[PPT] - Preliminary studies of the LFV decay at Belle II M. Garc a Hern PowerPoint Presentation

SLIDE 1

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

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Introduction Analysis Signal region Data Challenge Summary

Introduction

The τ → µγ is a Lepton Flavor Violating (LFV) process (Lτ (1 → 0) and Lµ (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

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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 × 1010 b, c and τ pairs over a period

f 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

n its branching ratio to

10−9.

(XXXII Annual Meeting) τ → µγ at Belle II 3/16 May 27, 2019 3 / 16

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Introduction Analysis Signal region Data Challenge Summary

SuperKEKB

Electrons HER (7 GeV) Positrons LER (4 GeV). Designed to collide at a ECM around the Υ(4S) resonance, ∼ 10.58 GeV

(XXXII Annual Meeting) τ → µγ at Belle II 4/16 May 27, 2019 4 / 16

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Introduction Analysis Signal region Data Challenge Summary

Belle II Detector

Belle II Detector [735 collaborators, 101 institutes, 23 nations]

electrons (7 GeV) positrons (4 GeV)

Vertex Detector

2 layers Si Pixels (DEPFET) +   4 layers Si double sided strip DSSD Belle II TDR, arXiv:1011.0352

EM Calorimeter

CsI(Tl), waveform sampling electronics

Central Drift Chamber

Smaller cell size, long lever arm

Particle Identification

Time-of-Propagation counter (barrel)

Prox. focusing Aerogel RICH (forward)

KL and muon detector

Resistive Plate Counter (barrel outer layers) Scintillator + WLSF + MPPC   (end-caps , inner 2 barrel layers)

(XXXII Annual Meeting) τ → µγ at Belle II 5/16 May 27, 2019 5 / 16

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Introduction Analysis Signal region Data Challenge Summary

Analysis

MC signal sample: τ − → µ−γ (PHSP) including beam background. MC background samples: q¯ q (q = b, c, s, d, u), τ +τ −, e+e−, µ+µ−, 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

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Introduction Analysis Signal region Data Challenge Summary

Selection requirements

Selection Muon ID Lµ > 0.95 Tag track ID Lπ µ < 0.80 γ’s & tracks quality −1 < ClusterTiming[ns] < 1 clusterE9E21 > 0.95 Pχ2 > 0.001 Kinematics Pµ > 1.0 GeV PCM µ/π < 4.5 GeV Eγ > 0.5 GeV 0.4 < cos θCM µ−γ < 0.8 cos θCM µ−π < 0 cos θR µ−τ < 0.4 Event kinematics pmiss > 0.4 GeV −0.866 < cos θmiss < 0.9535 0.4 < cos θCM π−miss < 0.98 pmiss > −5 ∗ m2 miss − 1 pmiss > 1.5 ∗ m2 miss − 1 ECM sum < 9.0 GeV ECM Tot < 12 GeV −0.5 < m2 ν Event shape 0.9 < |thrust|∗ < 0.98 ThrustB < 0.95 cos θT,Bz < 0.78 τ cc1 < 5

CM γ

µ

θ Cos 1 − 0.5 − 0.5 1 Events/0.021 10000 20000 30000 40000 50000 60000 Belle II 2019

Bkg c c u u d d s s

τ

+

τ B B

B

+

B γ

µ

→

τ

|thrust| 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 Events/0.004 500 1000 1500 2000 2500

3

10 × Belle II 2019

Bkg ) γ ee( γ

µ

→

τ

∗|thrust| = ΣN i | nthr · − → p i | ΣN i |− → p i | (XXXII Annual Meeting) τ → µγ at Belle II 7/16 May 27, 2019 7 / 16

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Introduction Analysis Signal region Data Challenge Summary

Signal region

Mτ − ≡ Mµγ =

E 2

µγ − P2 µγ and ∆E = E CM µγ − E CM beam are used to define a signal

region.

[GeV]

τ

M 1.66 1.68 1.7 1.72 1.74 1.76 1.78 1.8 1.82 1.84 E [GeV] ∆ 0.4 − 0.3 − 0.2 − 0.1 − 0.1 Belle II 2019

Bkg d d

τ

+

τ s s u u γ

µ

+

µ γ

µ

→

τ

(a) Mτ− − ∆E

[Gev]

τ

M 1.66 1.68 1.7 1.72 1.74 1.76 1.78 1.8 1.82 1.84 Events/0.002 5 10 15 20 25 30 35 Belle II 2019

Bkg 44 ν ν µ

>
τ

24 ν

ρ
>
τ

20 ν

π
>
τ

7 ν

> a1-
τ

14 ν ν

> e
τ

Another 3 625 γ

µ

→

τ

(b) Classification of τ-pair decays

(XXXII Annual Meeting) τ → µγ at Belle II 8/16 May 27, 2019 8 / 16

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Introduction Analysis Signal region Data Challenge Summary

Signal region

A 72◦ rotation is applied to the Mτ − ∆E plane to obtain a final signal region with no background events in 2.4 ab−1. The signal efficiency in this region is ǫsig = 4.34%.

E*sen(72) [GeV] ∆ *cos(72)+

τ

M 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 *sen(72) [GeV]

τ

E*cos(72)-M ∆ 1.8 − 1.75 − 1.7 − 1.65 − 1.6 − Belle II 2019 Bkg d d

τ

+

τ s s u u γ

µ

+

µ γ

µ

→

τ

(c) 2.4 ab−1

E*sen(72) [GeV] ∆ *cos(72)+

τ

M 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 *sen(72) [GeV]

τ

E*cos(72)-M ∆ 1.8 − 1.75 − 1.7 − 1.65 − 1.6 − Belle II 2019 Bkg d d

τ

+

τ s s u u γ

µ

+

µ γ

µ

→

τ

(d) 1 ab−1

(XXXII Annual Meeting) τ → µγ at Belle II 9/16 May 27, 2019 9 / 16

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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 nexpected

bkg

= 0, this sets an upper limit (sensitivity) of nevents < 2.3 at 90% CL and: 1 ab −1 B(τ − → µ−γ) = nevents 2Lintǫsignalσ(e+e−→τ +τ −) < 2.883 × 10−8 @ 90%CL, 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

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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, ǫsig = 3.88%):

E*sen(72) [GeV] ∆ *cos(72)+

τ

M 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 *sen(72) [GeV]

τ

E*cos(72)-M ∆ 1.8 − 1.75 − 1.7 − 1.65 − 1.6 − DataChallenge Belle II 2019

Tau LFV Tau Generic

Figure: Rotated Mτ− - ∆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

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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) BELLE (2008) 535 fb−1 < 4.5 × 10−8 BABAR (2009) 9.6 × 108 τ < 4.4 × 10−8 BELLE II (Book 2018) MC (1 ab−1) < 2.726 × 10−8 This analysis MC (1 ab−1) < 2.883 × 10−8 This analysis Data Challenge (assuming 0 observed events) (1 ab-1) < 3.225 × 10−8

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

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Introduction Analysis Signal region Data Challenge Summary

References I

Benjamin W. Lee and Robert E Shrock. Natural suppression of symmetry violation in gauge theories: Muon- and electron-lepton-number nonconservation.

Phys. Rev. D16 1444-1473 (1977).

Ta-Pei Cheng and Ling-Fong Li. Muon-number-nonconservation effects in a gauge theory with v + a currents and heavy neutral leptons.

Phys. Rev. D 16, 5 1425-1443 (1977).

Emi Kou, Phillip Urquijo, The Belle II collaboration, and The B2TiP theory community. The Belle II physics book.

Prog. Theor. Exp. Phys. (2018).

Andrea Brignole and Anna Rossi. Lepton flavour violating decays of supersymmetric higgs bosons. Phys.Lett. B566 (2003) 217-225 hep-ph/0304081 DFPD-03-TH-11. Monika Blanke, Andrzej J. Buras, Bj¨

rn Duling, Anton Poschenrieder, and Cecilia Tarantino.

Charged lepton flavour violation and (g − 2) µ in the littlest higgs model with t-parity: a clear distinction from supersymmetry. JHEP 0705, 013 (2007), arXiv:hep-ph/0702136.

G. Cvetiˇ

c, C. Dib, C. S. Kim, and J. D. Kim. On lepton flavor violation in tau decays.

Phys. Rev. D66, 034008 (2002), arXiv:hep-ph/0202212.

Chong-xing Yue, Yan-ming Zhang, and Lan-jun Liu. Nonuniversal gauge bosons z-prime and lepton flavor violation tau decays.

Phys. Lett. B547 252-256 (2002).

(XXXII Annual Meeting) τ → µγ at Belle II 13/16 May 27, 2019 13 / 16

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Introduction Analysis Signal region Data Challenge Summary

References II

Takeshi Fukuyama and Tatsuru Kikuchi. Lepton flavor violating processes and muon g − 2 in minimal supersymmetric so(10) model.

Phys. Rev. D68, 033012 (2003), arXiv:hep-ph/0304190.

The Belle collaboration. New search for τ → µγ and τ → eγ decays at Belle.

Phys. Lett. B 666 16-22 (2008).

The Belle collaboration. Upper bound on the decay τ → µγ from the Belle detector.

Phys. Rev. Lett. 92 (2004).

(XXXII Annual Meeting) τ → µγ at Belle II 14/16 May 27, 2019 14 / 16

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Introduction Analysis Signal region Data Challenge Summary

BACKUP

(XXXII Annual Meeting) τ → µγ at Belle II 15/16 May 27, 2019 15 / 16

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Introduction Analysis Signal region Data Challenge Summary MC10 samples Cross section [nb] B ¯ B (mixed /charged) 1.05 ± 0.10 u¯ u(γ) (uubar) 1.61 d ¯ d(γ) (ddbar) 0.40 s¯ s(γ) (ssbar) 0.38 c¯ c(γ) (ccbar) 1.30 τ+τ−(γ) (taupair) 0.919 Low multiplicity e+e− (ee) 300 ± 3 µ+µ− (mm) 1.152 e+e−e+e− (eeee) 39.8 ± 3 e+e−µ+µ− (eemumu) 19.1 ± 0.1 γγ (gg) 25.3 ± 0.1

Table: Summary of the cross section for the MC samples.

(XXXII Annual Meeting) τ → µγ at Belle II 16/16 May 27, 2019 16 / 16