1. Tau physics with BaBar Roger Barlow BaBarFest QMUL, 1 st April 2009

2. Overview BaBar was a tau factory Cross section 0.9 nb 850 Million tau leptons Compared to LEP: + More statistics + Better detector – Less hemisphere separation -Higher background from q-qbar Compared to CLEO and ARGUS + Way better in every respect 1st April 20092Roger Barlow: Tau Physics with BaBar τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ τ

3. Catalogue 1st April 20093Roger Barlow: Tau Physics with BaBar Search for Second-Class Currents in τ − → ωπ − ν Search for Lepton Flavour Violating Decays τ -  l - K 0 S with the BABAR Experiment Search for Lepton Flavor Violating Decays τ +-  l +- ω Measurement of the τ -  η π - π + π - ν τ branching fraction and a search for a second-class current in the τ -  η'(958) π - nu τ decay Improved Limits on the Lepton-Flavor Violating Decays τ -  l - l + l - Measurement of the τ -  K - π 0 ν τ branching fraction Search for Lepton Flavor Violating Decays τ +-  l +- π 0, l +- η, l +- η’ Exclusive Branching-Fraction Measurements of Semileptonic tau Decays into Three Charged Hadrons, into φ π - ν τ, and into φ K - ν τ Search for the decay τ -  3π - 2π + 2π 0 ν τ Search for Lepton Flavor Violation in the Decay τ +-  e +- γ Search for Lepton-Flavor and Lepton-Number Violation in the Decay τ -  l - + h +- h' - Search for the decay τ -  4π - 3π + (π 0 ) ν τ Study of the τ -  3h - 2h + ν τ decay Search for Lepton Flavor Violation in the Decay τ +-  μ +- gamma Search for Lepton Flavor Violation in the Decay τ  lll 8 on LFV 6 Branching Ratios 1 general

4. Lepton Flavour Violation Lepton Flavour Violation Exists in Standard Model ( ν oscillations) Enhanced Beyond the Standard Model (Charged Higgs, SUSY, etc) to 10 -7, 10 -9.. Large Tau mass makes it good place to look (Higgs link) τ  μμμ,μμe,μee,eee (6 combinations) No events. ~10% efficiency, small backgrounds, limits 1-2 10 -8 τ  μω, eω limits ~10 -7 τ  μγ, eγ τ  μh + h -, eh + h - τ  μπ/η/η’, eπ/η/η’ Τ  Λh 1st April 20094Roger Barlow: Tau Physics with BaBar No neutrinos Can use visible energy and mass to isolate signal (like B physics)

5. Properties Mass measured with 3 prong topology ‘pseudomass’ M τ 2 =M h 2 +2(E τ -E h )(E h -P h Cos θ ) Set Cos θ to 1 and get limit M τ = 1776.68 ±0.12 ±0.41 MeV M + -M - =-0.61 ± 0.23 MeV Lifetime 289.40 ± 0.91 ps 1st April 20095Roger Barlow: Tau Physics with BaBar

6. Branching Ratios B( τ → π π πντ ) = (8.83 ± 0.01 ± 0.13)%, B( τ → K − π − π + ντ ) = (0.273 ± 0.002 ± 0.009)%, B( τ → K − π − K + ντ ) = (0.1346 ± 0.0010 ±0.0036)% B( τ → K − K − K + ντ ) = (1.58 ± 0.13 ± 0.12) × 10 − 5, B( τ − → φπ − ντ ) = (3.42 ± 0.55 ± 0.25) × 10 − 5, B( τ − → φ K − ντ ) = (3.39±0.20±0.28)×10 − 5 Br(5pi)=(8.52 ±0.09±0.40)×10 -4 Br(5 prongs + π 0 π 0 ) < 3.4 10 -6 (@90%CL) 7 prongs not seen < 3 10 -7, 20096Roger Barlow: Tau Physics with BaBar

7. QCD Mass distribution (spectral function) These branching ratios measures α s (M τ )=0.344±0.005±0.007 Extrapolates to α s (M Z )=0.1212±0.0011 Comparison is nice check of QCD 1st April 2009Roger Barlow: Tau Physics with BaBar7 τ W ν q q Hadrons

8. Decays with Kaons 1st April 20098Roger Barlow: Tau Physics with BaBar Example: τ  K 0 s π ν Measure Br=(0.840±0.004±0.023)% |V us |=0.2146 ±0.0030 Incompatible with unitarity? Though BR (τ  K ν)/BR(τ  π ν) is OK This is also a good channel to look for CP violation due to charged Higgs

9. 2 nd class currents 1st April 20099Roger Barlow: Tau Physics with BaBar τ  ωπν Does this occur though the b 1 (1235) I G (J PC )=1 + (1 +- ) This would be a second class current – forbidden by isospin Would show up as flat component in angular distribution 2 nd class fraction < 0.7% (90% CL) Limits also from τ  ηπν, τ  η’πν

10. Future Many tau analyses not yet complete (many not even started) Many more results to come – Tau10 conference should be really exciting 1st April 2009Roger Barlow: Tau Physics with BaBar10