Jet Quenching at RHIC

1
Jet Quenching at RHIC

• Carl Gagliardi
• Texas AM University
• Outline
• Introduction to relativistic heavy ion
  collisions
• What have we learned in AuAu collisions?
• In the soft regime
• In the high-pT regime
• What did we see in dAu collisions?

2
Normal nuclear matter
Nucleons -- made of quarks and gluons -- are
close together, but only overlap occasionally
3
High density matter
Expect individual hadrons to loose their identity
at sufficiently high density. Quarks and gluons
become deconfined.
4
QCD phase diagram
e.g. two massless flavors (Rajagopal and Wilczek,
hep-ph/-0011333)
T gtgt LQCD weak coupling ? deconfined phase
(Quark-gluon plasma) T ltlt LQCD strong coupling ?
confinement ? phase transition at T LQCD ?
5
Lattice QCD at finite temperature
Ideal gas (Stefan-Boltzmann limit)
F. Karsch, hep-ph/0103314
Critical energy density
TC 175 MeV ? eC 1 GeV/fm3
6
RHIC the Relativistic Heavy Ion Collider
Built to search for and study the QGP
7
Two large detectors
STAR Solenoidal field, large-? tracking TPCs,
Si-vertex tracking TOF, large EM Cal 450
participants
PHENIX Axial field, high resolution rates 2
central arms, 2 forward muon arms TEC, RICH, EM
Cal, Si, TOF, ?-ID 500 participants
Coils
Magnet
E-M Calorimeter
Projection           Chamber
Time of    Flight
8
Two small detectors
BRAHMS 2 conventional spectrometers full phase
space coverage Magnets, TPCs, TOF, RICH 40
participants
PHOBOS Table-top 2-arm spectrometer full phase
space multiplicity measurement Magnet, Si
m-strips, Si multiplicity rings, TOF 80
participants
9
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
10
Experimental determination of centrality
11
Kinematics for colliders
Pseudo-rapidity
for m/p ltlt 1
Transverse momentum (pT) and (pseudo-)rapidity (y
or ?) provide a convenient description
12
pT distribution of charged particles
nucl-ex/0401006
Systematic Errors not shown
13
Bulk particle production in AuAu
PRL 91, 052303
130 GeV
200 GeV
19.6 GeV
Central
dNch/dh
Peripheral
h

• Central collisions at 200 GeV 5000 charged
  particles (!)
• mid-rapidity boost invariance
• Energy density boost invariant hydrodynamics
  (Bjorken)

(R A1/3, t 1 fm/c)
Central AuAu at 130 GeV e 4.6 GeV/fm3
(PHENIX, PRL 87, 052301)
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Particle spectra and radial flow

• Fit AuAu spectra to blast wave model
• ?S (surface velocity) drops with dN/d?
• T (temperature in GeV) almost constant.

Blue y0 Red y1
y 0
T
1 and 3 ? contours
preliminary
An explosive expansion?
pT (GeV/c)
15
Azimuthal anisotropy elliptic flow
py
px
Initial coordinate-space anisotropy
Final momentum-space anisotropy
Elliptic term
16
Hydrodynamic fits
Kolb and U. Heinz (2002)
Work well if you assume the QGP equation of
state predicted by lattice QCD
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Chemical equilibrium?
Thermal model Partition fn with params T, mB,
ms, mI3 Fit to ratios of antiparticle/particle
?, K, p, ?, ?, ?, K0, .
18
Phase diagram at chemical freeze-out
19
Doctors look inside their patients with X-rays
Grants Atlas of Anatomy, 7th edition, 1978.
Reproduced without permission.
20
Hard partonic collisions and energy lossin dense
matter
Bjorken, Baier, Dokshitzer, Mueller, Pegne,
Schiff, Gyulassy, Levai, Vitev, Zhakarov, Wang,
Wang, Salgado, Wiedemann,
Multiple soft interactions
Gluon bremsstrahlung
Opacity expansion
Strong dependence of energy loss on gluon density
?glue measure DE ? color charge density at
early hot, dense phase
21
Jets at RHIC
Find this.in this
pp ?jetjet (STAR_at_RHIC)
AuAu ? ??? (STAR_at_RHIC)
22
Partonic energy loss via leading hadrons
Energy loss ? softening of fragmentation ?
suppression of leading hadron yield
23
Initial-state multiple scattering Cronin
effect
Partons can undergo soft scatters prior to the
hard collision ? spreads the spectrum out in pT
24
Modified parton distributions Shadowing
Parton distributions in nuclei are different from
those in nucleons (e.g., EMC effect) ?
shadowing and anti-shadowing
1.3
R
1.0
0.7

NPA 713, 167
25
Modified gluon density Gluon saturation
Low Energy
x 2 pT / ?sNN
High Energy
CTEQ6M
Saturation sets in when gluons start to overlap
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Suppression of inclusive yield at 130 GeV
STAR, PRL 89, 202301
PHENIX, PRL 88, 022301

Both STAR and PHENIX see significant
suppression Limitation Ambiguities in the
reference spectra at 130 GeV
27
AuAu and pp inclusive charged hadrons
PRL 89, 202301
PRL 91, 172302
pp reference spectrum measured at RHIC
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Suppression of inclusive hadron yield
AuAu relative to pp
RAA
AuAu central/peripheral
RCP
PRL 91, 172302

• central AuAu collisions factor 4-5
  suppression
• pT gt 5 GeV/c suppression independent of pT

29
PHENIX observes a similar effect
PRL 91, 241803
PRL 91, 072301
p-p
Au-Au
30
So doPHOBOS (h 0.8) and BRAHMS (h 2.2)
PHOBOS PL B578, 297 and PRL 91, 072302
BRAHMS PRL 91, 072305 Also have h 0
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Azimuthal anisotropy and partonic energy loss
Anisotropy at high pT is also sensitive to the
gluon density of the medium.
32
Anisotropy vs. pT at 200 GeV
Preliminary
Preliminary
Significant anisotropy in AuAu at least up to
78 GeV/c
33
Jets and two-particle azimuthal distributions
pp ? di-jet

• trigger track with pTgt4 GeV/c
• Df distribution 2 GeV/cltpTltpTtrigger
• normalize to number of triggers

Phys Rev Lett 90, 082302
34
Azimuthal distributions in AuAu
AuAu peripheral
AuAu central
pedestal and flow subtracted
Phys Rev Lett 90, 082302
Near-side peripheral and central AuAu similar
to pp
Strong suppression of back-to-back correlations
in central AuAu
35
Other effects that might change RAA and RCP

• 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
• Hadronic re-interactions
• Baryon-meson differences

36
Theory vs. data
pQCD-I Wang, nucl-th/0305010 pQCD-II Vitev and
Gyulassy, PRL 89, 252301 Saturation KLM, Phys
Lett B561, 93
PRL 91, 172302
RCP
Final state
Initial state
pTgt5 GeV/c well described by gluon saturation
model (up to 60 central) and pQCDjet quenching
37
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
38
Final-state pre-hadronic plus hadronic
rescattering
Cassing, Gallmeister, and Greiner, hep-ph/0311358
May also have difficulty explaining high-pT
elliptic flow and near-side angular correlations
39
RAA different for charged hadrons and p0 ?
Charged Hadrons
p0
PHENIX PRL 91, 072301
STAR PRL 91, 172302
For pT less than 5 GeV/c, charged hadrons are
less suppressed in central AuAu collisions than
p0
40
Baryons vs. mesons
STAR PRL 92, 052302
PHENIX PRL 91, 172301
Significant proton-pion and lambda-kaon
differences, but the baryon enhancement ends by
pT 5-6 GeV/c
41
Is suppression an initial or final state effect?
Initial state?
Final state?
gluon saturation
How to discriminate? Turn off final state ? dAu
collisions!
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dAu vs. pp Theoretical expectations
43
dAu inclusive yields relative to binary-scaled
pp
STAR PRL 91, 072304

• dAu enhancement
• AuAu strong suppression

Suppression of the inclusive yield in central
AuAu is a final-state effect
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PHENIX, PHOBOS, BRAHMS find similar results
PHOBOS
PHENIX

BRAHMS
PRL 91, 072302/3/5
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Azimuthal distributions
PRL 91, 072304
pedestal and flow subtracted
Near-side pp, dAu, AuAu similar Back-to-back
AuAu strongly suppressed relative to pp and dAu
Suppression of the back-to-back correlation in
central AuAu is a final-state effect
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The strong suppression of the inclusive yield and
back-to-back correlations at high pT observed in
central AuAu collisions are due to final-state
interactions with the dense medium generated in
such collisions.
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Phys Rev Lett 91, 072302/3/4/5
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Whats next Can we turn the suppression off?
X.-N. Wang, PL B579, 299
Extrapolation down from the RHIC undershoots the
SPS measurements by a factor of 3. Just
completed a brief run at 62 GeV.
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Have we found the Quark Gluon Plasma at RHIC?

• We now know that AuAu collisions generate a
  medium that
• is dense (pQCD theory up to 100 times cold
  nuclear matter)
• is dissipative
• exhibits strong collective behavior

This represents significant progress in our
understanding of QCD matter

• 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
  ?

There is still work to do
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Mid-rapidity vs. forward rapidity
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
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Modified gluon density Gluon saturation
Mid Rapidity
Forward Rapidity
CTEQ6M
Saturation sets in when gluons start to overlap
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RdAu at different pseudo-rapidities
Statistical errors dominate over systematic at
?2 and 3 Systematic error (not shown) 15 The
values for ?0 were published in PRL 91, 072305
Possible evidence that gluon saturation
is important for forward physics at RHIC
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Baryons vs. mesons vs. centrality
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Baryons vs. mesons yet another view
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Quark coalescence at intermediate pT
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Relative charge dependence
STAR PRL 90, 082302 0-10 most central AuAu pp
minimum bias 4ltpT(trig)lt6 GeV/c 2ltpT(assoc.)ltpT(tr
ig)
Strong dynamical charge correlations in jet
fragmentation ? Compare and -- charged
azimuthal correlations to - azimuthal
correlations
AuAu
pp
Same correlated particle production for pTgt4
GeV/c in pp and central AuAu
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High pT correlations AuAu vs pp
PRL 90, 082302 (2003)
Peripheral Au Au
Central Au Au
Back-to-back jets are suppressed in central
collisions
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Away-side distribution and thermalization
Even a mono-jet should produce an away side
excess due to momentum conservation.
Borghini et al. PRC 62, 034902 (2000)
The away-side excess is similar to the shape
expected from statistical momentum conservation.
No punch-through for 6 lt pTtrig lt 10 GeV/c.
60
HBT versus reaction plane
STAR Preliminary

• HBT versus reaction-plane
• geometrical analog of v2
• R(?) reveals anisotropic source
• probe of dynamical evolution

• Strong radial flow
• HBT R(mT) flow-induced x-p correlations
• extensive systematics
• non-identical particle correlations shiftin
  emission points

61
Collision timescale
?c
?eq
Hydrodynamical model
Blast-wave fit
Buda-Lund
Rlong Sinyukov fit
HBT(?)
0
5
10
15
Evolution time (fm/c)
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Back-to-back correlations vs. reaction plane
preliminary 20-60 central
Near-side correlations independent of
orientation Back-to-back correlations
suppressed more strongly when the path length is
longer
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Baryon enhancement in dAu?
nucl-ex/0309012
Central AuAu
dAu
pp

• proton/charged hadron consistent in dAu, pp,
  ee- (gluon jets)
• baryon enhancement in central AuAu is final
  state effect