HIC03 1 STAR STAR Jets and high p T hadrons at RHIC: recent results from STAR Peter Jacobs Lawrence Berkeley National Laboratory Inclusive spectra Single-particle azimuthal asymmetry (“elliptic flo Two-particle azimuthal distributions Results from the d+Au run What have we learned at RHIC?
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STAR HIC031 Jets and high p T hadrons at RHIC: recent results from STAR Peter Jacobs Lawrence Berkeley National Laboratory Inclusive spectra Single-particle.
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HIC03 1STARSTAR
Jets and high pT hadrons at RHIC:recent results from STAR
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
HIC03 13STARSTAR
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?
HIC03 14STARSTAR
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?)
HIC03 15STARSTAR
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
HIC03 16STARSTAR
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
HIC03 17STARSTAR
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?
HIC03 18STARSTAR
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)
HIC03 19STARSTAR
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
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
HIC03 23STARSTAR
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
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
• 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
HIC03 30STARSTAR
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
HIC03 31STARSTAR
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)
HIC03 32STARSTAR
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