Heavy-Ion Physics

Raimond Snellings

XXIII Physics in Collision

Zeuthen, GermanyJune 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

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

We normally think of 4 phases: Plasma Gas Liquid Solid

Phase diagram of waterPhase diagram of nuclear matter

F. Karsch, hep-lat/0106019

QCD on the Lattice

3/5.1~

200150~

fmGeV

MeVT

c

c

•Z. Fodor and S.D. Katz, hep-lat/01060002

Schematic Space-Time Diagram of a Heavy Ion Collision

space

timeSchematic Time Evolution

e

Hard Scattering

AuAu

E

xpan

sion

----

------

-

Hadronization

Freeze-out

jet J/

QGP?Thermalization?

ep K

CERN SPS: A New State of Matter?

J/ suppression indication of deconfinement? Strangeness enhancement Melting of the

NA50

Are hadronic scenarios ruled out? Co-mover absorption? canonical suppression?

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

A New Era for Heavy Ion Physics: The Relativistic Heavy Ion Collider at BNL

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)

`

Hadron PID over broad rapidity acceptance

Two conventional beam line spectrometers Magnets, Tracking Chambers, TOF, RICH

Charged Hadrons in Central Spectrometer

Nearly 4 coverage multiplicity counters

Silicon Multiplicity Rings

Magnetic field, Silicon Pad Detectors, TOF

Electrons, Muons, Photons and Hadrons Measurement Capabilities

Focus on Rare Probes:

J/, high-pT

Two central spectrometers with tracking and electron/photon PID

Two forward muon spectrometers

Hadronic Observables over a Large Acceptance

Event-by-Event Capabilities

Solenoidal magnetic field Large coverage Time-

Projection Chamber Silicon Tracking, RICH,

EMC, TOF

Online Level 3 Trigger Display

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

p

s

2 tx

Event Characterization Cannot directly measure the impact parameter b! but we can distinguish peripheral collisions from central

collisions!

Ncoll

Npart

Impact Parameter (b)

•b

5% Central

STAR

Soft Physics

Particle Yields Spectra shapes Elliptic Flow

Particle distributions (PHOBOS)

dN

ch/d

19.6 GeV 130 GeV 200 GeV

PHOBOS PreliminaryCentral

Peripheral

•central collisions at 130 GeV: 4200 charged particles !•mid rapidity plateau

Energy Density (Bjorken estimate)

dy

dE

RdyR

dE

dzR

dE

V

E T

TT

T

T

T

222

1~~

3/6.4

2

0/

fmGeVo

yT

BJ R

dydE

Bjorken Estimate

R

0ddET 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)

0j,0T μBμ

μνμ

)mp

v(fpd

)dn(Thermal~

pddn

HYDRO

Particle spectra at the SPS

Rather well described by Hydro motivated fit

Particle ratios: chemical potentials and freeze-out temperature

pdedn E 3/)(~ Tμ Assume distributions described by

one temperature T and one ( baryon) chemical potential One ratio (e.g.,p / p ) determines / T A second ratio (e.g., K / ) provides T

Then predict all other hadronic yields

and ratios:

TμTμ

Tμ/2

/)(

/)(

ee

e

p

pE

E

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?

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

)(tan,)(2cos 12

x

yr p

pv

1

2

3

3

cos212

1

nrn

tt

nvdydpp

Nd

pd

NdE

y

x

coordinate space

py

px

Momentum space

Elliptic Flow at the SPS (NA49 and CERES)

•Clearly deviates from ideal hydrodynamic model calculations

Hydrodynamic limit

STAR

PHOBOS

Hydrodynamic limit

STAR

PHOBOS

Compilation and Figure from M. Kaneta

Integrated Elliptic Flow

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

p+p->0 + X

hep-ex/0305013 S.S. Adler et al.

Hard probes

At RHIC energies different mechanisms are responsible for different regions of particle production.

Rare process (Hard Scattering or “Jets”), a calibrated probe

Hard Scatterin

g

“Well Calibrated”

Thermally-shaped Soft Production

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”

32

2,glu

jetGLV eR S

EE C d r og

LL

hadrons

q

q

hadrons leadingparticle

leading particle

schematic view of jet production

22

4R s

BD eM gluS LC

E v

2 24ln

3TG F SD

EE C T L

Jets at RHICp+p jet+jet

(STAR@RHIC)Au+Au X

(STAR@RHIC)

find this in this

Find partonic energy loss with leading hadrons

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

Binary collision scaling p+p reference

Energy loss softening of fragmentation suppression of leading hadron yield

Relative to UA1 p+p

nucl-ex/0305015

BRAHMS preliminary

Measurements of jet suppression

nucl-ex/0304022

Binary scaling

Participant scaling

Elliptic Flow at higher-pt

M. Gyulassy, I. Vitev and X.N. Wang

•R.S, A.M. Poskanzer, S.A. Voloshin, •STAR note, nucl-ex/9904003

STAR preliminary

Back to back “jets” at the SPS (CERES)

Cronin Effect:

Multiple Collisions broaden high PT

spectrum

•Centrality 24-30%

•Centrality 11-15%

•SPS. CERES: Away side jet broadening, no disappearance

near side

away side

peripheral central

Disappearance of back to back “jets”

PRL 90, 082302 (2003)

2( )

22( ) 2(1 cos(2 ))

Au Au

p p

D

D B v

•In central Au+Au collisions the away-side “jet” disappears !!

High-pt phenomena: Initial state or final state effect?

pT>5 GeV/c: well described by KLM saturation model (up to 60% central) and pQCD+jet quenching

nucl-ex/0305015

Final state

Initial state

Theory expectations for d+Au

If Au+Au suppression is initial state (KLM saturation: 0.75)

1.1-1.5

pT

RA

B

1

Inclusive spectraIf Au+Au suppression is final state

~2-4 GeV/c

All effects strongest in central d+Au collisions

pQCD: no suppression, small broadening due to Cronin effect

0 /2 (radians)

0

High pT hadron pairs

saturation models: suppression due to mono-jet contribution?

suppression?

broadening?

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

“PHENIX Preliminary” results, consistent with

STAR data in submitted paper

?

Central Au+Au

d+Au

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

Elliptic Flow reduces spatial anisotropy -> shuts itself off

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

Structure Functions