b physics at LEP

1
b physics at LEP

• Andrea Sciabà
• INFN-CNAF

XXXIII International Symposium on Multiparticle
Dynamics September 5-11, 2003 Krakow
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b physics

• Goals
• CKM matrix
• B oscillations (Vtd, Vts)
• B semileptonic BRs (Vcb, Vub)
• b lifetimes
• CP violation
• masses
• Electroweak
• sin2 ?W (asymmetries)
• Rb (sensitive to new physics, mtop)
• QCD processes
• fragmentation
• two-photon events, gluon splitting, etc.
• New physics (CP violation, rare decays, etc.)

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b physics at LEP

• LEP data sample features and advantages
• 3.6106 Z0 ? bb events
• very clean environment
• high heavy quark tagging power due to silicon
  microvertex detectors
• production of many species of b hadrons
• high boost of b hadrons
• but many results being superseded by pp
  colliders and b factories

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b physics at LEP

• LEP data still contributes significantly to
• b semileptonic BRs
• Bs oscillations
• Asymmetries, Rb (no more measurements until Z
  factories)
• Bs, ?b, excited b hadron masses
• Fragmentation
• heavy quark production in 2-photon collisions
• gluon splitting into heavy quarks
• Only a few topics will be covered here

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Bs oscillations
Bs oscillations Basic concepts

• Main motivation
• ?md ? Vtd gives a large error
• ?md/ ?ms ? Vtd with a 3 times better
  theoretical error!

• 15 theor. error

Bs,d0
? Vcb2
SU(3) 5 error

• Oscillation probability
• fit A for any assumed value ? of ?ms
• ? ltlt?ms ? A ? 0
• ? ??ms ? A ? 1
• ? excluded at 95CL if A1.65sAlt1

A
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Strategy

• Flavour tagging at production
• jet charges, vertex charges, kaons, leptons, etc.
• typical mistag ? ? 25
• Flavour tagging at decay
• lepton or D charge
• identify final state (b??, b?c??, etc.)
• Distinguish Bs from other hadron species
• Measure Bs decay length, momentum
• ?t (m/p) ?L ? (?p/p) t
• Maximum likelihood fit to amplitude A at given ?ms

Production flavour tagging
Decay flavour tagging
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Oscillation analyses

• Exclusive analyses
• Fully reconstructed Bs (A, D)
• Semi-exclusive analyses
• Ds?- pairs (A, D, O)
• Dsh- pairs (D)
• F?- pairs (D)
• Inclusive analyses
• semileptonic decays (A, D, O)
• inclusive vertices (D)

Very low statistics High purity Very high p.t.
resolution!
Low statistics Fair purity Good p.t. resolution
High statistics Low purity Low p.t. resolution
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Fully reconstructed Bs (ALEPH)
total
Well reconstructed
Missing ?s or ?s

• ? and ?0 reconstruction ? efficiency and purity
  increase
• ???? ? 180 ?m??p/p? ? 0.5 3

Add a photon
?t?0.08 ps
Excellent proper time resolution!
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Ds?- pairs (DELPHI) (updated)
Semileptonic modes
Hadronic modes

• Event-by event
• proper time resolution
• Bs purity

Sensitivity ?ms 8.6 ps-1 ?ms gt 4.9 ps-1 at 95
CL
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Inclusive ? analysis (ALEPH)

• Topological reconstruction of the D vertex
• Extensive event-by-event treatment of
• proper time resolutions and corrections
• sample composition
• flavour tags
• ? huge sensitivity enhancement!

Sensitivity ?ms 13.6 ps-1 ?ms gt 11.9 ps-1 at
95 CL
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Oscillation results

• Almost final LEP results on Bs oscillations
• Lower limit on ?ms 14.4 ps-1
• Now, we must wait for Tevatron to measure ?ms

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b quark fragmentation

• The fragmentation function models the
  non-perturbative confinement of the quark into
  colourless hadrons
• different choices of the variablenot
  accessible experimentallyISR, FSR, hard gluon
  emission effects not unfolded
• Model-dependent analyses (fit to a given
  fragmentation model)
• Model-independent analyses (xB spectrum
  reconstruction)

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Inclusive analyses

• Event selection based on
• b-tagging
• secondary vertex reconstruction
• Direct xB reconstruction (OPAL)
• neural networks to distinguish particles from b
  decay and from hadronization
• sE?5 GeV
• xB, z reconstructed using neural networks
  (DELPHI), input variables correlated to
• kinematics (track rapidity, multiplicity, etc.)
• jet/total energy
• b-tagging

• preliminary

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Exclusive analyses (ALEPH)

• B ? D()l? decays
• Much lower statistics
• 3400 events
• E? reconstructed from missing energy
• sE 8

• xB spectra not efficiency-corrected

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Model dependent results

• Favoured models
• Lund / Lund-Bowler
• Kartvelishvili
• Disfavoured models
• Peterson et al.
• Collins

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Model independent results

• Mean values for xB
• OPAL 0.71930.0016(stat.)0.0038(syst.)
• DELPHI 0.71530.0007(stat.)0.0049(syst.)
• ALEPH 0.7160.006(stat.)0.006(syst.)
• All measurements consistent
• bias w.r.t. old measurements due to
  theassumption of the Peterson model

• -0.0033

• -0.0052

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AbFB measurements (I)

• Forward-backward asymmetry at the Z pole
• Related to sin2?W
• AbFB most sensitive to sin2?W
• Need to measure
• cos? (from thrust axis)
• quark charge
• quark flavour
• QCD corrections to be calculated

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AbFB measurements (II)

• Use a lepton tag
• lepton charge correlated to the quark charge
• need to distinguish
• correct for the dilution by B mixing

• Use the jet/vertex/kaon charge
• b-tagging to enhance purity
• purity from data
• quark charge estimator Q
• charge separation d from data
• effect of B mixing included

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AbFB with lepton tag (DELPHI)
New!

• b-tagging (?EVT)
• Track I.P., sec.vertex mass and energy, track
  rapidities
• Flavour composition vs. ?EVT from data with
  standard double counting method
• Lepton p and p?
• Charge tagging
• Lepton charge, opposite jet charge as cross check
• Uses Q? Qopp (-/ right/wrong correlation)
• Probability for a lepton of coming from b??,
  b?c ? ?, c ? ?, bckgnd function of (p, p?),
  (?EVT, Q? Qopp)
• 2-dim fit of the obs asymmetry to AbFB, AcFB over
  bins of (cos?T, Pb??-Pb?c ? ?, Pc??)

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AbFB with lepton tag (OPAL)

• Neural networks to distinguish b?? (NETb) and c??
  (NETc)
• Lepton p and p?, lepton jet energy, (?p?)jet
• Jet decay length significances, lepton i.p.
  significance
• Likelihood fit to the number of single-lepton and
  double-lepton events
• Bins of cos?, NETb and NETc
• Background fraction fitted from data
• average mixing parameter ? fitted from data using
  same-sign dilepton events
• AbFB and AcFB fitted

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AbFB with incl. tag (DELPHI)
New!

• high purity b-tagging (b-tag)
• lifetime and sec vertex info, rapidities
• b-tag selection efficiency from data counting
  events with 0,1,2 tagged hemi ? sample
  composition (b, c, uds)
• charge tagging (flavhem)
• jet charge, sec. vertex charge
• flavour tag combining P(same QB) for all tracks
• prob. of charge mistag from data
  (unlike/like-sign double tagged events)
• for charm events, mistag from data using charge
  of reconstructed D (new!)
• fit to no. of single and double tagged events in
  bins of (cos?, b-tag)
• Main systematics from jet charge hem. correlations

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AbFB with incl. tag (OPAL)

• b events tagged with
• NN (secondary vertex info)
• leptons (p, p?, isolation)
• flavour composition from data (singly and doubly
  tagged hemispheres) ? Pf
• charge tagging estimator Q from
• jet charge (always)
• sec. vertex charge (if any)
• kaon charge (from b?c ?s) (if any)

• all charge estimators available

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AbFB summary

• AbFB measurements all LEP almost finalized
• well compatible (also with SLD)
• sin2?lepteff 2.9s discrepancy with ALR (SLD)
• need to wait for linear colliders

SLD Ab
LEP AbFB
LEPSLD A?
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Open b production in ?? events

• Test of perturbative QCD
• at LO, only direct and single resolved terms (of
  the same order)
• more robust calculations because of the large
  quark mass
• 2-3 orders of magnitude suppression w.r.t open
  charm because of larger mass, smaller charge
• photons mostly quasi-real ? anti-tagged events
• Analysis strategy
• select anti-tagged photon-photon events
• reconstruct lepton candidates
• reconstruct jets
• measure lepton p? w.r.t. jet axis

• direct term

• single resolved term

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???bbX in L3

• 627 pb-1, 189 GeV lt ?s lt 209 GeV
• muons AND electrons ? large statistics
• Fit to p? distribution
• p? spectra from MC (PYTHIA for ? ?)
• Nb, Nc, Nuds left free
• Consistent e and ? results
• Also measurement of
• consistent with previous L3 results

• muons

• 435 events

• electrons

• 3850 events

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???bbX in DELPHI (new)

• 463 pb-1, 189 GeV lt ?s lt 209 GeV
• only muons
• Fit to p? distribution
• p? spectra from MC (PYTHIA for ? ?)
• Nc fixed to LEP average for ?(ee-?cc)
• Nuds measured selecting hadrons instead of muons
• Nb left free
• Study of K-lepton correlations (first time!)
• b?K-?-X c?K-?X
• increased purity of c (K??) and b (K?) events
• measured cross sections

651 events
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???bbX in OPAL

• 371 pb-1, 189 GeV lt ?s lt 202 GeV
• only muons
• Fit to p? distribution
• p? spectra from MC (PYTHIA for ? ?)
• Nc fixed to OPAL measurement for ?(ee-?cc) with
  D mesons
• Nuds measured selecting hadrons instead of muons
• Nb left free
• xT2pT/Wvis distribution shows the consistency of
  the 11 ratio of direct to single-resolved
  contributions

444 events
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???bb(cc) summary

• Consistent results among LEP experiments
• Good agreement with Drees et al. model for charm
  data
• EXCESS for beauty data 4? discrepancy
• No satisfactory explanation at present

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Conclusions

• LEP HF physics program almost finalized
• Bs oscillations (?msgt 14.4 ps-1 at 95 CL)
• heavy quark asymmetries (difficult
  interpretation)
• Heavy quark fragmentation function well measured
  and consistent (both in average and shape)
• Large discrepancy in ???bbX rate (but not in
  ???ccX rate)
• Impossible to show many other results