon collisions

Reaction plane

Non-central collision

“peripheral” collision (b ~ bmax)“central” collision (b ~ 0)

Nbin Npart

Peripheral EventFrom real-time Level 3 display.

STARSTAR

color code energy loss

Mid-Central EventFrom real-time Level 3 display.

STARSTAR

Central EventFrom real-time Level 3 display.

STARSTAR

Charged Particle Multiplicityd

19.6 GeV 130 GeV 200 GeVPHOBOS Preliminary

Central

Peripheral

Central at 130 GeV: 4200 charged particles !

Total multiplicity per participant pair scales with Npart

For the most central events:

PHENIX

Energy Density at RHIC

Bjorken ~ 4.6 GeV/fm3

~30 times normal nuclear density~1.5 to 2 times higher than at SPS (s = 17 GeV)~ 5 times above critical from lattice QCD

Bjorken formula for thermalized energy density

time to thermalize the system (0 ~ 1 fm/c)~6.5 fm

What is the energy density achieved?

How does it compare to the expected phase transition value ?

dydz 0

130 GeV

Hydrodynamics: Modeling High-Density Scenarios

Assumes local thermal equilibrium (zero mean-free-path limit) and solves equations of motion for fluid elements (not particles)

Equations given by continuity, conservation laws, and Equation of State (EOS)

EOS relates quantities like pressure, temperature, chemical potential, volume direct access to underlying physics

Works qualitatively at lower energybut always overpredicts collectiveeffects - infinite scattering limitnot valid there

RHIC is first time hydro works!

lattice QCD input

RHIC Spectra - an Explosive Source

data: STAR, PHENIX, QM01model: P. Kolb, U. Heinz

• various experiments agree well

• different spectral shapes for particles of differing mass strong collective radial flow

heavyT

purely thermalsource

explosivesource

T,mT1/

heavy• very good agreement with hydrodynamic

prediction

Single Particle Spectra and Radial Flow

Au+Au @ 130 GeV, central and peripheral (STAR, PHENIX):

Hydrodynamicseven works forperipheralcollisions up tob ~ 10 fm!

(Heinz & Kolbhep-ph/0204061)

Problem withpions at low pT

> 0required

= 0.6 fm/c, emax (b=0) = 24.6 GeV/fm3, <e>(=1 fm/c) = 5.4 GeV/fm3

Tmax(b=0) = 340 MeV, Tch = 165 MeV, Tfo = 130 MeV

Tfo and <r> vs. s

r increases continously

saturates around AGS energy

Strong collective radial expansion at RHIC high pressure high rescattering rate Thermalization likely

Slightly model dependenthere: blastwave model (Kaneta/Xu)

Azimuthal Anisotropy of Particle Emission: Elliptic Flow

Almond shape overlap region in coordinate space

y2 x2 y2 x2

Anisotropy in momentum space

SPS, RHIC

Interactions

2cos2 vx

cos212

nvdydpp

v2: 2nd harmonic Fourier coefficient in dN/d with respect to the reaction plane

Time Evolution: When Does Elliptic Flow Develop?

Equal energy density lines

P. Kolb, J. Sollfrank, and U. Heinz

Elliptic flow observable sensitive to early evolution of system

Mechanism is self-quenching

Large v2 is an indication of early

thermalization

Zhang, Gyulassy, Ko, PL B455 (1999) 45

Au+Au at b=7 fm

Charged Particle v2 vs. Centrality

midrapidity : |h| < 1.0

Hydrodynamic model

Nch/Nmax

AGSPRL 86 (2001) 402

Hydrodynamical models can describe data at low pT (~2 GeV/c) compatible with early equilibration

Contrast to lower collision energies where hydro overpredicts elliptical flow

Peripheral Central

STAR PRL87 (2001)182301

Models to Evaluate Tch and B: Statistical Thermal Models

Compare particle ratios to experimental data

Qi : 1 for u and d, -1 for u and d

si : 1 for s, -1 for s

gi : spin-isospin freedom

mi : particle mass

Tch : Chemical freeze-out

temperatureq : light-quark chemical potential

s : strangeness chemical potential

s : strangeness saturation factor

Particle density of each particle:Statistical Thermal ModelF. Becattini; P. Braun-Munzinger, J. Stachel, D. MagestroJ.Rafelski PLB(1991)333; J.Sollfrank et al. PRC59(1999)1637

Assume: • Ideal hadron resonance gas • thermally and chemically equilibrated fireball at hadro-chemical freeze-out

Recipe:• grand canonical ensemble to describe partition function density of particles of species i

• fixed by constraints: Volume V, ,

strangeness chemical potential S,

isospin• input: measured particle ratios• output: temperature T and baryo-chemical potential B

Statistical Models work well at RHIC

Statistical Models: from AGS to RHIC

Different implementation ofstatistical modelFact: all work well at AGS, SPS and RHIC

Tch [MeV] B [MeV]

AGS s = 2-4 GeV 125 540

SPS s = 17 GeV 165 250

RHIC s = 130-200 GeV 175 30

Does the success of the modeltell us we are dealing indeed with locally chemically equilibrated systems? this+flow If you ask me YES!neutron stars

Baryonic Potential B [MeV]

early universe

ical Tem

0 200 400 600 800 1000 1200

RHIC quark-gluon plasma

hadron gas

deconfinementchiral restauration

Lattice QCD

atomic nuclei

Slight variations in the models, but roughly:

Summary on “Soft” (pT < 2 GeV/c) Physics

Particle production is large Total Nch ~ 5000 (Au+Au s = 200 GeV) ~ 20 in p+p

Nch/Nparticipant-pair ~ 4 (central region) ~2.5 in p+p

Vanishing baryon/antibaryon ratio (0.7-0.8) close to net baryon-free but not quite (net proton dN/dy~10)

Energy density is high 4-5 GeV/fm3 (model dependent) lattice phase transition ~1 GeV/fm3, cold matter ~ 0.16 GeV/fm3

System exhibits collective behavior (radial + elliptic flow) strong internal pressure that builds up very early

The system appears to freezes-out very fast explosive expansion (HBT, correlation studies)

Particles ratios suggest chemical equilibrium Tch170 MeV, b<50 MeV near lattice phase boundary

Large system at freeze-out 2 size of nuclei

Overall picture: system appears to be in equilibrium but explodes and hadronizes rapidly

Products of parton fragmentation (jet “leading particle”).

Early production in parton-parton scatterings with large Q2.

Direct probes of partonic phases of the reaction

Sensitive to hot/dense medium: parton energy loss (“jet quenching”).

Info on medium effects accessible through comparison to scaled "vacuum" (pp) yields (“binary scaling”):

Production yields calculable via pQCD:

High-pT Particles @ RHIC – Jet Tomography

leading particle

),()()( 2/

2,, ccchcdabbbBbaaAa

hardhXAB QzDQxfQxf

Jets in Heavy Ion Collisionsee q q

(OPAL@LEP)pp jet+jet

(STAR@RHIC)

Au+Au ??? (STAR@RHIC)

Hopeless task? No, but a bit tricky…

Partonic Energy Loss: Theory

• Elastic scattering (Bjorken 1982):

• Gluon radiation is factor ~10 larger:

• Thick plasma (Baier et al.):

glueSglue

sRBDMS

22~ plasmas Tdz

• Thin plasma (Gyulassy et al.):

ELogrdCE jet

glueSRGLV 23 2

Linear dependence on gluon density glue• measures gluon density • is continuous function of energy density not a direct signature of deconfinement

Energy Loss in Cold Matter

Modification of fragmentation functions in e-Nucleus scattering:dE/dx ~ 0.5 GeV/fm for 10 GeV quark

Existing data is extensively studied but p+A measurements at RHICare desperately needed Run III (2003) d+Au

Wang and Wang, hep-ph/0202105

High-pT Hadrons: Au+Au at RHIC

Preliminary sNN = 200 GeV

Measuring Hadron Suppression

ddpNdpR

<Nbinary>/inelp+p

N-N cross section

1. Compare Au+Au to nucleon-nucleon cross sections2. Compare Au+Au central/peripheral

Nuclear Modification Factor:

If no “effects”: R < 1 in regime of soft physics R = 1 at high-pT where hard scattering dominates Suppression: R < 1 at high-pT

Leading Hadrons in Fixed Target Experiments

Multiple scattering in initial state(“Cronin effect”)

p+A collisions: Central Pb+Pb collisions at SPS

SPS: any parton energy loss effects buried by initial state multiple scattering, transverse radial flow,…

tppA ppA

Hadron Suppression: Au+Au at 130 GeV

Phenix: PRL 88 022301 (2002) and charged hadrons, central collisions

STAR: nucl-ex/0206011Charged hadrons, centrality dependence

Clear evidence for high pT hadron suppression in central nuclear collisions

Hadron Suppression: Au+Au at 200 GeV

Preliminary sNN

= 200 GeV

PHENIX preliminary

200 GeV preliminary data: suppression of factor 4-5 persists to pT=12 GeV/c

Phenix peripheral and central over measured p+p

STAR charged hadrons: central/peripheral

Hadron Suppression: Central Au+Au (Data vs. Theory)

Parton energy loss : dE/dx ≈ 0.25 GeV/fm (expanding)

dE/dx|eff

≈ 7 GeV/fm (static source)

~ 15 times that in cold Au nuclei

Opacities: <n> = L/≈ 3 – 4

Gluon densities:

dNg/dy ~ 900

S.MioduszewskiPHENIX Preliminarynucl-ex/0210021

All models expect a moderate increase of RAA at higher pT

What does it tell us about the medium ?

Elliptic “Flow” at High-pT: Theory

Snellings; Gyulassy, Vitev and Wang (nucl-th/00012092)

Jet propagation through anisotropic matter (non-central collisions)

• Finite v2: high pT hadron correlated with reaction plane from “soft” part of event (pT<2 GeV/c)• Finite asymmetry at high pT sensitive to energy density

STAR @ 130 GeV

STAR @ 200 GeV

• Jet core×0.5 × 0.5 study near-side correlations (~0) of high pT hadron pairs

• Complication: elliptic flow high pT hadrons correlated with the reaction plane (~v22)

• Solution: compare azimuthal correlation functions forshort range particles in jet cone + backgroundlong range background only

• Azimuthal correlation function:

• Trigger particle pT trig> 4 GeV/c

• Associate tracks 2 < pT < pTtrig

Caveat: Away-side jet contribution subtracted by construction,needs different method…

< 0.5 > 0.5

2-Particle Correlations at High-pT: Direct

Evidence for Jets

),()(11

)(2 NdefficiencyN

Ctrigger

Near-side correlation shows jet-like signal in central Au+Au

2 Particle Correlations at High-pT: Back-to-Back Jets?

• away-side (back-to-back) jet can be “anywhere”

• Ansatz: correlation function: high pT-triggered Au+Au event =

high pT-triggered p+p event + elliptic flow+ background

))2cos(21()()( 2222 vAppCAuAuC

A: from fit to “non-jet” region v2 from reaction plane analysis

0<||<1.4

p+punlike signlike sign

p+p measured in RHIC detectors

Suppression of Back-to-Back Pairs

Central Au + Au

Peripheral Au + Au

• Near-side well-described• Away-side suppression in central collisions

Away side jets are suppressed!

near side

away side

STAR Preliminary

))2cos(21()()( 2222 vAppCAuAuC

STAR Preliminary

High pT phenomena: suppression of inclusive rates, finite elliptic flow, suppression of back-to-back pairs

compatible with extreme absorption and surface emission

Summary

Soft physics:• Low baryon density• System appears to be in equilibrium (hydrodynamic behaviour)• Explosive expansion, rapid hadronization

Hard physics:• Jet fragmentation observed, agreement with pQCD• Strong suppression of inclusive yields• Azimuthal anisotropy at high pT• Suppression of back-to-back hadron pairs• large parton energy loss and surface emission?

Coming Attractions:• d+Au: disentangle initial state effects in jet production (shadowing, Cronin enhancement) resolution of jet quenching picture

• J/ and open charm: direct signature of deconfinement? (Charm via single electrons: PHENIX, PRL 88, 192303 (2002))

• Polarized protons: G (gluon contribution to proton spin)• Surprises …

Results from RHIC Measurements of High Density Matter Thomas S. Ullrich Brookhaven Nation Laboratory...

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