Introduction Analysis Signal region Data Challenge Summary Preliminary studies of the LFV decay τ → µγ at Belle II M. Garc´ ıa Hern´ andez , I. Heredia de la Cruz mgarciah@fis.cinvestav.mx iheredia@fis.cinvestav.mx XXXIII Annual Meeting of the Division of Particles and Fields, SMF (RADPyC) May 27, 2019 Belle (XXXII Annual Meeting) τ → µγ at Belle II 1/16 May 27, 2019 1 / 16
Introduction Analysis Signal region Data Challenge Summary Introduction The τ → µγ is a Lepton Flavor Violating (LFV) process ( L τ (1 → 0) and L µ (0 → 1)) and one of the most promising candidates to search for physics beyond the Standard Model (SM) at Belle II. This process is very suppressed within the SM, B ∼ 10 − 45 [1, 2, 3], however, it has been studied in different new physics models and predictions had been made on the range of 10 − 8 − 10 − 10 [1, 2, 3, 4, 5, 6, 7, 8]. The current limit is 4 . 4 × 10 − 8 at 90 % C.L. reported by Belle with an integrated luminosity of 535 fb − 1 [9]. We performed a sensitivity study of the upper limit on the branching ratio of τ → µγ using simulated MC samples for 1 ab − 1 under the Belle II framework. (XXXII Annual Meeting) τ → µγ at Belle II 2/16 May 27, 2019 2 / 16
Introduction Analysis Signal region Data Challenge Summary τ at Belle II The Belle II experiment is perfectly suited to study τ physics. It will collect 50 times more data than Belle, equivalent to 5 × 10 10 b , c and τ pairs over a period of 8 years [3]. The physics data taking has already started this year and we have already re-discovered the τ lepton. For the LVF decay τ → µγ , we expect to reduce the upper limit on its branching ratio to � 10 − 9 . (XXXII Annual Meeting) τ → µγ at Belle II 3/16 May 27, 2019 3 / 16
Introduction Analysis Signal region Data Challenge Summary SuperKEKB Electrons HER (7 GeV) Positrons LER (4 GeV). Designed to collide at a E CM around the Υ(4 S ) resonance, ∼ 10 . 58 GeV (XXXII Annual Meeting) τ → µγ at Belle II 4/16 May 27, 2019 4 / 16
Introduction Analysis Signal region Data Challenge Summary Belle II Detector KL and muon detector Resistive Plate Counter (barrel outer layers) Scintillator + WLSF + MPPC (end-caps , inner 2 barrel layers) EM Calorimeter CsI(Tl), waveform sampling electronics Belle II Detector [735 collaborators, 101 institutes, Particle Identification 23 nations] electrons (7 GeV) Time-of-Propagation counter (barrel) Prox. focusing Aerogel RICH (forward) Vertex Detector 2 layers Si Pixels (DEPFET) + 4 layers Si double sided strip DSSD positrons (4 GeV) Central Drift Chamber Smaller cell size, long lever arm Belle II TDR, arXiv:1011.0352 (XXXII Annual Meeting) τ → µγ at Belle II 5/16 May 27, 2019 5 / 16
Introduction Analysis Signal region Data Challenge Summary Analysis MC signal sample: τ − → µ − γ (PHSP) including beam background. q ( q = b , c , s , d , u ), τ + τ − , e + e − , µ + µ − , MC background samples: q ¯ e + e − e + e − , e + e − µ + µ − and γγ . Two charged tracks and zero net charge in the event. We apply similar selection requirements as in previous Belle analyses [10, 9], including new variables (e.g. event shape). (XXXII Annual Meeting) τ → µγ at Belle II 6/16 May 27, 2019 6 / 16
Introduction Analysis Signal region Data Challenge Summary Selection requirements Events/0.021 Belle II 2019 Bkg 60000 c c Selection u u d d 50000 s s Muon ID L µ > 0 . 95 τ + τ - 0 B 0 B L π 40000 + - Tag track ID µ < 0 . 80 B B τ - → µ - γ 30000 γ ’s & tracks quality − 1 < ClusterTiming [ ns ] < 1 clusterE 9 E 21 > 0 . 95 20000 P χ 2 > 0 . 001 10000 Kinematics P µ > 1 . 0 GeV 0 − − 1 0.5 0 0.5 1 PCM µ/π < 4 . 5 GeV θ CM Cos µ - γ E γ > 0 . 5 GeV × 3 10 0 . 4 < cos θ CM 2500 µ − γ < 0 . 8 Events/0.004 Belle II 2019 Bkg cos θ CM γ ee( ) µ − π < 0 2000 τ - → µ γ - cos θ R µ − τ < 0 . 4 1500 Event kinematics pmiss > 0 . 4 GeV − 0 . 866 < cos θ miss < 0 . 9535 1000 0 . 4 < cos θ CM π − miss < 0 . 98 500 pmiss > − 5 ∗ m 2 miss − 1 0 pmiss > 1 . 5 ∗ m 2 miss − 1 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 |thrust| ECM sum < 9 . 0 GeV ECM Tot < 12 GeV − 0 . 5 < m 2 ν 0 . 9 < | thrust |∗ < 0 . 98 Event shape Σ N nthr · − → ThrustB < 0 . 95 i | � p i | ∗| thrust | = cos θ T , Bz < 0 . 78 Σ N i |− → p i | τ cc 1 < 5 (XXXII Annual Meeting) τ → µγ at Belle II 7/16 May 27, 2019 7 / 16
Introduction Analysis Signal region Data Challenge Summary Signal region � µγ and ∆ E = E CM µγ − E CM M τ − ≡ M µγ = E 2 µγ − P 2 beam are used to define a signal region. Events/0.002 E [GeV] Bkg 35 Bkg d d Belle II 2019 Belle II 2019 τ τ τ - -> µ ν ν 44 0.1 + - τ - ρ - ν -> 24 s s 30 ∆ τ - π - ν u u -> 20 0 µ µ γ τ ν + - - -> a1- 7 25 τ - → µ - γ τ - - ν ν -> e 14 Another 3 − 0.1 20 τ - → µ - γ 625 − 15 0.2 10 − 0.3 5 − 0.4 0 1.66 1.68 1.7 1.72 1.74 1.76 1.78 1.8 1.82 1.84 1.66 1.68 1.7 1.72 1.74 1.76 1.78 1.8 1.82 1.84 M [GeV] τ M [Gev] - τ - (a) M τ − − ∆ E (b) Classification of τ -pair decays (XXXII Annual Meeting) τ → µγ at Belle II 8/16 May 27, 2019 8 / 16
Introduction Analysis Signal region Data Challenge Summary Signal region A 72 ◦ rotation is applied to the M τ − ∆ E plane to obtain a final signal region with no background events in 2.4 ab − 1 . The signal efficiency in this region is ǫ sig = 4 . 34%. − − 1.6 1.6 *sen(72) [GeV] *sen(72) [GeV] Belle II 2019 Belle II 2019 Bkg Bkg − − 1.65 1.65 d d d d τ τ - + τ + τ - s s s s - - τ τ E*cos(72)-M E*cos(72)-M − u u − 1.7 1.7 u u µ + µ - γ µ µ - γ + τ → µ γ - - τ - → µ - γ − − 1.75 1.75 ∆ ∆ − − 1.8 1.8 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 ∆ ∆ M *cos(72)+ E*sen(72) [GeV] M *cos(72)+ E*sen(72) [GeV] τ - τ - (c) 2.4 ab − 1 (d) 1 ab − 1 (XXXII Annual Meeting) τ → µγ at Belle II 9/16 May 27, 2019 9 / 16
Introduction Analysis Signal region Data Challenge Summary Upper limit We performed the calculation of the upper limit on the branching ratio of τ → µγ , assuming that the background distributions follow a Poisson statistics. For n expected = 0, this sets an upper limit (sensitivity) of n events < 2 . 3 at 90% CL and: bkg 1 ab − 1 n events B ( τ − → µ − γ ) = < 2 . 883 × 10 − 8 @ 90%CL , 2 L int ǫ signal σ ( e + e − → τ + τ − ) consistent with 2 . 726 × 10 − 8 estimated in the Belle II Physic Book [3]. (XXXII Annual Meeting) τ → µγ at Belle II 10/16 May 27, 2019 10 / 16
Introduction Analysis Signal region Data Challenge Summary Data Challenge The Data Challenge sample is equivalent to 1 ab − 1 of collision simulated data, where the truth information was removed to be treated like real data. New physics signals have been injected in these samples (this information remains secret). Previous described selection criteria was applied to the Data Challenge samples (the signal efficiency varies slightly due to a different data preprocessing, ǫ sig = 3 . 88%): − 1.6 *sen(72) [GeV] Belle II 2019 Tau LFV DataChallenge Tau Generic − 1.65 - τ E*cos(72)-M − 1.7 − 1.75 ∆ − 1.8 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 ∆ M *cos(72)+ E*sen(72) [GeV] τ - Figure: Rotated M τ − - ∆ E plane The signal box remains blinded. The analysis is under review by the Belle II Collaboration. Several improvements are being implemented. (XXXII Annual Meeting) τ → µγ at Belle II 11/16 May 27, 2019 11 / 16
Introduction Analysis Signal region Data Challenge Summary Summary A sensitivity study of the τ → µγ decay in Belle II is performed using MC samples for an integrated luminosity of 1 ab − 1 . A signal region with NO background events was found after tight selection requirements in 1 ab − 1 (and up to 2.4 ab − 1 ). Collaboration B ( τ → µγ ) 535 fb − 1 < 4 . 5 × 10 − 8 BELLE (2008) 9 . 6 × 10 8 τ < 4 . 4 × 10 − 8 BABAR (2009) MC (1 ab − 1 ) < 2 . 726 × 10 − 8 BELLE II (Book 2018) MC (1 ab − 1 ) < 2 . 883 × 10 − 8 This analysis This analysis Data Challenge < 3 . 225 × 10 − 8 (assuming 0 observed events) (1 ab-1) Table: Summary of the branching ratio for τ → µγ . The selection criteria is being improved to increase the signal efficiency (e.g BDT selection). “Data Challenge” samples are being analized to improve the analysis techniques. (XXXII Annual Meeting) τ → µγ at Belle II 12/16 May 27, 2019 12 / 16
Recommend
More recommend
