STAR

Tom HumanicThe Ohio State

University

FNAL --Feb 2003

Physics at RHIC

Results from the RHIC STAR Experiment

Outline

• Motivation for studying Relativistic Heavy Ion Collisions

• RHIC and the STAR experiment

• Soft Physics from STAR

• Hard Physics from STAR

• Summary

Why heavy ion collisions?

• Study bulk properties of nuclear matter

The “little bang”

• Extreme conditions (high density/temperature) expect a transition to new phase of matter…

• Quark-Gluon Plasma (QGP)• partons are relevant degrees of freedom over

large length scales (deconfined state)

• believed to define universe until ~ s

• Heavy ion collisions ( “little bang”)• the only way to experimentally probe

deconfined state

• Study of QGP crucial to understanding QCD• low-q (nonperturbative) behaviour

• confinement (defining property of QCD)

• nature of phase transition

The “little bang”

Stages of the collision

• pre-equilibrium (deposition of initial energy)• rapid (~1 fm/c) thermalization (?)• high-pT observables probe this stage

QGP formation (?)

hadronic rescattering

hadronization transition(very poorly understood)

Chemical freeze-out: end of inelastic scatteringsKinetic freeze-out: end of all scatterings

• low-pT hadronic observables probe this stage

Does “end result” look about the samewhether a QGP was formed or not???

tim

etem

perature

The Phase Space Diagram

TWO different phase transitions at work!

– Quarks and gluons roam freely over a large volume

– Quarks behave as though they are massless

Calculations show that these occur at approximately the same point

Two sets of conditions:

High Temperature

High Baryon Density

Lattice QCD calc. Predict:

Tc ~ 150-170 MeV

c ~ 0.5-0.7 GeV/fm

Deconfinement transition

Chiral transition

Quark-GluonPlasma

Hadrons

• Beam energy up to 100 GeV/A (250 GeV for p);

• Two independent rings (asymmetric beam collisions are possible);

• Beam species: from proton to Au;

• Six interaction points: STAR, PHENIX, PHOBOS and BRAHMS

RHIC Data-Taking

Year 2000: Au + Au @ 130 GeV 2 weeks

Year 2001: Au + Au @ 200 GeV 15 weeks

Au + Au @ 20 GeV 1 day

p + p @ 200 GeV 5 weeks

Year 2003: 1st of January

d + Au @ 200 GeV 10 weeks

p + p @ 200 GeV (5) + 3 weeks

Russia: MEPHI – Moscow, LPP/LHE JINR–Dubna, IHEP-Protvino

U.S. Labs: Argonne, Berkeley, Brookhaven National Labs

U.S. Universities: Arkansas, UC Berkeley, UC Davis, UCLA, Carnegie Mellon, Creighton, Indiana, Kent State, MSU, CCNY, Ohio State, Penn State, Purdue,Rice, Texas A&M, UT Austin, Washington, Wayne State, Yale

Brazil: Universidade de Sao Paolo

China: IHEP - Beijing, IPP - Wuhan

England: University of Birmingham

France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes

Germany: Max Planck Institute – Munich University of Frankfurt

Poland: Warsaw University, Warsaw University of Technology

Institutions: 36 Collaborators: 415

The Ohio State U. GroupProfs: PostDocs: Students:T.Humanic D.Majestro S.Bekele M.Lisa B. Nilsen M.Lopez-

NoriegaE.Sugarbaker I. Kotov R.Wells

R.Willson

The STAR Collaboration

The STAR Detector

The STAR Detector

• Year 2000, year 2001, year-by-year until 2003, installation in 2003

ZCal

Silicon Vertex Tracker *

Central Trigger Barrel+ TOF patch

FTPCs (1 + 1)

Time Projection Chamber

Vertex Position Detectors

Magnet

Coils

RICH * yr.1 SVT ladder

Barrel EM Calorimeter

TPC Endcap & MWPC

Endcap Calorimeter

ZCal

STAR Time Projection Chamber

(TPC)

• Active volume: Cylinder r=2 m, l=4 m

– 139,000 electronics channels sampling drift in 512 time buckets

– active volume divided into 70M 3D pixels

On-board FEE Card:Amplifies, samples, digitizes 32 channels

Spectators – Definitely going down the beam line

Participants – Definitely created moving away from beamline

Triggering/Centrality

ImpactParameter

Spectators

Spectators

Zero-Degree Calorimeter

Participants

Several meters

Spectators – Definitely going down the beam line

Participants – Definitely created moving away from beamline

Triggering/Centrality

ImpactParameter

Spectators

Spectators

Zero-Degree Calorimeter

Participants

Several meters

• “Minimum Bias”ZDC East and West thresholds set to lower edge of single neutron peak.

REQUIRE:Coincidence ZDC East and West

• “Central”CTB threshold set to upper 15%

REQUIRE: Min. Bias + CTB over threshold

~30K Events |Zvtx| < 200 cm

Au-Au Event at 130 A-GeV

Peripheral EventFrom real-time Level 3 display.

Au- Au Event 130 A-GeV

Mid-Central EventFrom real-time Level 3 display.

Au -Au Event 130 A-GeV

Central EventFrom real-time Level 3 display.

STAR Pertinent Facts (130 GeV)

Field:

0.25 T (Half Nominal value)

worse resolution at higher p

lower pt acceptance

TPC:

Inner Radius – 50cm

(pt>75 MeV/c)

Length – ± 200cm

( -1.5 1.5)

Events:

~300,000 “Central” Events –top 8% multiplicity

~160,000 “Min-bias” Events

Needle in the Hay-Stack!

How do you do tracking in this regime?

Solution: Build a detector so you can zoom in close and “see” individual tracks

Good tracking efficiency

Clearly identify individual tracks

high resolution

Pt (GeV/c)

Particle ID Techniques - dE/dx

Particle ID Techniques - dE/dx

dE/dx PID range: ~ 0.7 GeV/c for K/ ~ 1.0 GeV/c for K/p

12

Kp

d

edE

/dx

(keV

/cm

)

0

8

4

12

Kp

d

edE

/dx

(keV

/cm

)

0

8

4

Kp

d

edE

/dx

(keV

/cm

)

0

8

4

dE/dx

Particle ID Techniques - dE/dx

dE/dx PID range: ~ 0.7 GeV/c for K/ ~ 1.0 GeV/c for K/p

12

Kp

d

edE

/dx

(keV

/cm

)

0

8

4

12

Kp

d

edE

/dx

(keV

/cm

)

0

8

4

Kp

d

edE

/dx

(keV

/cm

)

0

8

4

dE/dx

6.7%Design

7.5%With calibration

9 %No calibration

Resolution:

Even identified anti-3He !

Particle ID Techniques - Topology

Decay vertices

Ks + + -

p + -

p + +

- + -

+ + +

+ K -

“kinks”:

K +

Vo

Physics Measurements(ones in red will be shown)

•dN/dfor h- (||<= ~1.5) particle density, entropy

•Elliptic flow early dynamics, pressure

•p/p, / stopping

•Particle spectra temperature, radial flow

•Particle ratioschemistry

•Particle correlations geometry, collective flow

•High Pt jet quenching

__

•Neutral particle decays ,K0s, strangeness production

Transverse Energy

PHENIX Preliminary

Phenix Electromagnetic Calorimeter measures transverse energy in collisions

Central Events:

Lattice predicts transition at

~ 5.0 GeV/fm3

critical ~ 0.5-0.7 GeV/fm3

Have the Energy Density!!

dydE

RBjt

02 2

11

Soft Physics (pT < 2 GeV/c)

Soft Physics (pT < 2 GeV/c)

99.5%

The majority of produced particlesare low pT.

Do they interact and exhibt collective behaviour?

What are the bulk dynamics ?

Is there Thermalization?

Almond shape overlap region in coordinate space

y2 x2 y2 x2

2cos2 v

x

y

p

patan

Origin: spatial anisotropy of the system when created and rescattering of evolving systemprobe of the early stage of the collision

Look at “Elliptic” Flow

Elliptic Flow of Pions and Protonsfrom STAR (130 GeV)

• Hydrodynamic calculations: P. Huovinen, P. Kolb and U. Heinz

Mass dependence of v2(pt) shows a

behavior in agreement with hydro calculations,

which assumes a system in equilibrium

Charged particle elliptic flow 0< pt< 4.5 GeV/c from STAR(130

GeV)

Around pt > 2

GeV/c the data starts to deviate from hydro.

However, v2 stays

large.

Only statistical errors

Systematic error 10% - 20% for pt = 2 – 4.5 GeV/c

Kinetic Freeze-out and Radial Flow

Want to look at how energy distributed in system.

Look in transverse direction so not confused by longitudinal expansion

Kinetic Freeze-out and Radial Flow

Look at mt = (pt2 + m2 )

distributionA thermal distribution gives a linear distribution dN/dmt e-(mt/T)

mt

1/m

t d2N

/dyd

mt

Slope = 1/T

Want to look at how energy distributed in system.

Look in transverse direction so not confused by longitudinal expansion

Kinetic Freeze-out and Radial Flow

If there is transverse flow

Look at mt = (pt2 + m2 )

distributionA thermal distribution gives a linear distribution dN/dmt e-(mt/T)

mt

1/m

t d2N

/dyd

mt

Slope = 1/T

Want to look at how energy distributed in system.

Look in transverse direction so not confused by longitudinal expansion

Kinetic Freeze-out and Radial Flow

If there is transverse flow

Look at mt = (pt2 + m2 )

distributionA thermal distribution gives a linear distribution dN/dmt e-(mt/T)

mt

1/m

t d2N

/dyd

mt

Slope = 1/T

Slope = 1/Tmeas

~ 1/(Tfreeze out + 0.5moflow2)

Want to look at how energy distributed in system.

Look in transverse direction so not confused by longitudinal expansion

First 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

First 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

mT1/m

T d

N/d

mT

light

heavyT

purely thermalsource

First 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

mT1/m

T d

N/d

mT

light

heavyT

purely thermalsource

explosivesource

T,mT1/

mT d

N/d

mT

light

heavy

First 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

mT1/m

T d

N/d

mT

light

heavyT

purely thermalsource

explosivesource

T,mT1/

mT d

N/d

mT

light

heavy• good agreement with hydrodynamic

calculations

T = 190 MeV

T = 300 MeV

Tp = 565 MeV

mid-rapidity

mt slopes vs. Centrality

• Increase with collision centrality

consistent with radial flow: Tfreeze out=0.12 GeV, flow=0.6c

We’ve shown so far that for RHIC collisions:

•Some evidence that source is thermalized

•Particles kinetically freeze-out with common T

•Large transverse flow - common to all particle species

“HBT 101” - probing source geometry

5 fm

1 m source(x)

r1

r2

x1

x2

p1

p2

q = p2 – p1

T = U(x1,p1)exp{i(r1-x1)p1}U(x2,p2)exp{i(r2-x2)p2}

+ U(x1,p2)exp{i(r2-x1)p2}U(x2,p1)exp{i(r1-x2)p1}

Integrate * over (x)

e.g. ~ exp(-r2/2R2) C = 1 + exp(-q2R2)

“HBT 101” - probing source geometry

5 fm

1 m source(x)

r1

r2

x1

x2

p1

p2

12 ppq

1-particle probability

)xx(iq2

*21

*1T

*T

21e1UUUU

2-particle probabilityq = p2 – p1

T = U(x1,p1)exp{i(r1-x1)p1}U(x2,p2)exp{i(r2-x2)p2}

+ U(x1,p2)exp{i(r2-x1)p2}U(x2,p1)exp{i(r1-x2)p1}

Integrate * over (x)

e.g. ~ exp(-r2/2R2) C = 1 + exp(-q2R2)

“HBT 101” - probing source geometry

2

21

2121 )q(~1

)p(P)p(P)p,p(P

)p,p(C

C (Q

inv)

Qinv (GeV/c)

1

2

0.05 0.10

Width ~ 1/R

Measurable! F.T. of pion source

5 fm

1 m source(x)

r1

r2

x1

x2

p1

p2

12 ppq

1-particle probability

)xx(iq2

*21

*1T

*T

21e1UUUU

2-particle probabilityq = p2 – p1

T = U(x1,p1)exp{i(r1-x1)p1}U(x2,p2)exp{i(r2-x2)p2}

+ U(x1,p2)exp{i(r2-x1)p2}U(x2,p1)exp{i(r1-x2)p1}

Integrate * over (x)

e.g. ~ exp(-r2/2R2) C = 1 + exp(-q2R2)

“HBT 101” - probing the timescale of emission

K

RoutRside y,xx,x sideout

Decompose q into components:qLong : in beam directionqOut : in direction of transverse momentumqSide : qLong & qOut

(beam is into board) 22

s2o RR

beware this “helpful” mnemonic!

2l

2l

2s

2s

2o

2o RqRqRq

lso e1)q,q,q(C

RO2 = <(xOut - Tt)2>

RS2 = < xSide

2 >RL

2 = <(xLong – Lt)2>

HBT and the Phase Transition

withouttransition

“”

withtransition

c

Rischke & GyulassyNPA 608, 479 (1996)

Generic prediction of 3D hydrodynamic models

Primary HBT “signature” of QGP

~ emission

timescale

Phase transition longer lifetime; Rout/Rside ~ 1 + ()/Rside

Two-pion interferometry (HBT)

from STAR• Correlation function for identical

bosons:• 1d projections of 3d Bertsch-Pratt• 12% most central out of 170k

events• Coulomb corrected• |y| < 1, 0.125 < pt < 0.225

qout

STAR preliminary

STAR preliminary

qlong

fmR

fmR

fmR

Long

Side

Out

)21.012.007.7(

)16.009.047.5(

)23.011.086.5(

03.001.050.0

Radii dependence on centrality and kt

•Radii increase with multiplicity - Just geometry (?)

•Radii decrease with kt – Evidence of flow (?)

low kT central collisions

“multiplicity”

STAR preliminary

x (fm)

y (f

m)

Hydro attempts to reproduce data

Rout

Rside

Rlong: model waits too longbefore emitting

• KT dependence approximately reproduced correct amount of collective radial flow

• Right dynamic effect / wrong space-time evolution??? the “RHIC HBT Puzzle”

generichydro

model emission timescale too long

HBT excitation function

STAR Collab., PRL 87 082301 (2001)

•decreasing parameter partially due to resonances

•saturation in radii

•geometric or dynamic (thermal/flow) saturation

•the “action” is ~ 10 GeV (!)

•no jump in effective lifetime

•NO predicted Ro/Rs increase(theorists: data must be wrong)

•Lower energy running needed!?

midrapidity, low pT -

from central AuAu/PbPb

timeEvolution ofa heavy-ion

collision

Before collision(heavy nuclei)After collision:

QM formation??Hadronization

Strong hadronicrescattering

“Freezeout”(hadrons freely

stream to detectors)

In order to studyQM/hadronizationstage of collision

from freezeout hadrons, need to understand

rescattering stage first!

Hadronic rescattering model

(T. J. Humanic, Phys.Rev.C 57, 866, (1998))1) Assume a simple hadronization picture to set the initial geometry and momenta.

2) Put in a bunch of hadrons whose multiplicities are consistent with RHIC experiments (or predictions).

3) Let hadrons undergo strong binary collisions until the system gets so dilute (since it is expanding) that all collisions cease.

4) Record the time, mass, position, and momentum of each hadron when it no longer scatters. freezout condition.

5) Calculate hadronic observables pT distributions, elliptic flow, HBT, …

1/mTdN/dmT = mTexp(-mT/T)T = 300 MeV

parameters: initial temperature (T), hadronization proper time ()

(cylindrical)(thermal)

K, N, ’…..

(i,j)

z

r

z = sinh y ; t = cosh y = 1 fm/c

Comparison of the Rescattering model with RHIC data for pT distributions

As seen above, the qualitative shapes are the same forpT < 3 GeV/c !

Comparison of Recattering model with RHIC data for mT distributions

The slopes areseen to agree !

Elliptic Flow vs. pT from rescattering model

compared with STAR

Figure4.eps

Figure5.eps

flattening at high pT as in data

STAR HBT vs Rescattering Model

Figure6.eps

Rescattering qualitatively describes the centrality andmomentum dependences of the pion HBT data!!

*

*

*

*

*

*

*

*

*

*

*

*

Comparison of Rescatteringmodel with SPS and RHIC

data for pion HBT

( rescattering model * )

Model is seen to describethe beam energy dependenceof the HBT parameters well!

Conclusion for “soft” (i.e. low pT) RHIC physics:

Hadronic rescattering with a shorthadronization time ( = 1 fm/c)

describes dynamic features well!

Hard Physics: pT > 2GeV/c

Goal: Use jets to probe properties of medium

Some Basic Observables:

- Inclusive Spectra and RAA

- Azimuthal Anisotropy, v2

- Statistical & Correlations

STAR p+p Di-Jet

hadronsleadingparticle suppressed

q

q

?

The Experimental Challenge

p+p dijet Central Au+Au Event

Find this ………………………………………………….in

here

Inclusive Charged Hadron Production

STAR, PRL 89, 202301 (2002)

s = 130 GeV

s = 200 GeV

nucl-ex/0210026

Leading Particle Suppression: Theory

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

leadingparticle

Wang and Gyulassy: partonic energy loss proportional to gluon density, glue

effective softening of fragmentation suppression of leading hadron yield

Nuclear Modification Factor:

<Nbinary>/inelp+p

Partonic Energy loss in high density matter

hadrons

q

q

hadrons

leading particle

hadrons

hadronsleadingparticle suppressed

leading particle suppressed

q

q

(Nuclear Geometry)

Leading Hadron Suppression: Data

STAR p+p reference in the works…

Suppression saturates at 3~5 for pT > 6 GeV/c

RCP Central/Peripheral

RAA using UA1 NN Reference s = 200 GeV Preliminary

Suppression similar at 130 GeV (PRL 89, 202301 (2002))

nucl-ex/0210026

Azimuthal Correlations

Px (GeV/c)

Py

(GeV

/c)

-4 -3 -2 -1 0 1 2 3 4

-4

-3

-2 -

1

0 1

2

3

4

Pt

• Correlation with respect to leading particle (pT>4 GeV/c)

• Consider only particles above 2 GeV/c

• Small difference in relative pseudorapidity

|

Peripheral Au + Au

Central Au + Au

Ansatz: Au+Au = p+p + Elliptic Flow

High pT Azimuthal Correlations

nucl-ex/0210033

Near-side correlation shows jet-like signal in central/peripheral Au+Au Away-side correlation suppressed in central Au+Au

Surface Emission of Jets ?

?

• This is in accordance with 1) the measured suppresion of the inclusive spectra with respect to binary collisions, and 2) high-pT azimuthal correlations.– We only ‘see’ jets

emitted from the surface?

Suppression of away-side jet consistent with strong absorption in the bulk, with emission dominantly from

the surface

Summary of Au+Au Collisions at RHIC

Hard physics:• Strong suppression of inclusive yields• Azimuthal anisotropy at high pT

• Suppression of back-to-back hadron pairs

Soft physics:• System appears to be thermalized• Rapid hadronization, strong rescattering• Large radial flow, elliptic flow, and HBT results all explainable as resulting from hadronic rescattering

Large parton energy loss with surface

emission?

?

STAR STRANGENESS!

K0s

K+

(Preliminary)

The Collisions

The End Product