BaBar Collaboration

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• BaBar Collaboration
• Randall Sobie
• Institute for Particle Physics
• University of Victoria

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Introduction

• Tau semi-leptonic decays are an ideal place to
  study strong interaction effects
• The large sample of tau data from BABAR allows us
  to re-examine many of the well-known hadronic
  decays but to look in detail at the decays that
  were previously limited by statistics
• This work focuses on 2 of the 5-hadron final
  state decays of the tau lepton

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Data Samples

• BABAR has collected 244 /fb
• These analyses use 110-124 /fb
• Approximately 100 million tau-pair events
• Results quoted are preliminary

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BABAR Detector at SLAC
1.5 T solenoid
EMC
DIRC (PID)
e (3.1 GeV)
e- (9.0 GeV)
Drift Chamber
Silicon Vertex Tracker
Instrumented Flux Return
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Simulation

• KK2F generates the tau pair events
• Tauola decays the tau leptons
• 3-prong signal events arise from two separate
  classes
• The final states are generated using a phase
  space distribution
• events are generated using
  a phase space distribution

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Selection of signal candidates

• Event selection
• Reduce non-tau background (Bhabhas, dimuons,
  2-photon and multihadrons) using thrust,
  transverse momentum and missing energy
• Event split into 2 hemispheres in CM frame using
  thrust
• Each hemisphere is associated with a tau
• The decay products are collimated due to the
  relativistic momentum of the tau
• Look for 1-3 and 1-5 topology
• Tag hemisphere selection
• Require single track with track being either an
  electron or muon

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Signal hemisphere selection

• Tracks required to be pions
• 2 pi0 candidates
• 2 clusters M100-160 MeV and Egt215 MeV
• M(5pion) lt 1.8 GeV
• 9900 candidates selected
• Background 15
• Mainly tau decay background
• Efficiency 0.65

• Pion or kaon tracks allowed
• Veto electrons, photon conversions and pi0s
• M(5hadron) lt 1.8 GeV
• 15869 candidates selected
• Background 21
• tau decays with pi0s or K0s
• Efficiency 3-4.5

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5 pion mass distribution I
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5 pion mass distribution II
Background subtracted mass distribution. Note
that the background is scaled to reproduce the
mass distribution above 1.8 GeV. The
disagreement between data and MC is similar for
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Comparison with other measurements
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BABAR has searched for the production of
phi(1020) mesons
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Systematic uncertainties
Largest Sources
Background 9.3 0.9 1.8
Efficiency for pi0 finding 6.5 6.5 6.5
Efficiency for pi- identification 3.0 3.0 3.0
Tracking efficiency 2.4 2.4 2.4
Luminosity and cross section 2.3 2.3 2.3
Total 12.3 8.9 8.6
Largest Sources
Tracking Efficiency 3.1
Luminosity and cross section 2.3
pi0 contamination 2.0
Background 1.5
Lepton tag 1.0
Total 4.7
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Decay mechanism

• .

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The f1 can also decay to a 4-pion final
state BABAR observes a peak in the 4-pion mass
distribution that is consistent with the CLEO
result
CLEO result
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Summary I
BaBar has studied the tau decays
Channel Preliminary branching fraction

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Summary II

• The large data sets have been used to investigate
  the structure of the decays
• The data shows that a phase space distribution
  does not adequately describe the data in 3 and 5
  prong tau decays
• New limit on the decay mode
• Peak in the 4-pion mass distribution is
  consistent with the CLEO observation of