BELLE Experimental review of τ lepton studies at the B factories D. Epifanov (NSU, BINP) 27 June 2017 Outline: 1 Introduction 2 Precision studies of τ properties 3 CPV in hadronic τ decays 4 LFV τ decays Summary 5 Experimental review of τ lepton studies at the B factories 27 June 2017 D. Epifanov (NSU, BINP) 1/33
Introduction The world largest statistics of τ leptons collected by e + e − B factories (Belle and BABAR ) 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, etc. should be measured experimentally as precisely as possible in order to test SM and search for the effects of New Physics. 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 ℓ ν τ , τ − → ℓ − ¯ ν ℓ ν τ γ , τ − → ℓ − ℓ ′ + ℓ ′− ¯ ν ℓ ν τ ; 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. Experimental review of τ lepton studies at the B factories 27 June 2017 D. Epifanov (NSU, BINP) 2/33
Introduction: e + e − B factories BABAR detector σ ( b ¯ b ) = 1 . 05 nb N b ¯ b = 1 . 2 × 10 9 σ ( c ¯ c ) = 1 . 30 nb N c ¯ c = 2 . 0 × 10 9 σ ( ττ ) = 0 . 92 nb N ττ = 1 . 4 × 10 9 B factories are also charm and τ factories ! B factory experimental strategy is proved to be fruitful to search for New Physics. Experimental review of τ lepton studies at the B factories 27 June 2017 D. Epifanov (NSU, BINP) 3/33
Precision studies of τ at e + e − B factories Michel parameters in τ → ℓνν ( ρ , η , ξ , δ ) at Belle : arXiv:1409.4969 Study of the radiative leptonic decays τ → ℓννγ : BABAR : Measurement of B ( τ → ℓννγ ) ; PRD 91, 051103(R) (2015) Belle (prelim.): ¯ η = − 1 . 3 ± 1 . 5 ± 0 . 8 , ξκ = 0 . 5 ± 0 . 4 ± 0 . 2 ; arXiv:1609.08280 Lepton universality with τ → ℓνν and τ → h ν (h= π ,K) at BABAR : ` g µ ` g τ ´ ´ τ = 1 . 0036 ± 0 . 0020, h = 0 . 9850 ± 0 . 0054; PRL 105, 051602 (2010) ge g µ Tau lifetime: Belle : τ τ = ( 290 . 17 ± 0 . 53 ( stat ) ± 0 . 33 ( syst )) fs; PRL 112, 031801 (2014) BABAR (prelim.): τ τ = ( 289 . 40 ± 0 . 91 ( stat ) ± 0 . 90 ( syst )) fs; Nucl. Phys. B 144, 105 (2005) Tau mass: Belle : m τ = ( 1776 . 61 ± 0 . 13 ( stat ) ± 0 . 35 ( syst )) MeV/ c 2 ; PRL 99, 011801 (2007) BABAR : m τ = ( 1776 . 68 ± 0 . 12 ( stat ) ± 0 . 41 ( syst )) MeV/ c 2 ; PRD 80, 092005 (2009) Accuracy comparable with the most precision measurements done by BES and KEDR at the τ + τ − production threshold. Tau electric dipole moment (EDM): Belle : Re ( d τ ) = ( 1 . 15 ± 1 . 70 ) × 10 − 17 e · cm, Im ( d τ ) = ( − 0 . 83 ± 0 . 86 ) × 10 − 17 e · cm; Ldt = 29 . 5 fb − 1 ) We are working on EDM with full Belle statistics R PLB 551, 16 (2003) ( Hadronic contribution to a µ ( τ − → π − π 0 ν τ ): Belle : a ππ = ( 523 . 5 ± 1 . 1 ( stat ) ± 3 . 7 ( syst )) × 10 − 10 ; PRD 78, 072006 (2008) µ Experimental review of τ lepton studies at the B factories 27 June 2017 D. Epifanov (NSU, BINP) 4/33
Michel parameters In the SM charged weak interaction is described by the exchange of W ± with a pure vector coupling to only left-handed fermions (”V-A” Lorentz structure). Deviations from ”V-A” indicate New Physics. τ − → ℓ − ¯ ν ℓ ν τ ( ℓ = e , µ ) decays provide clean laboratory to probe electroweak couplings. The most general, Lorentz invariant four-lepton interaction matrix element: M = 4 G » –» – g N u i ( l − )Γ N v n (¯ u m ( ν τ )Γ N u j ( τ − ) X ¯ ¯ √ ν l ) , ij 2 N = S , V , T i , j = L , R i Γ S = 1 , Γ V = γ µ , Γ T = ( γ µ γ ν − γ ν γ µ ) √ 2 2 Ten couplings g N ij , in the SM the only non-zero constant is g V LL = 1 Four bilinear combinations of g N ij , which are called as Michel parameters (MP): ρ , η , ξ and δ appear in the energy spectrum of the outgoing lepton: d Γ( τ ∓ ) = 4 G 2 F M τ E 4 „ x ( 1 − x ) + 2 q 9 ρ ( 4 x 2 − 3 x − x 2 max x 2 − x 2 0 ) + η x 0 ( 1 − x ) d Ω dx 0 ( 2 π ) 4 E ℓ m ℓ ∓ 1 » 1 − x + 2 ´–« q q 3 P τ cos θ ℓ ξ x 2 − x 2 3 δ ` 4 x − 4 + 1 − x 2 , x = , x 0 = E max E max 0 0 In the SM: ρ = 3 4 , η = 0, ξ = 1, δ = 3 4 Experimental review of τ lepton studies at the B factories 27 June 2017 D. Epifanov (NSU, BINP) 5/33
Status of Michel parameters in τ decays Michel par. Measured value Experiment SM value ALEPH ALEPH 0.752+/-0.019 0.086+/-0.078 ρ DELPHI DELPHI 0 . 747 ± 0 . 010 ± 0 . 006 CLEO-97 3/4 0.790+/-0.038 0.06+/-0.11 L3 L3 1.2% 0.762+/-0.035 0.27+/-0.14 ( e or µ ) OPAL OPAL η 0.781+/-0.033 0.027+/-0.055 0 . 012 ± 0 . 026 ± 0 . 004 ALEPH-01 0 SLD 0.72+/-0.09 2.6% CLEO CLEO ( e or µ ) 0.747+/-0.012 0.015+/-0.087 ARGUS ARGUS 0.731+/-0.031 0.03+/-0.22 ξ ρ η 1 . 007 ± 0 . 040 ± 0 . 015 CLEO-97 1 0.750+/-0.011 0.048+/-0.035 4.3% ( e or µ ) ALEPH ALEPH 1.000+/-0.076 0.782+/-0.051 ξδ DELPHI DELPHI 0 . 745 ± 0 . 026 ± 0 . 009 CLEO-97 3/4 0.974+/-0.061 0.699+/-0.028 L3 L3 2.8% 0.70+/-0.16 0.70+/-0.11 ( e or µ ) OPAL OPAL 0.98+/-0.24 0.65+/-0.16 SLD SLD ξ h 1.05+/-0.35 0.88+/-0.27 0 . 992 ± 0 . 007 ± 0 . 008 ALEPH-01 1 CLEO CLEO 1.010+/-0.043 0.745+/-0.028 1.1% ARGUS ARGUS 1.03+/-0.11 0.63+/-0.09 (all hadr.) ξ ξδ 0.988+/-0.029 0.735+/-0.020 With × 300 Belle statistics we can improve MP uncertainties by one order of magnitude In BSM models the couplings to τ are expected to be enhanced in comparison with µ . Also contribution from New Physics in τ decays can be amplified by ( m τ m µ ) n . « 2 „ Type II 2HDM: η µ ( τ ) = m µ M τ tan 2 β η e ( µ ) = M τ ; η µ ( τ ) m e ≈ 3500 M 2 2 H ± κ W „ «ff g 2 W µ νγ µ ( 1 − γ 5 ) τ + 2 m τ ∂ ν νσ µ nu ( 1 − γ 5 ) τ Tensor interaction: L = ¯ τ ¯ , √ 2 − 0 . 096 < κ W τ < 0 . 037: DELPHI Abreu EPJ C16 (2000) 229. Unparticles: Moyotl PRD 84 (2011) 073010, Choudhury PLB 658 (2008) 148. Lorentz and CPTV: Hollenberg PLB 701 (2011) 89 Heavy Majorana neutrino: M. Doi et al. , Prog. Theor. Phys. 118 (2007) 1069. Experimental review of τ lepton studies at the B factories 27 June 2017 D. Epifanov (NSU, BINP) 6/33
Method, study of ( ℓνν ; ρν ) and ( ρν ; ρν ) events Effect of τ spin-spin correlation is used to measure ξ and δ MP . Events of the ( τ ∓ → ℓ ∓ νν ; τ ± → ρ ± ν ) topology are used to measure: ρ , η , ξ ρ ξ and ξ ρ ξδ , while ( τ ∓ → ρ ∓ ν ; τ ± → ρ ± ν ) events are used to extract ξ 2 ρ . τ helicity − µ− Φ 1 τ+ helicity 0.78 µ ν ν ρ ν + - ( ; ) Φ − 2 0.77 ν µ RH ξ 0.76 ν τ H ρ p τ+ τ− χ ( ) α LH ρ 0.75 MC π+ τ+ L H σ 0.74 1 ρ+ ψ σ ∆ η ≈ ∆ ρ 2 0.73 Fit result 4 R H ξ ρ ρ+ σ 3 0.72 π 0 η -0.12 -0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.1 − ν τ d σ ( ℓ ∓ νν, ρ ± ν ) 4 = A 0 + ρ A 1 + η A 2 + ξ ρ ξ A 3 + ξ ρ ξδ A 4 = X A i Θ i dE ∗ ℓ d Ω ∗ ℓ d Ω ∗ ρ dm 2 ππ d ˜ Ω π d Ω τ i = 0 Φ 2 d σ ( ℓ ∓ νν, ρ ± ν ) d σ ( ℓ ∓ νν, ρ ± ν ) ∂ ( E ∗ ℓ , Ω ∗ ℓ , Ω ∗ ρ, Ω τ ) ˛ ˛ Z z ) = ˛ d Φ τ ˛ ˛ F ( � = ˛ ˛ dE ∗ ℓ d Ω ∗ dp ℓ d Ω ℓ dp ρ d Ω ρ dm 2 ππ d ˜ ℓ d Ω ∗ ρ dm 2 ππ d ˜ Ω π d Ω τ ∂ ( p ℓ, Ω ℓ, p ρ, Ω ρ, Φ τ ) Ω π ˛ Φ 1 N Z P ( k ) , P ( k ) = F ( � z ( k ) ) / N ( � L = Y Θ) , N ( � z ) d � z , � Θ) = F ( � Θ = ( 1 , ρ, η, ξ ρ ξ ℓ , ξ ρ ξ ℓ δ ℓ ) k = 1 4 λ i ) P ℓ − ρ signal + λ 1 P ℓ − 3 π + λ 2 P π − ρ + λ 3 P ρ − ρ X + λ 4 P other P total = ( 1 − ( MC ) bg bg bg bg i = 1 MP are extracted in the unbinned maximum likelihood fit of ( ℓνν ; ρν ) events in the 9D phase z = ( p ℓ , cos θ ℓ , φ ℓ , p ρ , cos θ ρ , φ ρ , m 2 ππ , cos ˜ θ π , ˜ space � φ π ) in CMS. Experimental review of τ lepton studies at the B factories 27 June 2017 D. Epifanov (NSU, BINP) 7/33
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