On the trail of the quark-gluon plasma at RHIC
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Transcript of On the trail of the quark-gluon plasma at RHIC
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On the trail of the quark-gluon plasma at RHIC
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outline
Why collide heavy ions?QCD and the phase transition
the Relativistic Heavy Ion Collider + experiments
What have we learned so far?Thermalization & pressure build up – early!(medium-induced) modification of jetsThe control experiment: d+AuMedium effects on fragmentation function?
Conclusions
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The Physics of RHIC
Create very high temperature and density matteras existed ~1 sec after the Big Banginter-hadron distances comparable to that in neutron starscollide heavy ions to achieve maximum volume
Study the hot, dense mediumis thermal equilibrium reached?transport properties? equation of state?do the nuclei dissolve into a quark gluon plasma?
Collide Au + Au ions at high energys = 200 GeV/nucleon pair, p+p and d+A to compareAlso polarized p+p collisions to study carriers of p’s spin
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Quantum ChromoDynamics
Strong interaction field theory : colored quarks exchange gluons Parallels QED but gluons have color charge
unlike E&M where are uncharged they interact among themselves (i.e. theory is non-abelian):
curious properties
+ +…
short distance:force is weak (probe w/ high Q2, Calculate with perturbation theory)large distance: force is strong (probe w/ low Q2, calculations must be non-perturbative) leads to confinementHigh temperature: force becomes screened by produced color-charges
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QCD Phase Transition
Basic (i.e. hard) questionshow does process of quark confinement work?how nature breaks symmetries massive particles from ~
massless quarks transition affects evolution of early universe
latent heat & surface tension matter inhomogeneity in evolving universeequation of state compression in stellar explosions
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s = 200 GeV: start with pQCD & pp collisions
p-p hep-ex/0304038
Good agreementwith NLO pQCD
Works!A handle on initial NN interactions
also need:2
/( , )
a Nf x Q
2
/( , )ch a
D z Q
Parton distribution functions
Fragmentation functions
Xc(A)
pQCD
BFKL, DGLAP
G-sat.
>2
RHIC
Log Q2
-Log10 x
0
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In A+A: QCD in non-perturbative regime
T/Tc
Karsch, Laermann, Peikert ‘99
/T4
Tc ~ 170 ± 10 MeV (1012 °K)
~ 3 GeV/fm3
But, we look for physics beyond simple superposition of NN:
EquilibrationCollective effectsEnergy, color transport in dense mediumDeconfinement?
EOS
Lattice…
Physics is softLattice QCD says:Create these conditions to look for new physics
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Experimental approach
Central region has max temperature & density
Head-on = “central” collisions max volume
pT
Thermalization? particle spectra, yieldsPressure developed? particle/energy flowsMedium properties? effects upon probe particlesDeconfinement? c and anti-c remain bound as J/?
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RHIC at Brookhaven National Laboratory
RHIC is first dedicated heavy ion collider10 times the energy previously available!
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4 complementary experiments
STAR
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Is the energy density high enough?PRL87, 052301 (2001)
R2
2c
Colliding system expands:
dy
dE
cRT
Bj 22
11
02
Energy tobeam direction
per unitvelocity || to beam
4.6 GeV/fm3 (130 GeV Au+Au)
5.5 GeV/fm3 (200 GeV Au+Au)well above predicted transition!
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history of heavy ion collisions
PCM & clust. hadronization
NFD
NFD & hadronic TM
PCM & hadronic TM
CYM & LGT
string & hadronic TM
, e+e-, + Kpnd,
Real and virtual photons from q scattering sensitive to the early stages. Probe also with q and g produced early, & passing through the medium on their way out.
Hadrons reflect medium properties when inelastic collisions stop (chemical freeze-out).
high , pressure builds up
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Particle production (lots!)
Central Au+Aucollisions
(~ longitudinal velocity)
sum particles under the curve,
find ~ 5000 charged
particles in collision final state
(6200 in 200 GeV/A central
Au+Au)
In initial volume ~ Vnucleus
Rescattering should be important!
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Hadron pT spectra – all 4 experiments!
BRAHMS: 10% centralPHOBOS: 10%PHENIX: 5%STAR: 5%
200 GeV/A Au+Au
Protons show velocity boost to beam.Expect if pressure build-up due to rescattering
Data well fit with: Tfo = 110-120 MeV & <t> = 0.5-0.6
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Simple quark counting:K-/K+
= exp(2s/T)exp(-2q/T)
= exp(2s/T)(pbar/p)1/3
= (pbar/p)1/3
local strangeness conservation K-/K+=(pbar/p)
= 0.24±0.02 BRAHMS = 0.20±0.01 for SPS
Good agreement with statistical-thermal model of Beccatini et al. (PRC64 2001) w/T=170 MeV
At y=0
From y=0 to 3
PRL 90 102301 Mar. 2003
Evidence for equilibrated final hadronic stateBRAHMS
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More evidence for equilibrated final state
Observed hadron ratios in agreement with thermal ratios!T(chemical freeze-out) ~ 175 MeV
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Early state? a barometer called “elliptic flow”
Origin: spatial anisotropy of the system when created, followed by multiple scattering of particles in the evolving system spatial anisotropy momentum anisotropy
v2: 2nd harmonic Fourier coefficient in azimuthal distribution of particles with respect to the reaction plane
Almond shape overlap region in coordinate space
y2 x2 y2 x2
2cos2 vx
y
p
patan
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v2 measured by the experiments
STAR
v2=0.05
130 GeV: 0.075< pt < 2.0 200 GeV: 0.150< pt < 2.04-part cumulants
200 GeV: 0.2< pt < 2.0
Preliminary
200 GeV: Preliminary
- Consistent results- At 200 GeV better pronounced decrease of v2 for the most peripheral collisions.
STARPreliminary
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v2 predicted by hydrodynamics
STARPRL 86 (2001) 402
Hydro. CalculationsHuovinen, P. Kolb,U. Heinz
pressure buildup explosionhappens fast early equilibration !
Hydro can reproduce magnitudeof elliptic flow for , p. BUTmust add QGP to hadronic EOS!!
Similar conclusion reached byKo, Kapusta, Bleicher, others… rescattering must be very large!
central
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a unique probe for physics of hot medium
hadrons
q
q
hadronsleadingparticle
leading particle
schematic view of jet productionProbe: Jets from hard scattered quarks
Observed via fast leading particles orazimuthal correlations between the leadingparticles
But, before they create jets, the scattered quarks radiate energy (~ GeV/fm) in the colored medium
decreases their momentum (fewer high pT particles)“kills” jet partner on other side “jet quenching”
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Nuclear Modification of Hadron Spectra?
ddpdT
ddpNdpR
TNN
AA
TAA
TAA /
/)(
2
2
<Nbinary>/inelp+p
nucleon-nucleon cross section
1. Compare Au+Au to nucleon-nucleon cross sections2. Compare Au+Au central/peripheral
Nuclear Modification Factor:
If no “effects”: RAA < 1 in regime of soft physics RAA = 1 at high-pT where hard scattering dominates Suppression: RAA < 1 at high-pT
AA
AA
AA
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pp
AuAubinaryAuAuAA Yield
NYieldR
/
2/pp
AuAupartAuAupartAA Yield
NYieldR
/
Au-Au s = 200 GeV: high pT suppression!
nucl-ex/0304022
Au-Au nucl-ex/0304022
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jet correlations: Au+Au vs p+pSTAR PRL 90, 082302 (2003)
Central Au + Au
Peripheral Au + Au
22 2 2( ) ( ) (1 cos(2 ))D Au Au D p p B v
Back-to-back jets are suppressed in central collisions!
near side
away side
peripheral central
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Suppression: a final state effect?
Hadronic absorption of fragments: Gallmeister, et al. PRC67,044905(2003)Fragments formed inside hadronic medium
Parton recombination (up to moderate pT)Fries, Muller, Nonaka, Bass nucl-th/0301078Lin & Ko, PRL89,202302(2002)
Energy loss of partons in dense matterGyulassy, Wang, Vitev, Baier, Wiedemann…
PCM & clust. hadronization
NFD
NFD & hadronic TM
PCM & hadronic TM
CYM & LGT
string & hadronic TM
Hadron gas
1AuAuR Absent in d+Au collisions!d+Au is the “control” experiment
Not technically quite a Final state effect…
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Suppression: an initial state effect?
Gluon Saturation (color glass condensate)
Wavefunction of low x gluons overlap; the self-coupling gluons fuse, saturating the density of
gluons in the initial state. (gets Nch right!)
• Multiple elastic scatterings (Cronin effect) Wang, Kopeliovich, Levai, Accardi
• Nuclear shadowing
Gribov, Levin, Ryshkin, Mueller, Qiu, Kharzeev, McLerran, Venugopalan,
Balitsky, Kovchegov, Kovner, Iancu …
probe rest frame
r/ggg
dAu AuAuR R RdAu~ 0.5D.Kharzeev et al., hep-ph/0210033
1dAuR
decreases dAuR
Broaden pT :
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PHENIX Preliminary 0
PHOBOS Preliminary
STAR Preliminary
Experiments show NO suppression in d+Au!
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Do see Cronin effect!
“Cronin” enhancement more pronounced in the charged hadron measurement
Possibly larger effect in protons at mid pT
Implication of RdAu? RHIC at too high x for gluon saturation…
(h++h-)/2
0
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0 RAA vs. predictions
PHENIX Preliminary
shadowing
anti-shadowing
Theoretical predictions:
d+Au: I. Vitev, nucl-th/0302002 and private communication.
Au+Au: I. Vitev and M. Gyulassy, hep-ph/0208108, to appear in Nucl. Phys. A; M. Gyulassy, P. Levai and I. Vitev, Nucl. Phys. B 594, p. 371 (2001).
Initial state: mult. scatt.,shadowing + final state dE/dx (Au+Au)
Also: Kopeliovich, et al (PRL88, 232303,2002)
predict RpA~1.1 max at pT=2.5 GeV projectile as color dipole
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Centrality Dependence
Dramatically different and opposite centrality evolution of AuAu experiment from dAu control.
Jet Suppression is clearly a final state effect.
PHENIX Preliminary results, consistent with PHOBOS data in submitted paper
Au + Au Experiment d + Au Control
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Back-to-back jets observed in d+Au
• jet pair production also looks independent of Ncoll
• observe no (big) suppression in back-to back jets!
• probably some jet broadening due to initial multiple scattering…
ST
STAR
PHENIX preliminary
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Particle mix at RHIC is different!
p/ ~1 at high pT
in central collisionsHigher than in p+por jets in e+e-collisions
Hydro. expansion at low pT
+ jet quenching at high pT?
Medium modified fragmentationfunction?
Vitev&Gyulassy nucl-th/0104066
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Do the baryons scale with Ncoll?
hard/soft process interplay? Quark recombination?Medium modification of fragmentation function?
Baryon yields not suppresed Ncoll at pT = 2 – 4 GeV/c
Au+Au
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To help sort it out, study initial state effects
=
d+Au PHENIX preliminary
low pT
high pT What is initial state multiplescattering mechanism?
How is production of mesons and baryons affected? fragmentation function…
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0-20%most central Ncoll=779
20-40%most central Ncoll=296
40-90%most central Ncoll=45
Proton
R.L. Thews, M. Schroedter, J. Rafelski Phys. Rev. C63 054905 (2001): Plasma coalesence modelfor T=400MeV and ycharm=1.0,2.0, 3.0 and 4.0.
L. Grandchamp, R. Rapp Nucl.
Phys. A&09, 415 (2002) and Phys. Lett. B 523, 50 (2001):Nuclear Absorption+ absoption in a high temperature quark gluon plasma
A. Andronic et. Al. Nucl-th/0303036
Deconfinement? Does colored medium screen c+cbar?
Can’t tell yet, but don’t see EXTRA (thermal) J/
Look at J/
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conclusions
Rapid equilibration! Strong pressure gradients, hydrodynamics worksConstituent scattering cross section is very large
EOS is not hadronic The hot matter is “sticky” – it absorbs energy
See energy loss, disappearance of back-to-back jetsd+Au data says: final state, not initial state effect
So, the stuff is dense, hot, ~ equilibrated AND NEW!
OK, why not announce QGP discovery?J/ suppression or not? Next runTinitial? direct photon analysis underway by PHENIXProperties not as expect for plasma – looks like gluon liquid
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A few mysteries…
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Hydro describes single + multi-particles
• How to increase R without increasing Rout/Rside???
EOS?initial T & r profiles? emissivity?
But FAILS to reproducetwo-particle correlations!
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Elliptic flow of high momentum particles
pT (GeV/c)
v2baryons cross mesons(not expected from hydro)
Still flowing at pT = 8 GeV/c? Geometry? v2 a bit too big…Mix of flow + jets???
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Total charm quark cross section at RHIC
Cross section fits into expected energy dependenceNo evidence for strong energy loss of charmed quarks…
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Centrality dependence of charm quarks
Compare themeasurement to (PYTHIA) an event generator tuned for pp collisions…
no largesuppressionas for lightquarks!
Spectra of electrons from c e + anything
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Why no big energy loss for heavy quarks?
no x4 suppressionfrom peripheral to central,as predicted fordE/dx=-0.5GeV/fm!
But (we squirm) - Is 40-70% peripheral enough? error bars still big!
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is d+Au same as p+Au?
p
n ZDC
Neutron tagged eventsenhance peripheral collisions
<Ncoll> = 5.0 / 3.6Could be Ncoll dependenced+Au looks very similar to p+Au
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Particle Composition at high pT
(h++h-)/20 ~ 50% greater in central than peripheral at mid pT
similar again for pT>5 GeV/c
Central
Peripheral
Run 2001/2002 Au-Au 200 GeV:s
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Locate RHIC on phase diagram
Baryonic Potential B [MeV]
0
200
250
150
100
50
0 200 400 600 800 1000 1200
AGS
SIS
SPS
RHIC
quark-gluon plasma
hadron gas
neutron stars
early universe
thermal freeze-out
deconfinementchiral restauration
Lattice QCD
atomic nuclei
fit yields vs. mass (grand canonical ensemble)
Tch = 175 MeV B = 51 MeV
These are the conditions when hadrons stop interacting
T
Observed particles “freeze out” at/near the deconfinement boundary!
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Au+Au at sNN=200GeVv2 of mesons & baryons to higher pT
Consistent between PHENIX and STAR
pT < 2 GeV/c v2(light) > v2(heavy) Explained by hydro. expansion
pT > 2.5 GeV/c v2(light) < v2(heavy) Can it be explained by some
combination of geometry + jet quenching?
Quark coalescence?S. Voloshin, nucl-ex/0210014R. Fries et al., nucl-th/0301087D. Molnar et al. nucl-th/0302014
In-medium fragmentation fn?
Model: P.Huovinen, et al., Phys. Lett. B503, 58 (2001)
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QCD Phase Transition
Basic (i.e. hard) questionshow does process of quark confinement work?how nature breaks symmetries massive particles from ~
massless quarks transition affects evolution of early universe
latent heat & surface tension matter inhomogeneity in evolving universeequation of state compression in stellar explosions
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J/ suppression was observed at CERN at s=18 GeV/A
Fewer J/ in Pb+Pb than expected!Interpret as color screening of c-cbar
by the mediumInitial state processes affect J/ tooso interpretation is still debated...
collaboration
J/yield
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Adler et al., nucl-ex/0206006
A puzzle at high pT
Still flowing at pT = 8 GeV/c? Unlikely!!
Nu Xu
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Vitev: they can get v2 right
C. Adler et al. [STAR Collab.], arXiv: nucl-ex/0206006
K. Filimonov [STAR Collab.],arXiv: nucl-ex/0210027
b=7 fmb~7 fm
• There is a quantitative difference Calculations/fits with flat or continuously growing
2 .v const 2 / .ln Tv p
Check against high-pT data (200 AGeV)
Same for 0-50%
• The decrease with pT is now supported by data• For minimum bias this rate is slightly slower
See: N.Borghini, P.Dinh, J-Y.Ollitrault, Phys.Rev. C 64 (2001)
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Npart/2
Nbinary
PHENIX 130
BRAHMSPRL88(02)
? 2003 ?
STAR 130
hch
15% too many particles, baryons over-quenched, but predicted the suppressionBUT: dE/dx =2 GeV/fm or 0.5 GeV/fm or not linear with x?
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kT dependence of R
Centrality is in top 30%
•Broad <kT> range : 0.2 - 1.2 GeV/c •All R parameters decrease as a function of kT consistent with collective expansion picture. • Stronger kT dependent in Rlong have been observed.
kT : average momentum of pair
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Comparison of kaon to pion
In the most 30% central
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Comparison with hydrodynamic model
Recent hydrodynamic calculation by U.Heinz and P. F. Kolb(hep-ph/0204061)
kT dependence of Rlong indicates the early freeze-out?
Hydro w/o FS
Hydro at ecrit
• Assuming freeze out directly at the hadronization point. (edec = ecrit)
• Standard initialization and freeze out which reproduce single particle spectra.
Centrality is in top 30%
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kT dependence of Rout/Rside
A. EnikizonoQM2002
C.M. Kuo, QM2002 poster (PHOBOS) 200 GeV:
.)(25.009.016.1 syst @0.25 GeV/c
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HBT PUZZLE
P.Kolb
Small Rout implies small
Large Rside implies large RSmall Rbeam impliessmall breakup ~10 fm/c
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Jet Evidence in Azimuthal Correlations at RHIC
near-side correlation of charged tracks (STAR)trigger particle pT = 4-6 GeV/c distribution for pT > 2 GeV/c
signature of jets
also seen in (0) triggered events (PHENIX)trigger particle pT > 2.5 GeV/c distribution for pT = 2-4 GeV/c
M. Chiu, PHENIX Parallel Saturday
QM2002 summary slide (Peitzmann)
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Identifying Jets - Angular Correlations
Remove soft background by subtraction of mixed event distribution
Fit remainder:Jet correlation in ; shape taken from PYTHIAAdditional v2 component to correct flow effects
PHENIX Preliminaryraw differential yields
2-4 GeV
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Verify PYTHIA using p+p collisions
(neutral E>2.5 GeV + 1-2 GeV/c charged partner)
||<.35 ||>.35
ake cuts in to enhance near or far-side correlationsBlue = PYTHIA
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In Au+Au collisions
1-2 GeV partner
(neutral E>2.5 GeV + charged partner)
||<.35 ||>.35
1/N
trig d
N/d
1/N
trig d
N/d
Correlation after mixed event background subtraction
Clear jet signal in Au + AuDifferent away side effect than in p+p
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Jet strengthSee non-zero jet strength as partner pT increases!
jets or flow correlations? fit pythia + 2v2vjcos(2)
partner = .3-.6 GeV .6-1.0 GeV/c 2-4 GeV/c
1/N
trig d
N/d
v2
vj
1-2 GeV/c
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How many particles are produced?
dNch/d = 640
Rises somewhat faster than Npart
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Charged hadron correlations - small
•Fit charged correlations with v2 + Gaussian (fixed pT)•Jet signal visible via
Width of near-side Gaussian decreases with pT
No significant centrality dependence on near-side
Cor
rela
tion
wid
th
jT
pT Correlation width jT/pT
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How do high pT yields scale?
vs. binary collisions:continuous decrease as
function of centralityfactor ~ 3.5 from
peripheral to central vs. participants:
first increase, then decrease as function of centrality
for Npart > 100 have 3 change (scaling or no?)
surface emission? re-interactions?accident?
18% scaling uncertainty from corrections
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Opaque, expanding source would mean:
2222222 2)()( xtso YXRR
)(outX
)(sideY
29.13
5)(
)(
spheres
shellhalfs
R
R
65.012
5)(
)(
sphereo
shellhalfo
R
R
Opaque Expanding
Rischke RIKEN workshop (2002): Such strong xt correlations probably require a lack of boost-invariance...
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Energy Dependence
Assumptions:in Lab in C.M.
Energy density (Bjorken):
2% most central at sNN=200 GeV:
5.5 GeV/fm3
From AGS, SPS to RHIC:
Transverse energy and charged particle multiplicity densities per participant consistent with logarithmic behaviour
d
dX
dy
dX
d
dX
dy
dX2.1
dy
dE
Rt
2
1
cfm
AfmR
/1
18.1 3/1
PHENIX preliminary
PHENIX preliminary
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So, is there jet quenching?
Suppression observed to 8 GeV/c! (in 3 independent measurements)
Theory agrees with data when quark, gluon energy loss is included
NB: 2 examples here, others also must add some kind of medium modification of the fast quarks/gluons
In initial or final state?
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Look at “transverse mass” mT2 = pT
2 + m02
— is distribution e-E/T?i.e. Boltzmann distribution from thermalized gas?
hadron spectra: , K, p and antiprotons
130GeV/A
yes !
Protons are flatter velocity boost to beamResult of pressure built up
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Spectral shapes
<pT> for radially expanding hadrongas with Tth and <>
<pT> in pp with “Tch” = 170 MeV and <>=0pp no rescattering, flow or equilibrium
STAR
preliminaryF. Wang
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min bias 200 GeV Au+ Au
v2 at high pT
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Note pbar/p behavior
Centrality dependence only for pT > 3 GeV/c
Peripheral collisions have quite a few protons at mid-y
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Anti-particle/particle ratios vs. rapidity
At y=0 (central coll.)
pbar/p = 0.75 ±0.04
K-/K+ = 0.95 ±0.05 = 1.01 ±0.04
Nearly baryon-free at central y, but not complete transparency
Larger contribution of protons nearer rapidity of the Au beams
PRL 90 102301 (Mar. 2003)
BRAHMS