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
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?
HIC03 2STARSTAR
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
HIC03 28STARSTAR
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? …
HIC03 29STARSTAR
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
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
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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)
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
STAR preliminary
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