Andr Mischke

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André Mischke for the STAR Collaboration
Quark Matter ? Jaipur ? February 4-10, 2008
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Outline

• Motivation
• Correlation technique
• Gluon splitting process at RHIC
• - comparison to MC_at_NLO simulations
• Data analysis in pp collisions
• - relative B?e contribution
• Summary

Quark Matter 2008, Jaipur, 04/10/08
Andre Mischke (UU)
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Motivation

• Study dynamical properties of the QGP e.g.,
  initial gluon density, drag coefficient
• Heavy quarks
• - primarily produced in initial state of the
  collision ? well calibrated probes
• - expected to lose less energy due to suppression
  of small angle gluon radiation (so-called
  dead-cone effect)
• Surprise in central AuAu Electrons from D and
  B decays are strongly suppressed
• Models implying D and B energy loss are
  inconclusive yet
• Goals
• Access to underlying production mechanisms
• Separate D and B contribution experimentally

D/B crossing point
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Correlation technique

• Identification and separation of charm and bottom
  production events using their decay topology and
  azimuthal angular correlation of their decay
  products
• electrons from D/B decays are used to trigger on
  charm/bottom quark pairs
• associate D0 mesons are reconstructed via their
  hadronic decay channel (probe)

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Electron tagged correlationsbottom production
Charge-sign requirement on trigger electron and
decay Kaon gives additional constraint on
production process
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LO PYTHIA simulations

• Near-side
• B decays (dominant)
• Away-side
• charm flavor creation (dominant)
• small B contribution

• Away-side
• B decays (dominant)
• small charm contribution

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NLO Process Gluon splitting

• FONLL/NLO calculations only give single
  inclusive distribution ? no correlations
• PYTHIA is not really adequate for NLO
  predictions
• STAR measurement of D in jet ? access to charm
  content in jets
• Gluon splitting rate consistent with pQCD
  calculation
• ? Small contribution from gluon splitting at top
  RHIC energy

Mueller Nason PLB 157, 226 (1985)
P29 X. Dong, Charm Content in Jets
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MC_at_NLO predictions for charm production

• NLO QCD computations with a realistic parton
  shower model
• Away-side peak shape remarkable agreement
  between LO PYTHIA and MC_at_NLO
• Near-side GS/FC ? 5? small gluon splitting
  contribution ? in agreement with STAR measurement

• S. Frixione, B.R. Webber, JHEP 0206 (2002) 029
• S. Frixione, P. Nason, and B.R. Webber, JHEP
  0308 (2003) 007
• private code version for charm production

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STAR experiment
Solenoidal Tracker at RHIC Large acceptance
magnetic spectrometer
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Electron identification

• Quality cuts
• well developed shower in EM calorimeter
• 0. lt p/Etower lt 2.
• 3.5 lt dE/dx lt 5.0 keV/cm (pT dependent)
• High electron purity up to high-pT
• ? Clean electron sample

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Photonic electron background

• Most of the electrons in the final state are
  originating from other sources than heavy-flavor
  decays
• Dominant photonic contribution
• gamma conversions
• ?0 and ? Dalitz decays
• Exclude electrons with low invariant mass minv lt
  150 MeV/c2
• ? Non-photonic electron excess at high-pT
• Photonic background rejection efficiency is ?70

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(K?) invariant mass distribution
?D0D0

• D0 reconstruction dE/dx cut (3?) around Kaon
  band

Quark Matter 2008, Jaipur, 04/10/08
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Azimuthal correlation of non-photonic electrons
and D0 mesons
expected D0/e ratio for charm flavor creationand
a track reconstruction efficiency of 80-90

• First heavy-flavor correlation measurement at
  RHIC
• Near- and away-side correlation peak ? yields
  are about the same

Quark Matter 2008, Jaipur, 04/10/08
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Relative B?e contribution
like-sign e-K pairs

• Model uncertainties not included yet
• Good agreement between different analyses
  P215 S. Sakai, e-h correlation in pp
• Data consistent with FONLL within errors

Comparable D and B contributions ? AuAu
suggests significant suppression of non-photonic
electrons from bottom in medium
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Summary and conclusions

• First heavy-flavor particle correlation
  measurement in pp collisions at RHIC
• MC_at_NLO simulations Small gluon-splitting
  contribution
• Azimuthal correlation of non-photonic electrons
  and D0 mesons
• access to production mechanisms
• ? allows separation of charm and bottom
  production events
• efficient trigger on heavy-quark production
  events
• significant suppression of the combinatorial
  background in D0 reconstruction
• Contribution to non-photonic electrons from
  bottom is significant (?50 at pT 3-7 GeV/c)
• Correlation technique is a powerful tool for
  comprehensive energy-loss measurements of
  heavy-quarks in heavy-ion collisions (e.g. IAA)

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Backup slides
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D0D- cross section measurement at the Tevatron
D0 or D
??
D-
B. Reisert et al., Beauty 2006, Nucl. Phys. B
(Proc. Suppl.) 170, 243 (2007)

• Within errors near- and away-side yields are the
  same ? gluon splitting as important as flavor
  creation
• Near-side yield PYTHIA underestimates gluon
  splitting

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PYTHIA event generator

• Parameter settings
• version 6.222 (Jan. 2004)
• MSEL 4 or 5 charm or bottom
• PMAS(4,1) 1.3 or 4.5 mc or mb
• PARP(91) 1.5 ltkTgt
• PARP(31) 3.5 k factor
• MSTP(33) 1 common k factor
• MSTP (32) 4 Q2 scale
• MSTP(51) 7 CTEQ5L PDF
• PARJ(13) 0.594 D/D spin factor
• CKIN(3) 1
• PARP(67) 4 ISRFSR
• MSUB(81) MSUB(82) sub-processes
• MSUB(84) 1

• kt ordering in shower
• String hadronization
• default FF (Peterson)
• B mixing included

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MC_at_NLO event generator

• Version 3.3 (Dec. 2006)
• CTEQ6M PDF
• HERWIG event generator (version 6.510, Oct.
  2005)
• - parton showering
• - hadronization
• - particle decays
• Parameter settings
• mc 1.55 GeV/c2
• mb 4.95 GeV/c2

• HERWIG
• Angular-ordered shower
• Cluster hadronization

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Model comparison
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D0 yield versus ??(e, hadron pair)

• Calculate ?? between non-photonic electron
  trigger and hadron pair pT
• Extract D0 yield from invariant mass
  distribution for different ?? bins

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D in jet measurement
Run V pp, jet patch triggered data1.7 M jets gt
8 GeV/c

• Magnitude at high z region is suppressed due to
  trigger, and it is consistent with MC simulation
  for only direct flavor creation process
• Excess at low z region is expected to be from
  gluon splitting process

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