Leptonic Decays of B Mesons

1
Leptonic Decays of B Mesons

• Quincy Wong
• Candidacy Exam
• The Ohio State University
• Department of Physics
• 2/28/2005

2
Overview

• Motivations for studying leptonic B decays
• Analysis strategies in BABAR
• B0 ? ee-
• B0 ? mm-
• B ? tnt
• Conclusion

3
B ? lnl Theory

• Standard Model (SM) Feynman diagram for B ? lnl
• SM predictions
• BR(B ? ene) 9.0 X 10-12
• BR(B ? mnm) 4.5 X 10-7
• BR(B ? tnt) 9.0 X 10-5

Decay constant
Helicity suppression factor
4
B0 ? ll- Theory
l
b
d
l-

• Standard Model Feynman diagrams for B ? ll-
• SM predictions
• BR(B0 ? ee-) 1.9 X 10-15
• BR(B0 ? mm-) 8.0 X 10-11
• BR(B0 ? tt-) 3.1 X 10-8

Inami-Lim function 1.06
Weinberg angle
5
Motivation

• Measurements of Standard Model parameters
• Decay constant
• CKM matrix elements
• Probes of new physics
• Non-SM contributions to branching ratios

6
The BABAR Detector I

• ee- ? U(4S) CMS energy 10.58 GeV mass of
  U(4S)
• U(4S) ? BB U(4S) decays at rest

Electromagnetic Calorimeter Energy measurements
of electrons and photons
1.5 T Solenoid
DIRC Charged particles ID
e (3.1 GeV)
Drift Chamber Momentum and energy measurements of
charged particles
e- (9 GeV)
Silicon Vertex Tracker Vertex detector
IFR/LSTs Muons/neutral hadrons detectors
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The BABAR Detector II
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B0 ? ee-, mm- Analysis Strategy I

• Preselection criteria
• Events with at least 2 high momentum tracks
• Track requirements momentum, of drift chamber
  hits and distance from beam spot
• B candidate
• Pair of opposite-charged tracks vertexed to form
  a B candidate
• Both tracks should originate from the same point
  in space

9
B0 ? ee-, mm- Analysis Strategy II

• Events are selected according to two kinematic
  variables, DE and mES
• Backgrounds
• Design cuts to suppress backgrounds
• Apply cuts on simulated Monte Carlo sample
• Optimize each cut to maximize signal to noise
  ratio

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B0 ? ee-, mm- Analysis Strategy III

• Selected events are displayed in DE vs mES plot
• Search for signals in the signal box
• Estimate signal box background using Grand Side
  Band
• Blind analysis
• Data in blinding box are hidden until the end
  of the analysis
• Prevent experimenters bias

Signal Box
Grand Side Band
Blinding Box
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B0 ? ee-, mm- Backgrounds

• Real electron pairs in qq production
• ee- ? qq ? ee-X
• Two-photon events
• ee- ? ee-gg ? ee-
• Radiative Bhabha events
• ee- ? ee-g
• Muon misidentifications
• B0 ? pp- (10-6) as B0 ? mm- (10-11)

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B0 ? ee-, mm- Background Suppression

• qq continuum
• cos qT Angle between the thrust axis of the B
  candidate and that of the remaining tracks
• mroet mass of all tracks not associated with the
  B candidate (non-leptonic ? assumed to be pions)
• Two-photon events Total energy of all tracks
• Bhabha events Multiplicity
• Bhabha event 2 charged tracks
• Signal event likely gt 5 charged tracks

f
e
e-
B
e
e-
f
B
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B ? tnt Analysis Strategy

• Neutrino analyses require additional constraints
• Reconstruct the B- (tag B) in the event
• Hadronic final state (e.g. B- ? D0p-)
• Semileptonic final state (e.g. B- ? D0e-ne)
• Remainder of the event belong to the B (recoil
  B) and are studied to search for B ? tnt
• Very low signal efficiency (10-4) because of the
  reconstruction of tag B

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B ? tnt Hadronic Tags I

• B- ? D0X-, X is a hadronic system composed of
  pions and kaons (3.4)
• D0 ? D0p0 (62)
• D0 ? K-p (3.8)
• ? K-pp0 (13)
• ? K-pp-p (7.5)
• ? Ks0pp-
• Ks0 ? pp- (69)
• Tag B is reconstructed with requirements on DE
  and mES

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B ? tnt Hadronic Tags II

• After tag B is reconstructed, search the
  remaining of the events for
• t ? enent (18)
• ? mnmnt (17)
• ? pnt (11)
• Signal selection
• Total charge of the event 0
• All leptons or pions should satisfy e/m/p ID
  requirements
• Reject events with Ks0 and p0
• Constraints on total momentum and missing mass
• Background suppression cuts and optimizations

16
B ? tnt Hadronic Tags III

• Signal region and side band defined by variable
  Eleft
• Eleft energy not associated with neutral
  particles from tag B

Normalized signal MC
Background MC
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B ? tnt Semileptonic Tags I

• B- ? D0l-nlX, X g, p0 or nothing, l e or m
  (10)
• D0 ? K-p
• ? K-pp0
• ? K-pp-p
• ? Ks0pp-
• Ks0 ? pp-
• Dl candidate
• D0 momentum
• Lepton momentum
• Daughter particles should meet particle ID
  criteria

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B ? tnt Semileptonic Tags II

• Search for B ? tnt in signal side
• t ? enent
• ? mnmnt
• Signal selection
• Exactly one charged track
• The charged track must be tagged as either an
  positron or muon
• Background suppression cuts and optimizations
• Define signal region in Eleft plot

19
B ? tnt Backgrounds

• BB- background
• Fake signal B, dominant and includes KL0
• Missing KL0
• B0B0 background
• Fake tag B, e.g. B0 ? K-p (10-5)
• ee- ? tt- events

20
B ? tnt Background Suppression

• ee- ? tt-
• cos qT
• Minimum invariant mass of any 3 tracks in the
  event
• Continuum (hadronic tags) DE and mES

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Current Status

• All upper limits _at_ 90 C.L.
• B0 ? ee-, B0 ? mm-
• BABAR 111.1 fb-1
• BELLE 78 fb-1
• B ? tnt
• BABAR 112.5 fb-1
• BELLE 140 fb-1

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Improvements

• More data
• New muon detectors LSTs
• Improved hadron IDs (2X)
• B ? tnt Intermediate resonance modes
• t ? rnt , r ? pp0
• t ? a1nt , a1 ? r0p, r0 ? pp-
• Additional invariant mass constraints no
  neutrinos
• Background control
• B0 ? ee-
• Bremsstrahlung recovery
• 4.4 signal recovered

23
Prospects and Conclusion

• Sensitive to new physics
• No events has been observed within the SM
  predictions
• BABAR aiming for 500 fb-1 by Summer 06

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Backup Slides