HIC03 1STARSTAR

Jets and high pT hadrons at RHIC:recent results from STAR

Peter JacobsLawrence Berkeley National Laboratory

• Inclusive spectra• Single-particle azimuthal asymmetry (“elliptic flow”)• Two-particle azimuthal distributions

• Results from the d+Au run

• What have we learned at RHIC?

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E-M Calorimeter

Time of    Flight

Projection           Chamber

The STAR detector

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Partonic energy loss in dense matter

Multiple soft interactions:

Strong dependence of energy loss on gluon density gluemeasure color charge density at early hot, dense phase

Gluon bremsstrahlung

Opacity expansion:

glueSmediumT

SR

kq

LqC

E

2

2

ˆ

ˆ4

L

ELogrdCCE jet

glueSaA 23 2

,

Bjorken, Baier, Dokshitzer, Mueller, Pegne, Schiff, Gyulassy, Levai, Vitev, Zhakarov, Wang, Wang, Salgado, Wiedemann,…

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Jets at RHIC

p+p jet+jet (STAR@RHIC)

Au+Au ??? (STAR@RHIC)

nucleon nucleonparton

jet

Find this……….in this

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Partonic energy loss via leading hadrons

-

Energy loss softening of fragmentation suppression of leading hadron yield

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

Binary collision scaling p+p reference

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Au+Au and p+p: inclusive charged hadrons

PhysRevLett 89, 202301 nucl-ex/0305015

p+p reference spectrum measured at RHIC

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Suppresion of inclusive hadron yield

• central Au+Au collisions: factor ~4-5 suppression • pT>5 GeV/c: suppression ~ independent of pT

nucl-ex/0305015

Au+Au relative to p+p Au+Au central/peripheralRAA RCP

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Azimuthal anisotropy wrt reaction plane (“Elliptic Flow”)

Coordinate space: initial asymmetry

Momentum space: final asymmetry

asymmetry generated early in collision, quenched by expansion observed asymmetry emphasizes early time

x

y

p

patan2cos2 vSecond Fourier coefficient v2:

multiple collisions (pressure)

py

px

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Azimuthal anisotropy at high pT: theory

Snellings; Gyulassy, Vitev and Wang (Phys Rev Lett 2537 (2001) )

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)

jet

jet

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Azimuthal anisotropy at high pT: data

STARSTAR

• pT<2 GeV: detailed agreement with hydrodynamics • pT >4 GeV: finite v2 in qualitative agreement with expectations from partonic energy loss

STAR preliminaryPhys Rev Lett 90, 032301

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Jets and two-particle azimuthal distributionsp+p dijet • trigger: highest pT track, pT>4 GeV/c

• distribution: 2 GeV/c<pT<pTtrigger

• normalize to number of triggers

triggerN.B. shifted horizontally by /2

relative to previous STAR plots!

Phys Rev Lett 90, 082302

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Azimuthal distributions in Au+Au

Au+Au peripheral Au+Au central

Near-side: peripheral and central Au+Au similar to p+p

Strong suppression of back-to-back correlations in central Au+Au collisions

pedestal and flow subtracted

Phys Rev Lett 90, 082302

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What might all this mean?

?

Conjecture: core of reaction volume is opaque to jets

surface emission

Consequences: near-side fragmentation independent of system suppression of back-to-back jets suppression of inclusive rates strong elliptic flow at high pT

Compelling picture, but is it right?

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Is suppression an initial or final state effect?

Initial state? Final state?

partonic energy loss in dense medium generated in collision

strong modification of Au wavefunction (gluon saturation at low Bjorken x?)

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nucl-ex/0305015RCP

Inclusive suppression: theory vs. data

pQCD-I: Wang, nucl-th/0305010pQCD-II: Vitev and Gyulassy, PRL 89, 252301Saturation: KLM, Phys Lett B561, 93

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

Final state

Initial state

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Is suppression an initial or final state effect?

Initial state?gluon

saturation

Final state?partonic energy

loss

How to discriminate? Turn off final state d+Au collisions

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d+Au vs. p+p: Theoretical expectations

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

1.1-1.5

pT

RA

B

1

Inclusive spectra

If 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?

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ZDC-d

FTPC-Au

Au d

ZDC-Au

d+Au event selection

Centrality tags:1. FTPC-Au: charged particle multiplicity in -3.8<<-2.82. ZDC-d: neutron spectator from deuteron

Minimum bias trigger: ZDC-Au (953% of hadronic)

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Centrality tags

FTPC-Au cut selects more central collisions

Modeling FTPC-Au multiplicity: convolute Au-participant (Glauber) with UA5 mult. distribution (pbar+p @ 200 GeV, 2.5<<3.5)

min. bias <Nbinary>=central 20% <Nbinary>=12.91.0

ZDC-d neutron spectator tag:• measured: 191% of hadronic

• Glauber: 183% of hadronic

peripheral events

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Inclusive charged particle spectra

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Inclusive yield relative to binary-scaled p+p

Suppression of the inclusive yield in central Au+Au is a final-state effect

• d+Au : enhancement Au+Au: strong suppression

• pT=4 GeV/c: cent/minbias = 1.110.03 central collisions enhanced wrt minbias

ddpdT

ddpdNR

Tpp

AB

TAB

AB /

/

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Azimuthal distributions

pedestal and flow subtracted

Near-side: p+p, d+Au, Au+Au similarBack-to-back: Au+Au strongly suppressed relative to p+p and d+Au

Suppression of the back-to-back correlation in central Au+Au is a final-state effect

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The strong suppression of the inclusive yield and back-to-back correlations at high pT previously observed in central Au+Au collisions are due tofinal-state interactions with the dense medium generated in such collisions.

?X

STAR collaboration, nucl-ex/0306024:

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Have we found the Quark Gluon Plasma at RHIC?

We now know that Au+Au collisions generate a medium that

• is dense (pQCD theory: many times cold nuclear matter density)

• is dissipative

• exhibits strong collective behavior

Not yet, there is still work to do

We have yet to show that:

• dissipation and collective behavior both occur at the partonic stage

• the system is deconfined and thermalized

• a transition occurs: can we turn the effects off ?

This represents significant progress in our understanding of strongly interacting matter

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Russia:MEPHI - MoscowLPP/LHE JINR - DubnaIHEP - Protvino

U.S. Laboratories:ArgonneBerkeleyBrookhaven

U.S. Universities: UC BerkeleyUC DavisUC Los AngelesCarnegie Mellon Creighton UniversityIndiana UniversityKent State UniversityMichigan State UniversityCity College of New YorkOhio State UniversityPenn. State UniversityPurdue UniversityRice UniversityTexas A&MUT AustinU. of Washington Wayne State UniversityYale University

Brazil: Universidade de Sao Paulo

China: IHEP – BeijingIMP - LanzouIPP – WuhanUSTCSINR – ShanghaiTsinghua University

Great Britain: University of Birmingham

France:IReS StrasbourgSUBATECH - Nantes

Germany: MPI – MunichUniversity of Frankfurt

India:IOP - BhubaneswarVECC - CalcuttaPanjab UniversityUniversity of RajasthanJammu UniversityIIT - BombayVECC – Kolcata

Poland:Warsaw University of Tech.

The STAR Collaboration

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Extra slides

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200 GeV Au+Au: and K0s

STAR preliminary

K0s

Power-law shape except for central

STAR preliminary

bar

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Central/peripheral: dependence on particle species

STAR preliminary

• 1.5<pT<3.5 GeV/c: scales ~Nbinary, K0s suppressed

• pT~ 6 GeV/c: , K0s, charged hadrons scale similarly

• “baryon enhancement”: partonic flow? modified fragmentation? Cronin effect? …

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Important complication: non-flow

STAR Preliminary

• Non-flow: any two-particle correlation not due to correlation with the reaction plane: jets, resonances, momentum conservation, ….• Borghini, Ollitraut et al: measure non-flow via higher order correlations (cumulants)

Non-flow

Finite flow at pT~7 GeV/c

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Two-particle azimuthal distributions

• near-side and back-to-back peaks seen in minbias and central d+Au• p+p d+Au minbias d+Au central:

• growth of off-jet pedestal• small, if any, growth of back-to-back width

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Near-side correlations II

),()(11

)(2 NdefficiencyN

Ctrigger

< 0.5 > 0.5

Azimuthal correlation function:

• Trigger particle pT trig> 4 GeV/c

• Associate tracks 2 < pT < pTtrig

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

Central Au+Au

N.B. Away-side jet contribution subtracted by construction, needs different method…

Phys. Rev. Lett. 90, 032301 (2003)

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Reality check: charge-sign dependence

||<0.5 - ||>0.5 (scaled)

0<||<1.4

Au+Au

p+p

System

(+-)/(++ & --)

p+p 2.7+-0.6

0-10% Au+Au

2.4+-0.6

Jetset 2.6+-0.7

DELPHI, PL B407, 174 (1997)

• Compare same-sign (++, --) and opposite-sign (+-) pairs• Known jet physics: charge ordering in fragmentation

Opposite/same correlation strength similar in Au+Au, p+p, JETSETpT~3-4 GeV are jet fragments

STAR preliminary