Diapositive 1

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Study of the dimuon mass spectra in Pb-Pb
collisions with the ALICE muon spectrometer

• OUTLINE
• Introduction
• ALICE muon spectrometer
• Muon trigger performance
• Beauty physics with the ALICE muon spectrometer
• simulation inputs
• upsilon yields for one month of Pb-Pb collisions
• Conclusion

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ALICE physics goalPPR vol I (CERN/LHCC 2003-049)

• study of hot and dense nuclear medium, the Quark
  Gluon Plasma (QGP), predicted by QCD
• In a QGP, QCD predicts the suppression of
  quarkonia (J/?, ?) by color screening
• In ALICE, quarkonia can be measured via their
  dimuon decays with the muon spectrometer at
  forward rapidity

Pb Beam
Pb Beam
IP
Trigger chambers
Dipole magnet

Tracking chambers
Muon filter
Front absorber
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Muon trigger (I)

• Search for (muon) tracks in X-Y roads of a given
  width
• Compute their deviations (pt cut)
• Muon trigger performs 2 pt cuts
• ptcut 1 GeV/c, optimized for J/? physics ?
  Low pt cut (Lpt)
• ptcut 2 GeV/c, optimized for ? physics ? High
  pt cut (Hpt)
• Method for muon trigger efficiency calculation
• Input (pt, ?) parameterization for physics
  continuum (p/K, D, B) quarkonia (?, J/?, ?)
• Output trigger response for single muons and
  dimuons
• ? Trigger efficiency for muons firing at least
  3/4 trigger planes

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Muon trigger (II) in Pb-Pb collisions _at_ 5.5 TeV
f (b1, b2) fcoll (b1, b2) Ptr (b1, b2)

• Trigger rates for a centrality class (b1, b2) ?

• fcoll Collision frequency
  fcoll (b1, b2) ltLgt sgeo(b1,b2)
• Ptr Trigger probability (poissonian behaviour)

For ltLgt 5 1026 cm-2s-1 in Pb-Pb collisions,
fcoll 4000 Hz (minimum bias)

• Particle multiplicity
• 1 central Pb-Pb collision in 4p (b lt 5 fm)
  86000 p/K, 204 D hadrons, 8 B hadrons
• For a centrality class, multiplicity of each
  source is assumed to be proportionnal to the
  number of hard processes ? Glauber model
• Trigger multiplicity number of triggered muons
  (or dimuons) for a given trigger pt cut

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Muon trigger (III)in Pb-Pb collisions _at_ 5.5 TeV
Unlike-sign dimuon trigger rates vs centrality
These rates fit the bandwidth of ALICE DaQ for
dimuon
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Beauty physics with ALICE muon spectrometer

• Interests for beauty physics (PPR vol II
  CERN/LHCC 2005-030)
• in p-p open beauty measurement will be a test
  of pQCD (_at_NLO)
• upsilon measurement will be used
  to test of production models (CEM, COM,...)
• in A-A study of medium effects
• upsilon measurement will be used to study color
  screening vs centrality pt
• energy loss will be studied from open beauty
  measurement vs centrality pt
• ? Upsilon yields measurement
• Method The upsilon yields are extracted from a
  fit of unlike-sign dimuon mass spectra with
• an exponential shape for the correlated
  continuum (beauty charm)
• a breit-wigner modified shape for upsilon
  states

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Simulation inputs (I)

• Muon sources
• upsilon family pt-distributions are extrapolated
  from CDF data quarkonia y-distributions are
  given by CEM model.
• (un-)correlated open charm open beauty are
  generated using Pythia
• p and K are generated from parameterizations of
  Hijing distributions
• Detector efficiency
• detector response (trigger/tracking efficiencies
  acceptance smearing) obtained from fast
  simulations.
• Cut used for analysis trigger high pt cut
  ptµ gt 1 GeV/c (on single muon)

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Simulation inputs (II)
Upsilon states (CEM model) heavy quark (pQCD _at_
NLO) cross section in p-p at 5.5 TeV
Unlike-sign dimuon mass spectra normalization
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Dimuon mass spectrum from upsilon states

• Fit with a sum of 3 modified Breit-Wigner
• f(Mµµ) ?2/(?2(Mµµ -M?)2)
• f(Mµµ) is a polynomial function with 3 free
  parameters to take into account detector effects
  on muon transport
• mean mass M? and mass resolution ? are extracted
  from fit
• upsilon mass resolution
• ? 100 MeV

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Global fit of correlated unlike-sign dimuon mass
spectra

• Case of central Pb-Pb collisions (b lt 3 fm)
• Assumption perfect subtraction of uncorrelated
  dimuon pairs ? sbin cor vNbin tot

w/o uncorrelated
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Yields vs. centrality

• large statistics for ?
• separation of the ? states is possible for each
  centrality class

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Conclusion

• Trigger performance in Pb-Pb
• low pt cut etr 71 for J/? unlike-sign
  dimuon trigger rates 330 Hz / 4000 Hz MB
• high pt cut etr 88 for ? unlike-sign
  dimuon trigger rates 65 Hz / 4000 Hz MB
• Upsilon yields from a fit of dimuon mass spectra
  
• upsilon states yield expected for one month of
  minimum bias Pb-Pb collisions
• N? 7400 N? 2000 N? 1000
• separation of upsilon states for each of the 5
  centrality classes
• large statistics low statistical errors (lt10
  ) for upsilon in 5 centrality classes

BEAUTY PHYSICS LOOKS VERY PROMISING WITH THE
ALICE MUON SPECTROMETER
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Thanks to all my ALICE colleagues who
participated to this work