1Carl Gagliardi – QNP 2004
Recent Results from RHIC
Carl GagliardiTexas A&M University
Outline• Introduction to relativistic heavy ion collisions• What have we learned in Au+Au collisions?
– In the “low-pT” regime– In the “intermediate-” and “high-pT” regimes
• What did we see in d+Au collisions?
2Carl Gagliardi – QNP 2004
Lattice QCD at finite temperature
F. Karsch, hep-ph/0103314
Critical energy density: 4)26( CC T
TC ~ 175 MeVC ~ 1 GeV/fm3
Ideal gas (Stefan-
Boltzmann limit)
3Carl Gagliardi – QNP 2004
RHIC: the Relativistic Heavy Ion Collider
4Carl Gagliardi – QNP 2004
Geometry of heavy ion collisions
Number of participants: number of incoming nucleons in the overlap region
Number of binary collisions: number of equivalent inelastic nucleon-nucleon collisions
Both depend on impact parameter or “centrality”
x
z
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Experimental determination of centrality
Paddles/BBCZDC ZDC
Au Au
Paddles/BBC Central
Multiplicity Detectors
Paddle signal (a.u.)
5% Central
STAR
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pT distribution of charged particles
Systematic Errors not shown
nucl-ex/0401006
7Carl Gagliardi – QNP 2004
Bulk particle production in Au+Aud
Nch
/d
19.6 GeV 130 GeV 200 GeV
PRL 91, 052303
CentralPeripheral
• Central collisions at 200 GeV: 5000 charged particles (!)• Energy density: boost invariant hydrodynamics (Bjorken)
d
dNp
Rdy
dE
Rch
TT
2
31122
(R ~ A1/3, = 1 fm/c)
Central Au+Au at 130 GeV: GeV/fm3 (PHENIX, PRL 87, 052301)
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Azimuthal anisotropy: “elliptic flow”
)](2cos[21 2 Rvd
dN
Elliptic term
xy z
Initial coordinate-space anisotropy
py
px
Final momentum-space anisotropy
x
y
p
patan
y 2 x 2
y 2 x 2 22
22
2
yx
yx
pp
ppv
9Carl Gagliardi – QNP 2004
Hydrodynamic fitsKolb and U. Heinz (2002)
Obtain reasonable simultaneous description with the QGP equation of
state predicted by lattice QCD.
Elliptic flowRadial flow
10Carl Gagliardi – QNP 2004
HBT interferometry versus reaction plane
Geometrical analog of v2 – confirms source anisotropy.
Note: Magnitudes of Rl and Ro/Rs smaller than predicted by hydrodynamic calculations.
nucl-ex/0312009STARSTAR
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Chemical equilibrium?
Thermal model: Partition fn with params T, B, s, I3
Fit to ratios of antiparticle/particle: , K, p, , , , K*0,
….
12Carl Gagliardi – QNP 2004
Phase diagram at chemical freeze-out
13Carl Gagliardi – QNP 2004
Hard partonic collisions and energy lossin 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,…
14Carl Gagliardi – QNP 2004
Jets at RHIC
p+p jet+jet (STAR@RHIC)
Au+Au ??? (STAR@RHIC)
nucleon nucleonparton
jet
Find this……….in this
15Carl Gagliardi – QNP 2004
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
16Carl Gagliardi – QNP 2004
Au+Au and p+p: inclusive charged hadronsPRL 91, 172302
17Carl Gagliardi – QNP 2004
Suppression of inclusive hadron yield
• central Au+Au collisions: factor ~4-5 suppression • pT > 5 GeV/c: suppression ~ independent of pT
PRL 91, 172302
Au+Au relative to p+p
Au+Au central/peripheral
RAA RCP
18Carl Gagliardi – QNP 2004
PHENIX observes a similar effect with π0
PRL 91, 072301
Binary scaling
Factor of 4-5
PRL 91, 241803
p-pAu-Au
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So doPHOBOS ( ~ 0.8) and BRAHMS ( ~ 2.2)
PHOBOS: PL B578, 297 and PRL 91, 072302
BRAHMS: PRL 91, 072305Also have = 0
BRAHMS
20Carl Gagliardi – QNP 2004
Azimuthal anisotropy and partonic energy loss
M. Gyulassy, I. Vitev and X.N. Wang
)](2cos[)(21 2 RTpvd
dN
Anisotropy at high pT is sensitive to the gluon density of the medium.
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Anisotropy vs. pT at 200 GeV
Significant anisotropy in Au+Au at least up to 7~8 GeV/c
PreliminarySTARSTAR Preliminary
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Jets and two-particle azimuthal distributionsp+p di-jet • trigger: track with pT>4 GeV/c
• distribution: 2 GeV/c<pT<pTtrigger
• normalize to number of triggers
trigger
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
pedestal and flow subtracted
Phys Rev Lett 90, 082302
?
24Carl Gagliardi – QNP 2004
Other effects that might change RAA and RCP
• Hadronic re-interactions
• Change in particle production mechanisms
• Initial- or final-state multiple scattering (“Cronin effect”)
• Nuclear modifications of the parton distributions (“shadowing” and “anti-shadowing”)
• Gluon saturation at high energy and low x
25Carl Gagliardi – QNP 2004
Final-state “pre-hadronic” plus hadronic rescattering
May also have difficulty explaining high-pT v2 and near-side angular correlations
Cassing, Gallmeister, and Greiner, hep-ph/0311358
26Carl Gagliardi – QNP 2004
Baryons vs. mesons
In central Au+Au collisions, baryons are substantially overproduced relative to mesons at intermediate pT
PHENIX: PRL 91, 172301
27Carl Gagliardi – QNP 2004
Power law:
Exponental:
Hadronization via constituent quark recombination/coalescence
Coalescence of partons into hadrons:
Where does coalescence win over fragmentation?
),()2
,2
;2
,2
()2()2( 3
33
,3
3
3qrq
PrRq
PrRW
rqddRd
Pd
dNMab
ba
M
Assume Wab(1;2) = wa(1)wb(2)
fragmenting parton:ph = z p, z<1
recombining partons:p1+p2=ph
TpTAew /~
DAeDwN TzPT /frag ~
TPTeAwwN /2reco ~
Tpw ~
TPN ~frag
2reco ~
TPN
From R. Fries, QM’04
28Carl Gagliardi – QNP 2004
Constituent quark scaling at intermediate pT
2 2 2 2an22 3
d 3p pM tt
B tt
pv p vv
pv p
At intermediate pT, different baryon and meson v2 values may reflect a common underlying partonic flow.
29Carl Gagliardi – QNP 2004
Limits of recombination/coalescence
PHENIX: PRL 91, 172301STAR: PRL 92, 052302
The “baryon enhancement” ends around pT ~ 5-6 GeV/c
STARSTAR
30Carl Gagliardi – QNP 2004
PRL 91, 172302RCP
Theory vs. datapQCD-I: Wang, nucl-th/0305010pQCD-II: Vitev and Gyulassy, PRL 89, 252301Saturation: KLM, Phys Lett B561, 93
pT>5 GeV/c: well described by gluon saturation model (up to 60% central) and pQCD+jet quenching
Final state
Initial state
31Carl Gagliardi – QNP 2004
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!
32Carl Gagliardi – QNP 2004
d+Au vs. p+p: Theoretical expectations
If Au+Au suppression is initial state(KLM gluon saturation: 0.75)
1.1-1.5
pT
RA
B
1
Inclusive spectra
If Au+Au suppression is final state
~2-4 GeV/c
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?
33Carl Gagliardi – QNP 2004
d+Au inclusive yields relative to binary-scaled p+p
ddpdT
ddpdNR
Tpp
AB
TAB
AB /
/
Suppression of the inclusive yield in central Au+Au is a final-state effect
STAR: PRL 91, 072304
• d+Au : enhancement
• Au+Au: strong suppression
STARSTAR
34Carl Gagliardi – QNP 2004
PHENIX, PHOBOS, BRAHMS find similar results
PRL 91, 072302/3/5
PHENIX
PHOBOS
BRAHMS
35Carl Gagliardi – QNP 2004
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
PRL 91, 072304
36Carl Gagliardi – QNP 2004
The strong suppression of the inclusive yield and back-to-back correlations at high pT observed in central Au+Au collisions are due to final-state interactions with the dense medium generated in such collisions.
37Carl Gagliardi – QNP 2004
Phys Rev Lett 91,072302/3/4/5
38Carl Gagliardi – QNP 2004
1 + ( pQCD direct x Ncoll) / ( phenix pp backgrd x Ncoll)
1 + ( pQCD direct x Ncoll) / phenix backgrd Vogelsang NLO
1 + ( pQCD direct x Ncoll) / phenix backgrd Vogelsang, mscale = 0.5, 2.0
AuAu 200 GeV Central 0-10%
Direct photons
Direct photons aren’t suppressed in central Au+Au collisions. Another demonstration that suppression is a final-state effect
preliminary
39Carl Gagliardi – QNP 2004
Back-to-back correlations vs. reaction plane
STARSTAR preliminary20-60% central
Near-side correlations: independent of orientation
Back-to-back correlations: suppressed more strongly when the path length is longer
40Carl Gagliardi – QNP 2004
Mid-rapidity vs. forward rapidity
s
pxx T
T
2~
Mid-rapidity spectra tend to sample quarks and gluons in the incident nuclei at:
In contrast, forward spectra tend to sample quarks at large x and gluons around:
yT exx ~
41Carl Gagliardi – QNP 2004
Statistical errors dominate over systematic at =2 and 3
Systematic error (not shown) ~15%
RdAu at different pseudo-rapidities
Possible evidence that gluon saturation isimportant for forward physics at RHIC
nucl-ex/0403005and preliminary
BRAHMS
42Carl Gagliardi – QNP 2004
Have we found the Quark Gluon Plasma at RHIC? We now know that Au+Au collisions generate a medium that:
• is dense (pQCD theory: up to ~100 times cold nuclear matter)
• is dissipative
• exhibits strong collective behavior
There is still work to do
We have yet to show:
• hydrodynamics can describe spectra, v2, and HBT simultaneously
• direct evidence the system is thermalized
• chiral restoration
• a transition occurs: can we turn the effects off ?
This represents significant progress in our understanding of QCD matter
43Carl Gagliardi – QNP 2004
44Carl Gagliardi – QNP 2004
Kinematics for colliders
2tanln y
||
||21 ln
pE
pEyRapidity:
TT dydpp
d
pd
dE
2
2
3
3
Invariant cross section:
Pseudo-rapidity: for m/p << 1
Transverse momentum (pT) and (pseudo-)rapidity (y or )provide a convenient description
45Carl Gagliardi – QNP 2004
Why is elliptic flow interesting?
Free-streaming does not convert spatial
anisotropy to momentum anisotropy
Developed early(t < 2 fm/c)
Zhang, Gyulassy, Ko, PL B455 (1999) 45
Sensitive to the physics of constituent interactions at early time
46Carl Gagliardi – QNP 2004
Particle spectra and radial flow
Fit Au+Au spectra to blast wave model:Fit Au+Au spectra to blast wave model:• S S (surface velocity) drops with dN/d(surface velocity) drops with dN/d • T (temperature in GeV) almost constant.T (temperature in GeV) almost constant.
Blue y=0 Red y=1
T
pT (GeV/c)
1 and 3 contours
y= 0
An explosive expansion?
BRAHMS preliminary
47Carl Gagliardi – QNP 2004
Suppression of inclusive yield at 130 GeV
PHENIX, PRL 88, 022301STAR, PRL 89, 202301
Both STAR and PHENIX see significant suppression
Limitation: Ambiguities in the reference spectra at 130 GeV
00 2 4
pT (GeV/c)
1
2
RA
A
48Carl Gagliardi – QNP 2004
RAA different for charged hadrons and 0 ?
STAR PRL 91, 172302PHENIX PRL 91, 072301
For pT less than ~5 GeV/c, charged hadrons are less suppressed in central Au+Au collisions than 0
Charged Hadrons 0
49Carl Gagliardi – QNP 2004
Baryons vs. mesons – yet another view
K* (and maybe ) follow K0S -- Ξ (and maybe Ω) follow Λ
50Carl Gagliardi – QNP 2004
Final state “pre-hadronic” and hadronic scattering
• Calculations based on the idea of “color transparency”
• Hard-scattered parton neutralizes its color, then propagates as a color-neutral object
• The object grows into a well-defined hadron during a “formation time” tf. The interaction cross section grows over the same period.
tf
Calculations use:
51Carl Gagliardi – QNP 2004
Modified gluon density: “Gluon saturation”
MidRapidity
ForwardRapidity CTEQ6M
Saturation sets in when gluons start to overlap
yT es
px 2
~
52Carl Gagliardi – QNP 2004
Relative charge dependenceStrong dynamical charge correlations in jet fragmentation
Compare ++ and -- charged azimuthal correlations to +- azimuthal correlations
STAR PRL 90, 0823020-10% most central Au+Au
p+p minimum bias4<pT(trig)<6 GeV/c
2<pT(assoc.)<pT(trig)
System (+-)/(++ & --)
p+p 2.7+-0.6
0-10% Au+Au 2.4+-0.6
Jetset 2.6+-0.7
Same correlated particle production for pT>4 GeV/cin pp and central Au+Au
Au+Au
p+p
53Carl Gagliardi – QNP 2004
High pT correlations: Au+Au vs p+pPRL 90, 082302 (2003)
Central Au + Au
Peripheral Au + Au
Back-to-back jets are suppressed in central collisions
22 2 2( ) ( ) (1 cos(2 ))D Au Au D p p B v
near side
away side
peripheral central
54Carl Gagliardi – QNP 2004
Collision timescale
0 5 10 15
Evolution time (fm/c)
Hydrodynamical model
Blast-wave fit
Rlong Sinyukov fit
ceq
HBT()
Buda-Lund
55Carl Gagliardi – QNP 2004
Baryon production in d+Au
nucl-ex/0309012 STARSTAR
Central Au+Au
d+Au
p+p
Baryon enhancement in central Au+Au is final state effect