Jets in Nuclear Collisions

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Jets in Nuclear Collisions

• Why jets in nuclear collisions?
• How do we find jets in nuclear collisions?
• Is hard scattering different in nuclear
  collisions than in ee- or pp collisions?
• What happens in the nuclear medium?
• Is jet transport fragmentation changed?
• What do we still want to know?

Barbara Jacak Stony Brook University June 29, 2004
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Why collide nuclei at ?s200 GeV/A?

• high energy nuclear collisions should create
  quark gluon plasma

Attractive potential Confinement at large distance
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how to probe the plasma?
System expands cools
e, pressure builds up
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Hard quarks gluons ? jets

• Hard scattering happens early
• affected by initial state nucleus
• Hard partons propagate
• fast quarks, gluons traverse the interesting
  stuff
• radiate gluons
• interact with QGP partons
• Fragmentation is last step
• - outside the medium

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Plasma physics of the quark gluon plasma?

• Want to know
• pressure, viscosity, energy gradients, equation
  of state,
• thermalization time extent
• determine from collective behavior
• Other plasma parameters
• radiation rate, collision frequency,
  conductivity, opacity, Debye screening length?
• what is interaction s of q,g in the medum?
• need short wavelength strongly interacting
  probe
• high momentum q,g provide just this!

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What is the effect of the medium?
before they create jets, the scattered quarks
radiate energy ( GeV/fm) in the colored medium
? decreases their momentum ? fewer high momentum
particles ? beam ? jet quenching
Approach calculate jet rate, test in pp, compare
jets in AA to pp
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QCD and EM Radiation
EM Radiation by scattering Interference between
initial and final state radiation
EM
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Energy Loss in Dense QCD Matter
Ivan Vitev, ISU
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Energy loss expected

• Since QCD is non-abelian, even 1 scattering in
  final state is sufficient to generate energy loss
• Remember that radiated gluon couples to medium!
• formation length of max E gluon lF 2E/m2 (
  m pT kick )
• DE E x L/l x L m2/2E m2 L 2 /2l

So
?
?
? formation time of radiated gluon (? gluon
interaction probability)
standard radiation with no interference
In normal, cold nuclei dE/dx 0.5
GeV/fm Prediction for RHIC ? 10x DE of cold
nuclei
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A closer look at the calculation
M.Gyulassy, P.Levai, I.V., Nucl.Phys.B594,
(2001) Phys.Rev.Lett.85, (2000)

• Radiative energy loss

• Significantly larger than the elastic
• for static nuclear matter

• Can be related to the density of
• gluons/quarks in the system or T

• Takes into account geometry, the
• small number of scatterings, finite
• kinematics

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How do we find jets in nuclear collisions?
In pp can look for hadrons in the characteristic
cone pattern
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3 Ways to Skin a Cat Jet

• Single Particle Spectra
• High pT dominantly from jets
• d?/dpT ? RAA, RdA
• nuclear modification factor

Trigger ? 0
2) 2-Particle Correlations dN/d(??)
Adler et al., PRL90082302 (2003), STAR
3) Jet Reconstruction d?/dET, Fragmentation
function
away-side
Nice work if you can get it!
near-side
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Jet physics in AuAu

• Trigger
• hadron with pT gt 2.5 GeV/c
• Biased, low energy, high z jets!
• Df of associated partners
• Count associated lower pT particles for each
  trigger
• conditional yield
• Near side yield number of jet associated
  particles from same jet in specified pT bin
• Away side yield jet fragments from opposing jet

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Subtract the underlying event
includes ALL triggers (even those with no
associated particles in the event)
combinatorial background large in AuAu
CARTOON
dN
1
flowjet
Ntrig d??
flow
jet
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Benchmark calculation of probe rate on a simple
system pp collisions
p0 rates dN/dpT2dy1dy2 ? dxa dxb dzc dzd
fa/N(xa,Q2) . fb/N(xb,Q2) . Dh1(zc,Q2) .
Dh2(zd,Q2) . dsab /dQ2dy
p0
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Now check that it works in AuAu

• Not so easy cannot use anything that should be
  affected by the medium!
• Try QCD direct photons

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pQCD in AuAu? direct photons
Probe calculation works!
AuAu 200 GeV/A 10 most central collisions
Preliminary
pT (GeV/c)
????measured / ????background
?measured/?background
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Is the message in the medium?

• Is there jet quenching as predicted from energy
  loss?
• ? count high pT particles (AA vs. pp)
• ? look at back-to-back jets
• How much energy do fast partons lose?
• What does it tell us about the medium?
• Where does the lost energy go?
• What does the presence of q and q in the QGP do
  to jet fragmentation?

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Technique to search for jet quenching

• Compare to baseline nucleon-nucleon collisions
  at the
• same energy
• To 0th order Au Au collisions start with
  collisions of quarks gluons in the individual
  N-N reactions
• ( effects of
• nuclear binding and
• collective excitations)
• Hard scattering (p transfer gt few GeV) processes
  scale as the number of N-N binary collisions
  ltNbinarygt
• so for pTgt 2 GeV/c expect YieldA-A YieldN-N .
  ltNbinarygt

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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
ltNbinarygt/sinelpp
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Au-Au ?s 200 GeV high pT suppressed!
PRL91, 072301(2003)
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look for the jet on the other side
STAR PRL 90, 082302 (2003)
Peripheral Au Au
Central Au Au
Trigger 4-6 GeV/c pT
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Path Length Dependence
Background Subtracted See J. Bielcikova et al.,
(nucl-ex/0311007) for background derivation
di-hadron, 20-60 Central
STAR Preliminary
Out-of-plane
In-plane
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Suppression a final state effect?
Hadron gas

• Hadronic absorption of fragments
• Gallmeister, et al. PRC67,044905(2003)
• Fragments formed inside hadronic medium
• Hadron source is soft, after all
• Recombination of flowing partons
• Fries, Muller, Nonaka, Bass nucl-th/0301078
• Lin Ko, PRL89,202302(2002), Hwa, et al.
• Energy loss of partons in dense matter
• Gyulassy, Wang, Vitev, Baier, Wiedemann

But absent in dAu collisions! ? dAu is the
control experiment
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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!)

Levin, Ryshkin, Mueller, Qiu, Kharzeev,
McLerran, Venugopalan, Balitsky, Kovchegov,
Kovner, Iancu
RdAu 0.5
D.Kharzeev et al., hep-ph/0210033
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Experiments show NO suppression in dAu!
PHENIX Preliminary p0
STAR Preliminary
PHOBOS Preliminary
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Centrality Dependence
Au Au Experiment
d Au Control
PHENIX preliminary

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

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Are back-to-back jets there in dAu?
Yes!
So this is the right picture for AuAu
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Property probed density

• Agreement with data
• Vitev, Gyulassy, Wang, others say dE/dx 7.5
  GeV/fm
• get dAu right too!
• initial gluon density
• dNg/dy 1100e 15 GeV/fm3
• hydro initial state same e
• 5-10 x ecritical

dAu
d-Au
Au-Au
NB Lowest energy radiation sensitive to infrared
cutoff.
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Recap

• Hard partons are excellent probes of QGP
• Can calculate their production rate with pQCD in
  AuAu
• (surprisingly) Can do jet physics in heavy ion
  collision
• See jet quenching in single particles
  back-to-back correlations
• Infer
• dE/dx 7.5 GeV/fm
• dNg/dy 1100
• e 15 GeV/fm3

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Turn to the fragmentation function
Standard picture If true fragmentation
independent of medium Baryon/meson at high pT
same in AuAu and pp
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Formation time of fragmentation hadrons

• Uncertainty principle relates hadron formation
  time to hadron size, Rh and mass, mh
• In laboratory frame tf Rh (Eh /mh)
• consider 2.5 GeV pT hadrons
• tf 9-18 fm/c for pions Rh0.5-1 fm
• tf 2.7 fm/c for baryons (Rh1 fm)
• Alternatively, consider color singlet dipoles
  from combination of q q from gluon splitting
• Using gluon formation time, can estimate
• tf 2Eh (1-z)/(kT2mh2)
• for z 0.6-0.8 and kT LQCD (tf baryons)
  1-2 fm/c

R(Au nucleus) 7 fm ? Baryon formation is NOT
outside the medium!
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We observe a puzzle
h/p0 ratio shows that p is enhanced only lt 5
GeV/c
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Are extras from the (soft) underlying event?
Hydro. expansion at low pT jet quenching at
high pT. Coalesce (recombine) boosted
quarks ? hadrons enhances mid pT hadrons
baryons especially
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Phase space filled with partonscoalesce into
hadrons
Use lowest Fock state, i.e. valence quarks

• ReCo of hadrons convolution of Wigner functions
• Where does ReCo win?

Wab(12) wa(1)wb(2)
fragmenting parton ph z p, zlt1
recombining partons p1p2ph
R. Fries
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Coalescence Model results
Fries et al Phys.Rev. C68 (2003) 044902
Greco, Ko, Levai PRC 68 (2003)034904

• particle ratios and spectra OK
• intermediate pT hadrons from coalescence of
  flowing partons NOT from jets, so no jet-like
  associated particles

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But baryons show jet-like properties too
Baryons at 2-4 GeV/c pT scale with Ncoll !
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So baryons seem jet-like!
Rcp

• baryons antibaryons not suppressed!?
• parton DE depends upon what fragmentation WILL
  be???
• baryon excess due to fragmentation function
  modification?
• Step 1 determine if baryons are from jets
• do we see hadronic partners from the same jet?
• Step 2 calculate effect of q,q in surrounding
  medium upon (soft part of) fragmentation function

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Step 1 use 2 particle correlations
Select particles with pT 2.5-4.0GeV/c Identify
them as mesons or baryons via Time-of-flight Find
second particle with pT 1.7-2.5GeV/c Plot
distribution of the pair opening angles
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Jets in PHENIX

• Large multiplicity of charged particles
• --solution find jets in a statistical manner
• using angular correlations of particles
• mixed events give combinatorial background
• 2 x 90 degree acceptance in phi and ?lt0.35
• --solution correct for azimuthal acceptance,
• but not for ? acceptance
• Elliptic flow correlations
• --solutions
• use published strength values
• and subtract
• (could integrate over 90
• to integrate all even
• harmonics to zero)

PHENIX PRL 91 (2003) 182301
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Subtracting combinatorial background
includes ALL triggers (even those with no
associated particles in the event)
CARTOON
dN
1
flowjet
Ntrig d??
flow
Associated particles from the underlying event.
Collective flow causes another correlation in
them
jet
associated particles with non-flow angular
correlations -gt jets! Treat as 2 Gaussians
B(12v2(pTtrig)v2(pTassoc)cos(2??))
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Identify Trigger Source of intermediate pT
baryons?

• jet partner equally likely for trigger baryons
  mesons
• Same side only slight decrease with centrality
• Away side partner rate as in pp confirms jet
  source of baryons!
• See disappearance of away-side jet for both
  baryons and mesons

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partners expected from recombination

• Yield of partners per trigger expected from
  recombination of purely thermal (soft)
  constituent quarks
• (dilutes jets)

pions
only soft protons
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pT spectra of same jet associated particles
Spectra in lab, rather than jet, frame Allows to
compare with inclusive spectra
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Compare slope to inclusive hadron spectra
Generally higher
Perhaps thermalized in most central
collisions? Calculations (step 2) desperately
needed!
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Conclusion about fragmentation function

• Its modified in the medium!
• AuAu jets richer in soft hadrons than pp or
  dAu
• AuAu jets baryon yield increases with medium
  volume
• Maybe some evidence that jet fragments are
  beginning to thermalize in the medium

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What do we still want to know?

• Quantitative information on medium modification
  of jet fragmentation
• Where does the energy radiated by fast partons
  go?
• Many soft gluons no (per observed multiplicity)
• A few semi-hard gluons? could be
• How is the lost energy propagated in the medium?
• Infer energy, color transport properties of QGP
• basic plasma physics!
• Is the lost energy thermalized in the medium?

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values
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kT, jT at RHIC from pp Data
Statistical Errors Only
di-hadron
J. Rak, Wed.
J. Rak, DNP03
PHENIX preliminary
Df
near-side
away-side
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Moment Analysis of QCD Matter
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Collective effects? Pressure 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
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v2 reproduced by hydrodynamics

• see large pressure buildup
• anisotropy ? happens fast
• ? early equilibration !

STAR
PRL 86 (2001) 402
central?
Hydrodynamics assumes early equilibration Initial
energy density is input Equation of state from
lattice QCD Solve equations of motion
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But at forward rapidity reach smaller x
y 3.2 in deuteron direction ? x ? 10-3 in Au
nucleus Strong shadowing, maybe even
saturation?
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Pions in 3 detectors in PHENIX

• Charged pions from TOF
• Neutral pions from EMCAL
• Charged pions from RICHEMCAL

Cronin effect gone at pT 8 GeV/c
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Centrality dependence of Cronin effect

• Probe response of cold nuclear matter with
  increased number of collisions.
• See larger Cronin effect for baryons than for
  mesons (as at Fermilab)

Qualitative agreement with model by Accardi and
Gyulassy. Partonic
Glauber-Eikonal approach
sequential multiple partonic collisions.
nucl-th/0308029
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Does Cronin enhancement saturate?

• A different approach
• Intrinsic momentum broadening in the excited
  projectile proton
• hpA average number of collisions

X.N.Wang, Phys.Rev.C 61 (2000) no upper
limit. Zhang, Fai, Papp, Barnafoldi Levai,
Phys.Rev.C 65 (2002) n4 due to proton

d

dissociation.