Andr

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Heavy-flavor particle correlations in STAR via
electron azimuthal correlations with open charm
mesons
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

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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
M. Cacciari et al., PRL95, 122001 (2005)
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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

E. Norrbin T. Sjostrand, Eur. Phys. J. C17,
137 (2000)
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

• Energy measurement
• - Barrel EMC
• fully installed and operational, ? lt 1
• Pb/scintillator (21 X0)
• dE/E 16/?E
• Shower maximum detector

• PID and tracking
• - TPC
• ? lt 1.5
• ?p/p 2-4
• ?dE/dx/dEdx 8
• - Magnet
• 0.5 Tesla

Dataset Run6 pp at ?s 200 GeV, L
9 pb-1 High ET trigger energy threshold
5.4 GeV
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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

dE/dx (keV/cm)
purity ()
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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

• Significant suppression of combinatorial
  background factor 200
• S/B 14 and signal significance 3.7

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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

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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

essentially from B decays only
?75 from charm ?25 from beauty
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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The STAR collaboration
52 institutes from 12 countries, 582 collaborators
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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