Open B-physics with CMS

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Open beauty production in pp collisions at 7 TeV
with CMS
Kajari Mazumdar Tata Institute of Fundamental
Research Mumbai, India. on behalf of CMS
Collaboration, LHC, CERN
ICPAQGP, GOA,
December 2010
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Open Beauty and Heavy Ions

• Heavy flavours are important probes of QGP in
  heavy ion collisions.
• Several issues
• Production time scale of heavy flavour shorter
  than that of the deconfined state.
• Lifetime of heavy flavours are larger than that
  of the medium ? b,c quarks fragment inside the
  hot medium.
• Hard partons are likely to loose energy while
  traversing through the dense medium.
• Does the energy quench depend on mass of the
  hadron?
• --gtStudy and compare the properties of J/?, ?,
  B-mesons .
• The tussle between the heavy meson dissociation
  and decay of c/b quark in dense medium may
  cause significant modification in measured
  b-cross-section in QGP ? compare with pp
  measurement.

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LHC accelerator and CMS experiment
pp collisions at LHC provides the important
normalization for HI to estimate the shadow
effect. LHC provides abundant rates for heavy
flavour production ? accurate measurements
possible in both pp and HI collision data.

• CMS recorded about 43 pb-1 integrated luminosity
  for pp collision at 7 TeV and 9 ?b-1 of Pb-Pb
  collision in 2010 .
• LHC instantaneous luminosity during
• April to October 1027 to 1032 /cm-2 / s-1
• CMS trigger strategy changed accordingly
• Results shown today uses only
• a fraction of the total pp collision data.
• HI collisions of 2010 just getting over.
• No result on open beauty measurement
• in HI data of CMS available as yet.

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CMS precise charged particle tracking,
robust lepton identification, finely
segmented calorimeter
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Open beauty double b-jet candidate
CMS B-physics programme for p-p collisions 1.
Measure differential and total cross-sections
for beauty production. 2. Study
exclusive B-decays. 3. Search for New Physics
in rare decays.
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Important subdetectors
B-physics studies mainly depend on

• Charged particle tracking in pixel layer and
  Si-strip detectors
• ? Long lifetime and large mass reconstruction
• Measurement of transverse momentum (pT)
• Primary and secondary vertex reconstruction
• Muon measurements 3 different sub-detector types
• Muon identification
• Muon pT measurement
• Semileptonic decays
• Particle Flow algorithm for jet reconstruction
  upto 20 GeV/c, with capability for b-jet
  identification.
• Minimum bias and inclusive single jet
  triggers

pT resolution _at_ 1 GeV/c 0.7 _at_ ? 0, 2 _at_
? 2.5
Muon trigger pTgt 3GeV/c impact parameter
d0lt 2cm dedicated quarkonia triggers
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Tracking performance

• Measure upto pT few hundred MeV/c
• Good efficiency and purity for pT gt1 GeV/c

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Muon identification
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Particle flow technique in CMS
Correlate charged tracks with energy deposits in
the calorimeter, correct For energy loss along
trajectory.

• Utilise . good separation of charged particles
  in large tracker volume.
• . good resolution for soft particles
  upto few hundred MeV/c.
• . very good electromagnetic
  calorimeter
• . in multijet events only 10 of
  energy correspond to neutrals
• Big improvement in energy resolution (factor of 2
  wrt measure-
• ments using only calorimetric info) and particle
  identification.
• ? Reduces jet scale uncertainty by factor of 2.

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Semileptonic decays, b ? ? X
B-hadron kinematics ? lepton is harder ? use
relative pT of muon wrt jet to identify b-event.
Consider events with pT ? gt 6 GeV/c, wrt
z-direction, ?lt 2.1

• Use shape templates from simulation, to get
  fraction of b,c and light quark contributions.
• validate with b-enriched data sample fb 441
• Consider charm decays in-flight decays from
  light quarks, gluons
• ?Template for misidentified hadron validated with
  data

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Estimation of b-quark production cross-section in
semileptonic decays
Trigger eff 82 ? reconstruction97 ?-jet
association77
Models for fake muon underlying
effect dominate experimental uncertainty 15-20
Visible rate
Uncertainty in luminosity 11
Larger discrepancy with theory at low pT? and
central region
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Measurement of inclusive b-jets

• Tag/identify b-jets to include higher momentum
  b-events.
• ? utilise hard fragmentation, large mass, large
  lifetime of b.
• Different systematics compared to semileptonic
  decays !
• Several algorithms for b-jet identification use
  discriminators based on track counting,
  secondary vertex, semi-leptonic decays, b-jet
  probability
• ? threshold values in each case decide sample
  purity, identification efficiency.

Secondary vertex tagging method? utilization of
atleast 3 tracks to reconstruct secondary
vertex?best efficiency and high purity.
B-tagging efficiency in data, using muon spectrum
in b-jets ? fraction of tagged, untagged events
obtained from fit to samples which pass and fail
discriminator
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b-jet candidate selection discrimination
between b and light-flavoured jets
Basic track qualities Number of pixel hits
gt2 Total number of silicon (pixelstrip)
hitsgt8 Chi2/ndof of track fit lt5 Transverse
momentum gt1 GeV/c Unsigned transverse impact
parameter dxy lt0.2 cm Unsigned longitudinal
impact parameter dz lt 17 cm Distance of closest
approach to the jet axis lt 0.07 cm Decay length lt
5 cm.
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b-tagging in data
Sample purity determined from secondary vertex
mass fit 73
B-fraction in tagged sample, using MC
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Estimation of cross-section
Stat. Uncertainty in cross-section 2 Syst.
Uncert. 20?take ratio wrt inclusive jets to
reduce systematics due to luminosity, energy
resolution, energy scale of jets.
Agreement good with Pythia for pTgt40 GeV/c Shape
difference with MC_at_NLO for large pT and in
forward direction.
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Exclusive B-meson decays in pp collision
Reconstructed modes B?J/? K , Bs?J/?
? Several on going analyses B0?J/? K0 , ?J/?
K0 ????Bs ? ?????????b???J/? ????
CMS DPS-2010-040
Maximum likelihood fit for single gaussian
first order polynomial
Fit 3 gaussians for signal exponential for
background
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Conclusion

• Discussed the capability of CMS experiment for
  open-b physics with pp collision data
  exclusive decays and measurement of b-production
• cross-section from low to reasonably high
  transverse momentum,
• using semileptonic decays and b-tagged jets.
• Agreement with theoretical predictions (LO
  NLO) reasonable, though some differences are
  significant in different kinematic regions.
• Measurements with larger statistics being
  finalised.
• CMS will carry forward b-physics programme in
  heavy ion collision
• data.
• Expect interesting results in near future.
• Stay tuned!

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BACKUP
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Tagging performance
Jet pt (30,50)
Jet pt (10,30)
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Event Displays
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