Modification of the jet properties at the Relativistic Heavy Ion Collider

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Modification of the jet properties at the
Relativistic Heavy Ion Collider

• Jan Rak
• Department of Physics and Astronomy
• Iowa State University

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Exploring the nature of strong interaction in
Heavy Ion collisions

• Relativistic Heavy Ion physics
• many-body Quantum Chromo Dynamics at high
  temperature and density.
• phase transition to Quark Gluon Plasma
  deconfined QCD matter.
• Relevance
• Quark-hadron phase transition in early Universe
• Cores of dense stars
• High density QCD
• High-pT particles as a probes of QCD medium at
  Relativistic Heavy Ion Collider HI in the
  new regime partonic degree of freedom
• Inclusive high-pT particles production jet
  quenching
• Two particles azimuthal correlations - nuclear
  modification of jet properties
• initial partons distribution and intrinsic
  momentum kT
• fragmentation function

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Collider era quantum field theory
Local gage invariance ? two identical theories
Asymptotic freedom
Landaus zero charge problem vacuum
rearrangement ?
Heavy ion coll. - QGP problem vacuum
rearrangement ?
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Deconfinement to Quark-Gluon plasma
Lattice QCD predicts a phase transition to a
quark-gluon plasma where the long range confining
force is screened.
T 150-200 MeV e 0.6-1.8 GeV/fm3
Lect. Notes Phys 583, 209 (2002)
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Partonic jets as a probe of QCD medium
schematic view of jet production

• Particle production _at_RHIC
• dnch/dh h0 670, Ntotal 7500 92 of
  (15,000) all quarks from vacuum !
• Jets _at_RHIC
• produced early ??1fm
• primarily from gluons
• 30-50 of particle production
• Observed via
• fast leading particles
• azimuthal correlations

Scattered partons radiate energy in colored
medium ? suppression of high pt particles jet
quenching
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Nuclear modification factor
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Nuclear Modification of Hadron Spectra
1. Compare AuAu to nucleon-nucleon cross
sections 2. Compare AuAu central/peripheral
Nuclear Modification Factor
nucleon-nucleon cross section
?Nbinary?/sinelpp
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High pT p-p and Au-Au ?0 Results
Au-Au nucl-ex/0304022
p-p hep-ex/0304038
Good agreement with NLO pQCD
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AuAu vs pp _at_ 200 GeV (?0)
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RAA High pT Suppression
Run 2001/2002 Au-Au ?sNN 200 GeV

Au-Au nucl-ex/0304022
Au-Au nucl-ex/0304022
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Suppression an initial state effect?
Color Glass Condensate hep-ph/0210033 Gribov,
Levin, Ryshkin, Mueller, Qiu, Kharzeev, McLerran,
Venugopalan, Balitsky, Kovchegov, Kovner, Iancu
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Initial/final state effects - CGC
Charged hadrons ratios to minimum bias
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pA (or dA) The control experiment
Nucleus- nucleus collision
Proton/deuteron nucleus collision

• Nuclear effects other than a dense medium are
  known to affect hadron spectra (e.g. shadowing,
  Cronin effect) in pA and dA collisions, which
  do not have a created medium.
• Could these initial state effects be causing the
  suppression of high-PT hadrons in AuAu
  collisions?
• If so, then we should see suppression of high-PT
  hadrons in dAu collisions.

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Centrality Dependence
Au Au Experiment
d Au Control
Preliminary Data
Final Data

• Dramatically different and opposite centrality
  evolution of AuAu experiment from dAu control.
• Jet Suppression is clearly a final state effect.

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Hard scattering
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Jet Fragmentation (width of the jet cone)
Partons have to materialize (fragment) in
colorless world
jT and kT are 2D vectors. We measure the mean
value of its projection into the transverse plane
?jTy? and ?kTy? .
?jTy? is an important jet parameter. Its
constant value independent on fragments pT is
characteristic of jet fragmentation
(jT-scaling). ?kTy? (intrinsic NLO radiative
corrections) carries the information on the
parton interaction with QCD medium.
pp
pA
AA
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Fragmentation Function (distribution of parton
momentum among fragments)
jet
In Principle
Fragmentation function
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xE in pp collisions
PHENIX preliminary
1/?xE? ? -4 to 5
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?z? extracted from pp data
xTtrigg2.pTtrigg/?s
We measured xE and
Only one unknown variable ?z? ? iterative
solution
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Inclusive jet fragmentation
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Di-jet fragmentation
Q2 gt Q2
zgtz
zltz
pTassoc
pTq
pTq
pTtrig
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Di-jet mean z
inclusive
conditional
Phase space effect
plateau
Deviation from power law
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Hydrodynamic flow
x ?plane
spatial asymmetry eccentricity
momentum asymmetry elliptic flow - v2
Sensitive to early pressure and dynamics of
initial system
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Method azimuthal correlation function
Now we know the ?z? - let us measure ?N and ?N.
Two particle azimuthal correlation function
Two particle correlations in HI elliptic flow
flow pairs 12v22 cos(2??) Intra-jet
pairs angular width ?N ? ?jTy? Inter-jet
pairs angular width ?A ? ?jTy? ? ?kTy?
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pp and dAu correlation functions
pp h-
3.0ltpTlt6.0
1.0ltpTlt1.5
Fixed correlation both pTtrigg and pTassoc are
in the same range Assorted correlation pTtrigg
and pTassoc different
Away side peak
Near-side peak
5.0ltpTtrigg lt16.0 GeV/c
dAu ???h?
1.5ltpTlt2.0
dAu
Jet function assumed to be Gaussian
1.0ltpTlt1.5
Fit const Gauss(0)Gauss(??)
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?N, ?A , ?jTy?, ?kTy? relations
Knowing ?N and ?A it is straightforward to
extract ?jTy? and ?ztrigg? ?kTy? In the
high-pT limit (pT gtgt?jTy? and pT gtgt?kTy? )
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?N ,?A ??jTy?, ?kTy? in pp data
PHENIX preliminary
PHENIX preliminary ?jTy? 359?11
MeV/c ?kTy? 964?49 MeV/c
Both ?jTy? and ?kTy? in very good agreement
with previous measurements PLB97 (1980)163
PRD 59 (1999) 074007
?A
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From pp to dAu
?kTy? carries the information on the parton
interaction with cold nuclear matter. ?jTy?
should be the same as in pp systematic cross
check
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?kTy? from pp and dAu
No significant kT-broadening seen in dAu data
?z? 0.75 value taken from pp data
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AuAu ?jTy? and ?z? ?kTy? from CF
(2.5?pTtrigg?4.0)?(1.0?pTassoc?2.5)
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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
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STAR jets and away-side quenching
Hint of surface emission ? Nucl-ex/0210033
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AuAu associated yields
(2.5?pTtrigg?4.0)?(1.0?pTassoc?2.5) GeV/c
Note pT is rather low associated particle
yields increase with centrality
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Jet remnants ?
High pT
Low pT
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pT distributions on near and away side
Away side energy from initial parton seems to
be converted to lower pT particles
reminiscent of energy loss predictions
Near side Overall enhancement from pp to AA
Apparent modification of the fragmentation
function ?
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Summary and conclusions

• Inclusive high-pT particles yield in pp, dAu and
  AuAu collisions
• Significant suppression, RAA? 0.2, found in AuAu
  collisions. Cronin like enhancement found in
  dAu charged hadron spectra and ?0. No room for
  initial state gluon saturation.
• Jet production and fragmentation
• Good agreement of the jet properties in pp
  collisions with other experiments
• dAu jT and kT consistent with pp
• In AuAu significant kT - broadening with
  centrality
• Yield of away side associated particles is
  suppressed at pTgt2GeV/c and shows rising trend
  with Npart below 2GeV/c. Remnant of high-pT jets
  - hint of jet-quenching balance ?

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QGP in Heavy Ion collisions ?

• What has been discovered
• existence of strongly coupled QCD medium is
  proven (hints seen at SPS)
• it looks like a ideal fluid, not a gas
• Is it a QGP QCD matter in the asymptotically
  free phase ?
• No. There is no collective manifestation of the
  asymptotic freedom. Is the strongly coupled
  medium still plasma?
• What next ?
• J/Psi suppression/enhancement run04 data
  analysis in progress
• Run LHC!