BELLE Tau lifetime and decays D. Epifanov The University of Tokyo 29 May 2014 Outline: 1 Introduction 2 τ lifetime at B-factories 3 0 Decays with KS at B-factories 4 LFV τ decays at LHCb 5 Summary 29 May 2014 Tau lifetime and decays D. Epifanov The University of Tokyo 1/25 Introduction The world largest statistics of τ leptons collected by e+e− B-factories (Belle and BaBar) and LHCb opens new era in the precision tests of the Standard Model (SM). Basic tau properties, like: lifetime, mass, couplings, electric dipole moment, anomalous magnetic dipole moment and other appear as free parameters in the SM (mass), which should be measured experimentally as precise as possible, or provide unique possibility to test SM and search for the effects of New Physics (anomalous magnetic moment). In the SM τ decays due to the charged weak interaction described by the exchange of W ± with a pure vector coupling to only left-handed fermions. There are two main classes of tau decays: − − − − Decays with leptons, like: τ → ℓ ν¯ℓντ , τ → ℓ ν¯ℓντ γ, − − ′+ ′− ′ τ → ℓ ℓ ℓ ν¯ℓντ ; ℓ,ℓ = e, µ. They provide very clean laboratory to probe electroweak couplings, which is complementary/competitive to precision studies with muon (in experiments with muon beam). Plenty of New Physics models can be tested/constrained in the precision studies of the dynamics of decays with leptons. Hadronic decays of τ offer unique tools for the precision study of low energy QCD. 29 May 2014 Tau lifetime and decays D. Epifanov The University of Tokyo 2/25 Introduction: B-factories (Belle and BaBar) BABAR detector ¯ 9 σ(bb)= 1.1 nb Nbb¯ = 1.3 × 10 9 σ(cc¯)= 1.3 nb Ncc¯ = 2.0 × 10 9 σ(ττ)= 0.9 nb Nττ = 1.4 × 10 B-factories are also charm- and τ-factories ! B-factory experimental strategy is proved to be fruitful to search for New Physics. 29 May 2014 Tau lifetime and decays D. Epifanov The University of Tokyo 3/25 Precision studies of τ properties at B-factories Tau lifetime: Belle: τ = (290.17 0.53(stat) 0.33(syst)) fs; PRL 112, 031801 (2014) τ ± ± BaBar(prelim.): τ = (289.40 0.91(stat) 0.90(syst)) fs; Nucl. Phys. B 144, 105 (2005) τ ± ± Tau mass: Belle: m = (1776.61 0.13(stat) 0.35(syst)) MeV/c2; PRL 99, 011801 (2007) τ ± ± BaBar: m = (1776.68 0.12(stat) 0.41(syst)) MeV/c2; PRD 80, 092005 (2009) τ ± ± Accuracy comparable with the most precision measurements done by KEDR and BES at the τ+τ− production threshold. Tau electric dipole moment (EDM): Belle: Re(d ) = (1.15 1.70) 10−17 e cm, Im(d ) = ( 0.83 0.86) 10−17 e cm; τ ± × · τ − ± × · PLB 551, 16 (2003) ( Ldt = 29.5 fb−1) We are working on EDM with full statistics − − 0 Hadronic contributionR to aµ (τ → π π ντ ): Belle: aππ = (523.5 1.1(stat) 3.7(syst)) 10−10; PRD 78, 072006 (2008) µ ± ± × Lepton universality: g BaBar: µ = 1.0036 0.0020, gτ = 0.9850 0.0054, h=π, K; ge τ ± gµ h ± PRL 105,` 051602´ (2010) ` ´ Michel parameters in τ → ℓνν(γ) (ρ, η, ξ, δ, η¯, κ): Belle: study is going on. Anomalous magnetic moment of τ (aτ ): In τ − ℓ−ν¯ ν γ it is not so promising to measure a at B-factories (see the talk: → ℓ τ τ D. Epifanov ”Tau physics at Super B-factory” at KEK-FF2014 Workshop). Technique to measure a in e+e− τ +τ − process is under discussion. τ → 29 May 2014 Tau lifetime and decays D. Epifanov The University of Tokyo 4/25 Measurement of the τ-lepton lifetime, motivation Precise measurement of the tau lifetime is necessary for the tests of lepton universality in the SM: ge = gµ = gτ τ µ e W W gτ g gµ g =e,µ e ν ν ν ν τ µ e − − 2 2 5 (L ℓ ν¯ νL(γ)) g g m Γ(L− ℓ−ν¯ ν (γ)) = B → ℓ = L ℓ L F (m , m ) ℓ L 4 3 corr L ℓ → τL 32MW 192π 2 3 mL α(mL) 25 2 Fcorr(mL, mℓ) = f (x) 1 + 1 + π 5 M2 2π 4 − „ W «„ „ «« f (x) = 1 8x + 8x3 x4 12x2 ln x, x = m /m − − − ℓ L − − (µ e ν¯e ν (γ)) = 1 B → µ 5 gτ τµ m Fcorr(mµ, me) gτ = (τ − µ−ν¯ ν (γ)) µ , = 1.0024 0.0021 (HFAG2012) µ τ 5 ge sB → ττ mτ Fcorr(mτ , mµ) ge ± 5 gτ τµ m Fcorr(mµ, me ) gτ = (τ − e−ν¯ ν (γ)) µ , = 1.0006 0.0021 (HFAG2012) µ τ 5 gµ sB → ττ mτ Fcorr(mτ , me ) gµ ± 29 May 2014 Tau lifetime and decays D. Epifanov The University of Tokyo 5/25 Measurement of the τ-lepton lifetime, motivation LEP EW WG (W → τ ντ)/(W → µ νµ) 1.0390 ± 0.0130 LEP EW WG (W → τ ν )/(W → e ν ) τ → ν ν × τ τ τ e HFAG Fit ( e e τ) µ/ τ ± 1.0008 0.0021 1.0360 ± 0.0140 HFAG Fit (τ → π ντ)/(π → µ νµ) 0.9961 ± 0.0030 HFAG Fit (τ → µ νµ ντ) × τµ/ττ HFAG Fit (τ → K ντ)/(K → µ νµ) ± 0.9857 0.0073 1.0027 ± 0.0021 HFAG Average (τ → π ντ, K ντ) 0.9949 ± 0.0029 τ → ν ν π ν ν HFAG Average ( e e τ, τ, K τ) 0.9998 ± 0.0020 HFAG-Tau HFAG-Tau Summer 2011 Summer 2011 0.9 1 0.9 1 |g /g | |g /g | τ µ τ e S. Schael et al. [ALEPH, DELPHI, L3, OPAL, LEP EWG] Phys. Rep. 532, 119 (2013) 2B(W → τν ) τ = 1.066 ± 0.025 B(W → µνµ)+ B(W → eνe) 2.6σ deviation from the Standard Model 29 May 2014 Tau lifetime and decays D. Epifanov The University of Tokyo 6/25 Measurement of ττ at Belle, method + + + + + We analyze e e− → τ τ − → (π π π−ν¯τ , π π−π−ντ ) events. + Φ − π 1 → CMS π P − 1 frame π Φ → θ 2 1 → − n V ν + 1 τ h1 → l1 → − ψ P V τ 1 ν1 01 − + e e dl + → → τ ψ Pν2 V 2 02 + → → l2 n V h - θ 2 π+ 2 2 → P + ν 2 π − τ π 2 2 2Eτ Eh −M −m ℓ 1,2 τ h1,2 x = cos ψ = βτ γτ 1,2 2pτ p h1,2 τ momentum direction is determined with two-fold ambiguity in CMS, for the analysis we use the average axis. Asymmetric-energy layout of experiment allows us to determine + τ τ − production point in LAB independently from the position of beam IP. + Possibility to test CPT conservation measuring τ − and τ lifetimes separately. 29 May 2014 Tau lifetime and decays D. Epifanov The University of Tokyo 7/25 Measurement of ττ at Belle, selections 1 6 Use the data sample of Ldt = 711 fb− with Nττ = 650 × 10 R Selection criteria: Event is separated into two hemispheres in CMS, Thrust>0.9. Each hemisphere contains 3 charge pions with the 1 net charge. ± 0 0 There are no additional KS , Λ, π candidates. Number of TOT additional photons Nγ < 6 with Eγ < 0.7 GeV. 2 2 P⊥(6π) > 0.5 GeV/c, 4 GeV/c < Minv(6π) < 10.25 GeV/c . 88 2 2 m) µ 87 Pseudomass M + 2(E E )(E P ) < 1.8 GeV/c , ( beam h h h τ h λ − − 86 − + 85 h = (3π) , (q3π) . 84 83 82 Cuts on the quality parameters of the vertex fits and tau axis -2 -1 10 10 1 value of dl cut (cm) reconstruction. 87.3 m) µ 87.2 ( τ Minimal distance between τ − and τ + axes in LAB c 87.1 87 dl < 0.02 cm. 86.9 86.8 86.7 -2 -1 10 10 1 value of dl cut (cm) 1148360 events were selected with ∼2% background contamination, + the main background comes from e e− → qq¯ (q =u, d, s). 29 May 2014 Tau lifetime and decays D. Epifanov The University of Tokyo 8/25 Measurement of ττ at Belle, fit of decay length 770.8 / 794 Decay length PDF P1 0.2692E+05 46.74 P2 -0.7470E-04 0.4629E-05 4 P3 0.3845E-02 0.4587E-04 10 P4 -0.1932E-01 0.1015E-02 P5 0.3237 0.7377E-02 ′ P6 -0.3029 0.1705E-01 −x /λτ ′ ~ ′ ~ (x) = e R(x x ; P)dx + uds R(x; P) + cb(x), m 10 3 P N − N P µ Z 2 x P 2 ( 1) 10 R(x; P~ ) = (1 2.5x) exp − , − · − 2σ2 „ « 10 Entries / 2.5 1/2 3/2 σ = P2 + P3 x P1 + P4 x P1 + P5 x P1 | − | | − | | − | 1 -0.1 -0.075 -0.05 -0.025 0 0.025 0.05 0.075 0.1 Free parameters of the fit: λ , , P~ = (P , ..., P5) x - x (cm) τ N 1 rec true λτ - estimator of cττ , cττ = λτ + ∆corr, ∆corr is determined from MC; R(x; ~P) - detector resolution function; uds - contribution of background from eN+e− qq¯ (q = u, d, s) (predicted by MC) → + − cb(x) - PDF for background from e e qq¯ (q = c, b) (fixedP from MC) → From the fit of experimental data λτ = 86.53 ± 0.16 µm, applying correction ∆corr = 0.46 µm we got: cττ = 86.99 ± 0.16 µm 29 May 2014 Tau lifetime and decays D.