Tom Ferbel

1
Lifetimes of b Hadrons

• Tom Ferbel
• Department of Energy and University of
  Rochester
• On Behalf of the DØ Collaboration
• Abstracts 195, 196, 201,
  199
• International Europhysics
  Conference HEP2005
  
• EPS (July 21st-27th 2005)
  in Lisboa, Portugal

2
Introduction

• Hadrons containing a b quark, such as Bd, Bs,Bc
  mesons and ?b baryons, when produced at high
  transverse momenta (pT) have relatively large
  boosts, which helps greatly in measuring their
  lifetimes.
• With Heavy-Quark Effective Field Theory
  predicting a clear hierarchy of hadronic
  lifetimes in the decays of b quarks, it is
  important to measure with precision the lifetimes
  of these hadrons.
• Absolute lifetimes are of major interest, but
  ratios are even more valuable to compare with
  theory, as many sources of systematic uncertainty
  cancel when any two lifetimes are measured in one
  experiment (or calculated at NLO to some given
  order of corrections in Mb-1).

3
Provides for your pleasure

• Abstract 196, new (and best) Bs0 lifetime based
  on 400 events/pb µ?DsX data (with
  Ds?fp?KKp).
• Abstract 195, ratio of B/B0 lifetimes based on
  just published PRL 94, 182001 (2005)
  reconstructed semileptonic decays in µ?D0X and
  µ?DX states. (With D?D0p?Kpp).
• Abstract 201, measurement of lifetimes of ?b0,
  Bs0 and Bd0 in exclusive modes ?b0? J/?(µµ) ?0
  (p p) and Bs0?J/?(µµ) f(KK), and
  Bd0?J/?(µµ) K0(Kp) and Bd0?J/?(µµ)
  Ks0(pp). First measurement of ?b in a fully
  reconstructed channel. ?b Bs0 PRL 94, 102001
  ibid, 042001 (2005)
• Abstract 199, lifetime and mass of
  Bc?J/?(µµ)µ? for reconstructed µµµ
  systems. Mass from fully reconstructed
  J/?(µµ)p decay is still in the works.

4

• Tracking is done via silicon microstrips and
  
• scintillating fibers in 2T field
• Muon system has drift tubes and scintillation
• trigger counters, with good coverage to ?2

5
Measuring Lifetimes

• Reconstruct primary and decay vertexes of
  B-hadron candidates
• Calculate the Proper Decay Length in transverse
  plane from signed transverse decay length Lxy

• For semileptonic decays, use decay length l
  for observed part of hadron, and a factored-in
  distribution to correct for total momentum.

6
Hadronic Modes

• Use dimuon (J/?) events, and then
• Search for a long-lived neutral decaying to two
  oppositely charged tracks, pointing to vertex of
  J/?
• ?b0? J/?(µµ) ?0(pp)
• Bd0? J/?(µµ) Ks0(pp)
• Or for two tracks originating at the J/? vertex
• Bs0? J/?(µµ) f(KK)
• Bd0? J/?(µµ) K0(Kp)
• Search for similar topologies at same time,
  analyze in similar way, so as to minimize errors
  on ratios of particle lifetimes

7
Semileptonic Modes

• No specific muon selection at trigger level, but
  offline require pTµ gt2GeV/c, and p gt3GeV/c (this
  depends on analysis).
• Bc?J/? µ?X uses dimuon decays of J/?, and third
  muon at decay vertex, to fit µµµ to mass and
  lifetime
• Bd0?µ?D(D0p)X and Bd ? µ?D0(Kp)X are used
  for direct measurement of ratio of lifetimes of
  Bd0 and Bd
• Bs0?Dsµ?X, where Ds? f(KK)p is used to
  extract Bs0 lifetime, with proper decay length
  l for observed part of signal smeared with a K
  factor distribution based on Monte Carlo.

8
Bs0? µ?DsX (with Ds?fp?KKp)
Mass distribution of Ds (fp) candidates when
significance on l exceeds 5 s.d. Dark points
correspond to correct Dsm charge combinations,
and crosses to wrong ones. Smooth curve is fit to
right-sign combinations. Peaks are for D and
Ds, respectively

9
Bs0? µ?DsX (with Ds?fp?KKp)
Mass distribution of (fp) candidates, whithout
significance criterion
10
Bs0? µ?DsX (contd)
Distribution in l for Bs0 signal and
superimposed fit (dashed), with combinatorial
background represented by dotted curve.
11

Results for Bs
t(Bs0) 1.4200.043 (stat) 0.057 (syst) ps
(Semileptonic mode) t(Bs0) 1.444 0.10/-0.09
(stat) 0.20 (syst) ps (Hadronic mode)
Both agree with previous world averages, and
our semileptonic result for Bs0 is currently most
precise at 1.420 0.071 ps
12
New Method for t(Bd)/t(Bd0)

• Ratio established by
• Fitting to the number of Dµ candidates (85
  from B0) as a function of visible proper decay
  length l.
• Fitting to the number of D0µ candidates (83
  from B) as a function of l.
• Finding the ratio of the numbers as a function of
  l.
• If this ratio is independent of l, the
  lifetimes are equal. Any difference manifests
  itself as a slope in this ratio vs l .
• This new DØ technique can be used in other
  measurements of ratios

13
t(Bd)/t(Bd0) from large clean sample of µ?D0X
and µ?DX, with D?D0p?Kpp
14
?b0 ? J/?(µµ) ?0 (p p)
15
Bc ?J/?(µµ)µ? Lifetime(See 95 17 events
excess in mass and in lifetime)
16
Results for Lisboa

• t(Bs0) 1.4200.043 (stat) 0.057 (syst) ps
  
• Best single measurement of flavor-specific
  t(Bs0), with dominant systematic uncertainty from
  estimate of background. Prediction for
  t(Bd0)/t(Bs0) 1.000.01. Also important in
  extraction of dGs/Gs.
• t(Bd)/t(Bd0) 1.080 0.016 (stat) 0.014
  (syst) ps Competitive and most
  precise measurement of ratio at Tevatron, mainly
  limited by model for parameterization of signal
  and background in the fit to data. Theory
  predicts 1.06 0.02.
• t(Lb0) 1.22 0.22/-0.18 (stat) 0.04
  (syst) ps
• t(Lb0 )/t(Bd0) 0.87 0.17/-0.14
  (stat) 0.03 ps
• First measurement of Lb0 in a fully reconstructed
  exclusive channel. Theory for ratio is 0.86
  0.06
• t(Bc) 0.45 0.12/-0.10 (stat) 0.12
  (syst) ps
• Based on first analysis with signal in excess of
  5 s.d. Analysis being updated and completed for
  publication.

17
Summary

• DØ has new and many emerging results in the area
  of b-hadronic lifetimes.
• DØ semileptonic Bs0 lifetime is best measurement
  available.
• Measurement of hadronic ?b lifetime is best for a
  fully reconstructed channel.
• Significant contributions to world averages.
• Unfortunately, all DØ results are consistent with
  HQET and with world averages. Have to try harder!