Production of beauty quarks in deep inelastic scattering at HERA

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Production of beauty quarks in deep inelastic
scattering at HERA
XI International Workshop on Deep Inelastic
Scattering St. Petersburg, 23-27 April 2003
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Outline

• Event topology
• Event selection
• Signal extraction and cross section calculation
• QCD predictions and related uncertainties
• Results total and differential cross sections
• Comparison with other measurements
• Summary and outlook

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Process topology in the ZEUS experiment
laboratory frame
(forward)
(rear)
Inner muon chambers
Central tracking detector
Outer muon chambers
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Summary of the event selection
Data sample ZEUS 1999-2000 ep (60 pb-1)
DIS cuts Q2gt2 GeV2 0.05ltylt0.7 40ltE-pzlt65 GeV box cut 12x24 cm
Muon cuts P(c2)gt1 pmgt2 GeV 30ºltqmlt160º
Jet cuts At least one jet in the Breit frame ETBreitgt6 GeV -2lthLablt2.5 Jet associated to the muon ETBreitgt4 GeV QED radiation cuts
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Extraction of the s.l. b-decays (1)

• After the selection cuts the data sample is a
  mixture of several processes
• Fake muons Punch-through, Sail-through, pions
  and kaons in-flight decays (cross checked with
  real data)
• Semi-leptonic decays of charmed hadrons
• Semi-leptonic decays of beauty hadrons

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Extraction of the s.l. b-decays (2)

• 836 events after all cuts
• Charm and light-quark ptrel distributions are
  added together according to the respective cross
  sections
• The b-fraction is extracted with the Binned
  maximum likelihood method
• fb (25.34.8)
• The algorithm takes in account the statistical
  errors of the data and MC samples in each bin

210 beauty events
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Comparison with the simulation
muon variables
Good description of the event kinematics with the
extracted fractions
jet variables
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QCD predictions NLO calculations

• The calculation of the NLO QCD predictions
  proceeds in three steps
• HVQDIS (B.Harris, J.Smith, hep-ph/9503484)
  gg?bb, gg?bbg, gq?bbq, etc.
• Fragmentation of the b-quark into a B-meson
• Semileptonic decay of the B-meson

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Results and QCD predictions
Measured cross section svis 38.77.7(stat.)6.1
(syst.)-5.0(syst.) pb NLO QCD (HVQDIS) sNLO
28 5.3 - 3.5 pb mR,F2 Q24mb2 mb4.75
GeV Peterson frag. function Proton parton
distribution CTEQ5F4
Model Program Cross Section (pb)
LO HVQDIS 14.4
MEDGLAP RAPGAP 17.1
CDM RAPGAP 16.3
MECCFM CASCADE 36.8
Data/NLO 1.4 but in agreement within exp. and
theoretical uncertainties
mR,F2 pT24mb2
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ds/dQ2
measured at HERA for the first time!
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ds/dlog(xbj)
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NLO QCD uncertainties

• more sources
• Hadronization (jet) 10
• Fragmentation (muon) 5-10

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A tricky business...
red ICHEP02 results
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Summary and conclusions

• We have measured a visible cross-section of
  b-quark production in DIS and single differential
  cross-sections (Q2, xbj) for the first time at
  HERA
• The cross section is measured with events
  containing the scattered positron, a hard jet in
  the Breit frame and a muon traversing the inner
  and outer muon chambers.
• Detailed analysis of the theoretical uncertaities
  has been performed

• The measured cross-section is a factor 1.4 above
  the NLO QCD predictions but in agreement within
  the experimental and theoretical uncertainties
• Similar result in the ZEUS measurement in
  photoproduction

• The description of the b-quark production is
  still an open question. Cannot draw a unique
  conclusion from the existing results.

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SPARE SLIDES
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Acceptance, efficiency, purity
MEPS CDM MEPS (e-p) MECCFM
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b-quark fragmentation
hep-ph/0204025
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Event simulation Monte Carlo

• Parton shower with
• CCFM-like evolution
• CASCADE
• Matrix Element with incoming off-shell gluon
• Angular ordering in the gluon ladder
• Direct (pointlike) component only
• Gluon density unintegrated in kT and extracted
  from a fit to the HERA structure function F2
• Lund string model

• Parton shower with DGLAP Evolution
• Matrix Element with incoming on-shell gluon
• KT ordering in the gluon ladder
• RAPGAP
• Lund string model
• Color dipole model and MEPS

...more samples generated to understand the
model dependance
Light quarks 120106 events (200 pb-1) Charm
quarks 7.5106 events (94 pb-1) Beauty
quarks 430103 events (540 pb-1)
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Visible vs extrapolated measurements

• Visible cross section the kinematic region is
  close to the detector acceptance
• Example ep ? bbx ? m Jet Jet X
• Model assumptions are not absorbed in the
  experimental result but in the corresponding QCD
  predictions ? published measurements are then
  safe
• Extrapolated cross sections
• Extrapolation to an unmeasured kinematic region
  the unmeasured phase space is taken from a Monte
  Carlo simulation (for example extrapolation to
  the jet or muon full phase space)
• It is nice to quote more general and inclusive
  measurements, but...
• Is the h and ET spectrum of the extrapolated
  region the same in the NLO as in the Monte Carlo?
• How large are the extrapolation factors? Are they
  reported in the published results?
• Is the extrapolation considered in the estimation
  of the systematic uncertainties?
• Extrapolation from detector to parton/hadron
  level the corresponding correction factors are
  extracted from a Monte Carlo simulation