Review of Standard Tau Decays from B Factories

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1 Review of Standard Tau Decays from B Factories Fabrizio Salvatore, RHUL Tau mass measurement Tau decays to strange particles: - Measurement of V us using τ s decays - τ KKπν τ, KKKν τ branching fractions - φ resonance in KKπ and KKK final states Tau decays in modes including η mesons Rare tau decays - τ 5π2π 0 ν τ PHIPSI08 International workshop on e + e - collisions from Phi to Psi

2 B Factories Detectors Asymmetric energy colliders: s =10.58 GeV at Υ(4s) peak σ ( BB) 1.1nb σ ( τ + τ ) 0. 9nb More than just B Factories! Ldt ~ 530 fb 1 BABAR TOF Counter Ldt ~ 800 fb 1 Belle PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 2

3 Tau pair B Factory Tau products are well collimated in the CM system Select events with one ν l,h leptonic (tag) and one hadronic tau decay τ tag τ sig e - e + Divide event into two hemispheres (thrust axis) PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 3

4 Tau mass and test of CPT In SM, high precision measurements of mass, lifetime and BF of τ can be used to test lepton universality Present limit on m τ dominated by BES (PRD 53 (1996) 20) Accuracy ~ 0.3 MeV; also obtained by KEDR collaboration (arxiv: [hep-ex]) Similar accuracy reached by Belle experiment Significant improvement expected by combining these high precision measurements Analysis of τ lepton decays allows to measure separately m τ + and m τ - and test CPT theorem Similar test from OPAL: (m τ + -m τ -)/m avg < 90% CL (PL B492 (2000) 23) Many systematic errors cancel in the difference Cannot be measured by threshold experiments Significant improvement with current τ statistics at B Factories PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 4

5 Tau mass results Analysis uses pseudo-mass technique: M min = M 2 X + 2( E beam E X )( E X P X ) 414 fb -1 M min m τ ; in absence of ISR and FSR, M min has an edge at m t Use τ - l - ν l ν τ ; τ + π + π - π + (π 0 )ν τ decays and fit M min F(X) = (p 3 +p 4 X)atan((X-p 1 )/p 2 ) + p 5 + p 6 X m τ = ±0.13±0.35 MeV Dominant systematic: beam and tracking uncertainties PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 5

6 Tau mass results Analysis uses pseudo-mass technique: M min = M 2 X + 2( E beam E X )( E X P X ) 414 fb -1 M min m τ ; in absence of ISR and FSR, M min has an edge at m t Use τ - l - ν l ν τ ; τ + π + π - π + (π 0 )ν τ decays and fit M min F(X) = (p 3 +p 4 X)atan((X-p 1 )/p 2 ) + p 5 + p 6 X 414 fb -1 m τ = ±0.13±0.35 MeV Dominant systematic: beam and tracking uncertainties Fit separately m τ + and m τ - m τ + -m τ - /m avg < PRL 99 (2007) PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 6

7 Tau decays to strange particles Hadronic tau decays provide clean laboratory for the study of the hadronic weak current For τ decays to states with Δs=1, isospin symmetry breaking and OPE can be used to determine V us and m s In τ s decays the uncertainty on V us and m s is dominated by experimental uncertainties: V us : ~ O(1%) [from 3-body leptonic kaon decays] m s : ~ O(10 MeV) [from Lattice QCD] Use large statistics collected at B Factories to reduce uncertainties V us 0.7% If spectral function is measured simultaneous fit of V us and m s PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 7

8 Before B Factories: BF (τ s)( PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 8

9 Before B Factories: V us and m s PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 9

10 Good K/π separation τ K - π 0 ν τ decay Discriminate Cabibbo suppressed τ s signal from τ d bg 1-1 topology for signal events: Tag & probe technique tag τ ν ν l + τ + τ - ν π 0 K - signal τ PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 10

11 Kπ invariant mass distribution BF (τ( - K - π 0 ν τ ) data 230 fb -1 ττ bkg qq μμ PRD 76 (2007) (R) ε (2.267±0.008)% N data N BG ± 159 PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 11

12 τ - K 0 π - ν τ decay 351 fb -1 BF(%) τ - K s π - ν τ τ - K 0 π - ν τ τ - K * ν τ, K * K s π ± 0.002(stat) ± 0.013(syst) ± 0.004(stat) ± 0.026(syst) ± 0.002(stat) ± 0.012(syst) ± 0.012(mod) PL B654 (2007) 65 PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 12

13 Comparison of BF and m(k*) with PDG Best description of K* mass spectrum obtained considering the K*(800)+K*(892)+K*(1410) and K*(800)+K*(892)+K*(1430) models m(k*(892) - ) ± 0.20(stat) ± 0.44(syst) ± 0.59(mod) MeV/c 2 Γ(K*(892) - ) 46.2 ± 0.6(stat) ± 1.0(syst) ± 0.7(mod) MeV PL B654 (2007) 65 PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 13

14 Exclusive BF (τ( - h - h + h - v τ ) KπK KKK πππ Kππ Data sample: 342 fb -1 PRL 100 (2008) PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 14

15 φ peak in τ KKπν τ and τ KKKν τ KKπ 342 fb -1 KKK Resonant decay is consistent with saturating τ -- K - K + K - ν τ channel B(τ -- K - K + K - ν τ ex. Φ) < 2.5 x 10-6 PRL 100 (2008) PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 15

16 Update on V us using B Factory results [1] Updated Results: [2] [2] [2] BABAR: τ K - π 0 ν τ PRD 76 (2007) (R) τ K π + π - ν τ PRL 100 (2008) [3] 3.5σ 3.2σ Belle: τ K 0 π - ν τ PLB 654 (2007) 65 [1] Mateo Palutan, Kaon 07 [2] Matthias Jamin, Moriond EW 07 [3] Swagato Banerjee arxiv: [hep-ex] Theory prediction B(τ Kν τ ) from B(K μν μ (γ)) assuming μ τ universality arxiv: v4 [hep-ex] PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 16

17 τ decays in modes with η meson Important for testing different theoretical models E.g.: Weiss-Zumino-Witten (WZW) anomaly, chiral theory, relations to σ e +e- following from (CVC) Studies of tau decay modes including η mesons have been previously reported by CLEO and ALEPH Low statistics: fb -1 used in the analysis Difficult to discriminate between different predictions Belle s analysis uses 485 fb -1 of data 100 times larger than any previous dataset Better systematics thanks to precise estimation of peaking background PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 17

18 Branching fraction results 485 fb -1 Improved accuracy in BF by factor 4-8 wrt previous measurements τ - K - η ν τ η γγ η π + π π 0 τ - K - π 0 η ν τ τ - π - π 0 η ν τ B(τ - K - η ν τ ) B(τ - K - π 0 ην τ ) B(τ - K*(892) - ην τ ) B(τ - π - π 0 ην τ ) (1.62 ± 0.05 ± 0.09)x10-4 (4.7 ± 1.1 ± 0.4)x10-5 (1.13 ± 0.19 ± 0.07)x10-4 (1.39 ± 0.03 ± 0.07)x10-3 Fit γγ distribution using Crystal Ball + polynomial for bg η γγ η γγ arxiv: v1[hep-ex]) PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 18

19 Comparison with predictions signal ττ bg B(τ - π - π 0 ην τ ) consistent with prediction based on CVC and experimentally measured e + e - π+π-η cross section Good agreement between data and MC (TAUOLA) arxiv: v1[hep-ex]) PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 19

20 Comparison with predictions signal ττ bg B(τ - π - π 0 ην τ ) consistent with prediction based on CVC and experimentally measured e + e - π+π-η cross section Good agreement between data and MC (TAUOLA) Central value of B(τ - K - ην τ ) and B(τ - K - π 0 ην τ ) slightly different from chiral theory prediction (Phys. Rev. D 55 (1997) 1436) arxiv: v1[hep-ex]) More tuning of MC needed Further studies of final state dynamics and resonance formation in progress PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 20

21 Rare tau decays τ (5π) 2π 0 ν τ decay BABAR has ~200 times more data Previous Experimental limit: B( τ 5h 2π 0 ν τ ) < % CL (CLEO 1994, 1.7 fb -1 ) Tiny phase space suppresses 8-body τ decays No BR prediction Decay is likely to go through τ 2ωπν τ (R. Sobie, PRD 60, (1999)) Select signal using pseudo-mass assume neutrino is mass-less approximate τ direction by 7 tracks Standard tag & probe technique m* τ2 =2(E beam E 7π )(E 7π P 7π )+m 7π 2 signal region: 1.3 < m* τ < 1.8 GeV/c 2 Improves rejection of qq background τ tag τ rec e - e + 5-prong τ decay PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 21

22 BF (τ( 5π2π 0 ν τ ) Background PDF 232 fb -1 τ 2ωπ ν τ Signal counting region Signal counting region Signal efficiency 0.66 ± 0.05 % Total expected bkg. Observed events Signal efficiency 1.53 ± 0.13 % Total expected bkg. Observed events Multi-pion mode PDG2004 BABAR τ 5π 2π 0 ν τ < < (PRD 74 (2006) ) τ 2ω πν τ N/A < (PRD 74 (2006) ) τ 7π (π 0 )ν τ < < (PRD 72 (2005) ) PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 22

23 Conclusions Exciting tau physics programme at the B Factories B Factories are also Tau Factories! Huge tau statistics allow unprecedented precision in measurements of properties of tau lepton Measurement of tau mass and test of CPT theorem τ decays to strange particles Measurement of BFs and new measurement of V us from tau decays First evidence of τ KKK ν τ decay mode Resonance in τ KKπ ν τ and τ KKK ν τ decays τ decays in modes containing η meson Tests various theoretical models (WZW, chiral, CVC) Important to study backgrounds for 2 nd class currents Rare tau decays PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 23

24 Conclusions Exciting tau physics programme at the B Factories B Factories are also Tau Factories! Huge tau statistics allow unprecedented precision in measurements of properties of tau lepton Measurement of tau mass and test of CPT theorem τ decays to strange particles Measurement of BFs and new measurement of V us from tau decays First evidence of τ KKK ν τ decay mode Resonance in τ KKπ ν τ and τ KKK ν τ decays τ decays in modes containing η meson Tests various theoretical models (WZW, chiral, CVC) Important to study backgrounds for 2 nd class currents Rare tau decays Many more results to follow PhiPsi08, 10 Apr 2008 Fabrizio Salvatore, RHUL 24