Aspect(s) of

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Aspect(s) of Jet Production with PHENIX
OR Can we really learn about QCD from heavy-ion
collisions?

• Matthew Nguyen
• Moriond 2009

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PHENIX _at_ RHIC
Colliding AuAu, pp, etc. _at_ a modest
of 200 GeV
PHENIX Central Arms
h lt 0.35
PHENIX specializes in rare probes Photons and
Leptons
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Jet Tomography
The medium A dense, thermalized,
effectively deconfined, strongly coupled Quark
Gluon Plasma formed in high energy nuclear
collisions
Parton beam
QGP Brick
q
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The Hard Probe Paradigm
In practice we have neither parton beams nor QGP
bricks Lifetime, System size order 10 fm Instead
use hard scattered partons
Medium modified FF
The question of energy loss of fast partons
led to the consideration of destructive
interference, the QCD LPM effect, previously
unsolved
A fundamental test of QCD radiation
Path-length through the medium
Scattering strength GeV2/fm
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The Baseline NLO vs. pp Collisions
Pions
Inclusive Direct g
200 GeV
Inclusive Isolated
PHENIX can identify p0 by direct reconstruction
out to gt 20 GeV Measure both inclusive and
isolated direct photon cross sections NLO pQCD
works at RHIC!
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Nuclear Modification
RAA Observed yield over expected yield
Yield in AA / yield in pp scaled by the
number of binary collisions
Head-on Collisions
Direct g
p0
Strong jet quenching, yield of high pT fragments
reduced by factor of 5! To first approx. direct g
are unmodified as expected for color neutral
objects
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Jet Reconstruction in HI Collisions
PHENIX
Reconstructed Jet
Out-of-cone area
Jet Reconstruction in Heavy-Ion Collisions is an
extremely active topic Backgrounds from soft
collisions are non-trivial even at the LHC,
particularly at small z
Out-of-cone area
?
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Two-Particle Correlations

• Essential features of jet production are evident
  via azimuthal correlations between particle
  pairs
• In pp collisions
• Approximate UE as a flat pedestal
• Fit double Gaussian constant to remove
  pedestal may be taken as definition
• In AA collsions
• Two-source model Jet combinatorial background
  from soft collisions
• Estimate background by event-mixing
• Background has its own azimuthal correlations
  due to collective behavior of medium It flows!

Per-Trigger (Conditional) Yield Yield of
Associated Particles Per Trigger
Per-Trigger Yield
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Modified Jet Shapes The Cone
Centrality
Di-hadron Correlations

• At intermediate pT, away-side peaks are displaced
  in central collisions
• Collective phenomenon, e.g., shock wave or
  modification to QCD bremsstrahlung in medium?

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Modified Jet Shapes The Ridge
Jet shape also modified on the near-side Dh
Limited to pT lt 4 GeV, same region as away-side
cone structure PHOBOS Ridge correlation extend
out to Dh gt 4!
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Modified Jet Shapes from Modified pQCD
Polosa and Salgado, PRC75, 041901 (2007)

• Uses standard perturbative methods (Sudakov Form
  Factors) calculates shower evolution
• Introduce modified splitting functions to account
  for multiple scattering in medium
• Large angle scattering is enhanced, reproducing
  conical emission

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Modified Leading Log Approach
CERN-PH-TH-2005-100
Borghini and Wiedemann
MLLA -- Resummation of interference effects in
shower evolution, used to calculate D(z) using
LPHD
z
0.37 0.14 0.05 0.02
0.007 0.002
Very different than fractional energy loss, may
be Q2 dependent Di-hadron measurements introduce
a trigger bias which makes their calculation
difficult. For quantitative comparisons we need
to look to full jet reconstruction or direct
photon correlations where the parton energy is
determined
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Direct g Correlations
200 GeV pp
7 lt pTg lt 9 GeV, 3 lt pTh lt 5 GeV
Inclusive g-h Decay g-h Direct g-h
Inclusive (direct decay) photon correlations
are measured Decay correlations are estimated
from measured p0-hadron and h-hadron
yields Direct g-h hadrons are obtained by
statistical subtraction of subtraction of decay
correlations from inclusive
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Isolated Direct g Distributions
Zhang, Owens, Wang, Wang arXiv0902.4000
Yield of hadrons per isolated, direct g
Relation to the FF
1/Ng dNg-h/dzT
CTEQ NLO KKP
To first order pp baseline well described by
NLO Work being done to quantify scale
uncertainties, sensitivity to kT effect, etc.
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Direct g-h in HI Collisions

• Expectation in HI depends on model
• Surface bias for single hadrons
• Tangential Bias for high pT di-hadrons
• Diffuse medium/few scattering model
• Punch-through
• g-jet surface biased at large zT, but probe
  progressively further into medium as zT decreases

Black Core / Corona vs. Diffuse Medium
Single hadron
di-hadron
g-hadron
ZOWW
ZOWW ? Model of energy loss using effective FFs
arXiv0902.4000 refs therein
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Jet Suppression Opposite Direct g
Ratio of AuAu Yields to pp expectation
Nuclear Modification
No isolation cut applied

• At intermediate zT g-h consistent with p0 RAA ?
  dominated by surface bias
• Just getting started, new higher statistics data
  soon,
• Will help us push to lower values of zT

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Conclusions

• QCD at high density/temperature accessible at
  RHIC
• Qualitatively new features in jet correlations
  attest to strong medium modifications to vacuum
  jet fragmentation
• Modeling such effects is challenging and requires
  better hard probe observables, jet reconstruction
  and direct photon correlations with high
  luminosity data from RHIC and large collision
  energy data from the LHC
• g-h data are now available from RHIC and will
  enable precision studies of jet fragmentation in
  medium as more statistics are accumulated

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Backup Slides
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Away-side head vs shoulder
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Di-hadron IAA
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Two Component Fits
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The Ridge and the Cone
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Jet Hadro-chemsitry
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Near and away ridge vs shoulder
Spectra and yields for ridge and shoulder are
similar, and show same trend with centrality
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pT Dependence of Di-hadron Corrleations
partner pT
Dip develops
Yield suppressed
Yield enhanced
trigger pT
Jet reemerges
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Photon Sources in AuAu
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World direct g data (pp)
Aurenche et al. Phys.Rev. D73 094007 (2006)

• PHENIX bridges the gap between Tevatron and fixed
  target data
• Highest energy data that removes decay background
  by direct reconstruction rather than calorimeter
  response

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Isolated Direct g Cross Section
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Direct g x-sec, Data/Fit
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g-jet in HI Cartoon