Prospects for � lepton physics at Belle II

Michel Hernández Villanueva Outline: Department of Physics Cinvestav, Mexico • Achievements of B-factories in � lepton physics. On behalf of the Belle II collaboration • The Belle II experiment.

15th International Workshop on • First results with early data. Tau Lepton Physics • Prospects of � lepton Amsterdam, Netherlands, Sep 24, 2018 physics

Rap God – Eminem B Factories

• B-Factory: Production of b pairs.

• � factory too! �(e+e- —> �(4s)) = 1.05 nb �(e+e- —> � �) = 0.92 nb

Michel H. Villanueva 2 � lepton physics results at B factories

Lint (fb-1)

Lifetime and CPT test CP-Violation

LFV limits

Mass and CPT test CP-Violation LFV limits Limits in Mass and CPT test τ → μγ SCC searches Belle Limits in τ → ℓℓℓ BaBar Electric Limits in dipole limit τ → μγ τ → ℓℓℓ

Michel H. Villanueva 3 Next gen: Belle II collaboration

• 800+ members, 108 institutions, 25 countries

• Located in KEK at Tsukuba, Japan

Mt. Tsukuba

Linac Michel H. Villanueva 4 Next gen: SuperKEKB

• Super B-Factory (And � factory too!)

�(e+e- —> �(4s)) = 1.05 nb �(e+e- —> � �) = 0.92 nb

• Integrated luminosity expected: 50 ab-1

(x50 than previous B factories)

4.6x1010 � pairs

@KEK Tsukuba, Japan

Michel H. Villanueva 5 Next gen: SuperKEKB

“Nano-beams”: vertical beam size is 50nm at the IP.

KEKB

• Challenges at L=8x1035 1/cm2/s:

- Higher background (Radiative Bhabha, Touschek, beam- SuperKEKB gas scattering, etc.).

- Higher trigger rates (High performance DAQ, computing).

Michel H. Villanueva 6 Belle II Detector

Michel H. Villanueva 7 SuperKEKB luminosityBelle II Schedule projection

Data taking in phase II was performed with all Goal of Be!e II/SuperKEKB subsystems, except vertex detectors.

)

-1 3 months

(ab They are being installed and they will be ready for phase III. Integrated luminosity 9 months/year 20 days/month

) -1 s -2 (cm Peak luminosity

Calendar Year Phase II Phase III (Partial Belle II) (Full Belle II)

Michel H. Villanueva 8 First collisions on Phase II

Most of the Belle II detector subsystems are working well. We have signals involving photons and charged tracks.

+ − Ks → π π

π0 → γγ

Michel H. Villanueva 9 �→3�� in Belle II early data

Candidates: 3 - 1 prong decay + − e e → (τ → 3 tracks)(τtag → track) hadrons

We are assuming pion hypothesis in signal side.

⌫⌧ • Thrust axis: nˆ thrust such that Vthrust is maximum. ⌧ 1800 Signal Belle II Simulation 1600 ⌧ nˆthrust Events 1400 ττ Tag 1200 uu+dd+ss cc 1000 eeγ 800 other ` ⌫¯` 600 ⌫⌧ 400 cm 200 i p~i nˆthrust Vthrust = | · cm | 0 p~i 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 P i | | thrust value P Michel H. Villanueva 10 �→3�� in Belle II early data

1000 Belle II 2018 (Preliminary) Data After selection cuts, we

Events have an agreement 800 MC total (stat ⊕ sys) Ldt = 291 pb-1 ± between distributions in ∫ τsig → 3π ν data an MC. 600 τsig → other qq Performance of the 400 other subsystems is good.

200

0 1.5 1 0.5 M ± [GeV] Data / MC 3π 0 0.5 1 1.5 2 2.5 3 3.5

M 3π± [GeV] M3� distribution @ 291 pb-1

Michel H. Villanueva 11 �→3�� in Belle II early data

Michel H. Villanueva 12 Measurement of � mass

• Measured in the decay mode Mmin distribution @ 291 pb-1: � → 3��,

600 using a pseudomass technique Belle II 2018 (Preliminary) Data

Events MC total (stat ⊕ sys) developed by the ARGUS 500 -1 Ldt = 291 pb ∫ τ → 3π±ν collaboration: sig 400 τsig → other qq 300 • M = M2 + 2(E − E )(E − P ) other min 3π beam 3π 3π 3π 200

100 • The distribution of the pseudomass 0 is fitted to a empirical edge function. 1.5 1 0.5 M [GeV] Data / MC min • A first measurement of m� at Belle II 0 0.5 1 1.5 2 2.5 3 3.5 M [GeV] is performed using the data collected min during the Phase II.

Michel H. Villanueva 13 Measurement of � mass

) 70 Our result, obtained from Belle II early 2 Belle II 60 data 2018 (Preliminary) 50 2 Data m� = (1776.4 ± 4.8 (stat)) MeV/c

40 -1 L dt = 291 pb ∫ Is consistent with previous Events / ( 5.0 MeV/c 30 experimental results. 20

10 2 mτ = (1776.4 ± 4.8) MeV/c Belle (2007) 0 BaBar (2009) 2 BES III (2007)

Pull 0 −2 1.7 1.72 1.74 1.76 1.78 1.8 1.82 1.84 ARGUS (1992) 2 Mmin (GeV/c ) Belle II (2018)

PDG average 1770 1775 1780 1785 2 mτ (MeV/c ) Michel H. Villanueva 14 Prospects of � lepton physics

• The enormous amount of e+e- collisions that are expected from the Belle II experiment features an unique environment for the study of � physics with high precision.

• Further details can be looked at “The Belle II Physics Book”, which is now available at: arXiv:1808.10567

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+extra µ K* e- φ µ- φ

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epciey h inlyedi evaluated is yield signal The respectively. , + ar eeae ih(G1 n without and (BGx1) with generated pairs -1 16 e µ e , - + - vnsadrdaied-un(for di-muon radiative and events e 0 µ µ ): td ntepeec fba background beam of presence the in study - + - . e- π π-

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. µ π π- 9 e- + K - π - See See Ami’s talk on Tuesday Br +

⇥ µ π K - + - e- K+ π- 10

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- + - � ⌧ e- K+ K- lhh M 8 µ K K LFV decays.

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⌧ e K K S0 S0

⌧ - µ µ K K F easotie nteCLEO, the in obtained decays LFV S S

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⌧ π µ π - - +

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( π µ K E - + - e K e K ar 11] r shown are [1511]) pairs F erhsbcmsa becomes searches LFV - + - K µ K itiuin h main The distribution. - )=4 = ))

π Λ Λ -

π Λ h K- Λ of New Physics. signature is a clear Observation of LFV - K Λ . ⌧ 5 Belle II LHCb Belle BaBar CLEO ⇥ F analyses LFV 1 . 10 8 µ (1

. or ) 2 ⇥ CP violation in �→Ks �(≥0�0)�

• The decay of the � lepton to final states containing a Ks meson will have a nonzero decay-rate asymmetry due to CP violation in the kaon sector. Γ(τ+ → π+K0ν¯ ) − Γ(τ− → π−K0ν¯ ) A = S τ S τ τ + + 0 − − 0 Γ(τ → π KSν¯τ) + Γ(τ → π KSν¯τ)

• The SM prediction1,2 is SM −3 Aτ = (3.6 ± 0.1) × 10

• BaBar measured: 2.8 � away BaBar ± ± −3 Aτ = (−3.6 2.3 1.1) × 10 from SM

An improved measurement of A is a priority at Belle II. J.P. Lees et.al (BaBar) � Phys.Rev D85 (2012) 031102 1 I. I. Bigi and A. I. Sanda, Phys. Lett. B 625, 47 (2005). 2 Y. Grossman and Y. Nir, JHEP 2012.4 (2012). Michel H. Villanueva 17 CP violation in �→Ks ��

• CPV that could arise from a charged scalar boson exchange. It can be detected as a difference in the decay angular distributions

•

• With 50 ab-1 data at Belle II, we expect 70 times improvement, i.e., |���| < (0.5 - 3.8) × 10−4, at 90% C.L. assuming the central value ��� = 0. M. Bischofberger et. al (Belle) Michel H. Villanueva 18 PRL 107 (2011) 131801 Michel Parameters

⌫⌧ When spin of � lepton is not determined, �, �, � and � are the experimentally accessible ⌫¯ parameters used in describing the phase ` space distribution of � leptonic decays. ⌧ ` In SM: � = 3/4, � = 0, � = 1 and � = 3/4.

With full dataset (50 ab-1), the statistical uncertainty is expected to be ~10-4.

Comparing with current Belle performance1 , systematic uncertainties will be challenging at Belle II (~10-3).

1 D. Epifanov, Nucl.Part.Phys.Proc. 287-288 (2017) 7-10

Michel H. Villanueva 19 Second class currents: �→��� decay

• Mechanisms in the SM: isospin violation

+

• The corresponding suppression of the SM contribution can make new physics visible.

670 fb-1 470 fb-1

Michel H. Villanueva 20 Estimated Upper Limits for �→��� 5 13 BaBar Upper Limit 10

× 12 3 coupled channels model ) ν 11 Other models π 10 η 9 →

3-1 prong τ 8 7 6 SM predictions: BR(�→���) ~ 10-5 5 BRV BRS BRV+S Model 4 (x105) (x105) (x105) 3 0.36 1.0 1.36 MDM, 1 resonance 2 Upper Limit of BR( 1 [0.2, 0.6] [0.2, 2.3] [0.4, 2.9] MDM, 1 and 2 resonances 0 0.44 0.04 0.48 Nambu-Jona-Lasinio 2×10−1 1 2 3 4 5 6 7 8 10 20 -1 Luminosity (ab ) 0.13 0.20 0.33 Analiticity, Unitarity

We have the capability of testing models in 0.26 1.41 1.67 3 coupled channels the first years of data taking.

Michel H. Villanueva 21 Summary

• The performance of the detector in the first months of data taking is good. Belle II is reconstructing e+e- → �+�- events.

• Semileptonic � decays provides a clean environment to study SM processes with QCD involved.

• SuperKEKB will produce a sample of � pairs 50 times larger than previous B-factories. Precision studies with � leptons involved will be performed.

• Systematic uncertainties will become dominant. Improvements with respect to the last generation of B-factories are required.

• � decays @ Belle II will provide very interesting results in the next decade. See “The Belle II Physics Book” at arXiv:1808.10567.

Michel H. Villanueva 22 Thank you

Michel H. Villanueva 23 Backup

Michel H. Villanueva 24 B-Factories

PEP-II KEKB SuperKEKB

Detector BaBar Belle Belle II

Start date 1999 1999 2016

End of 2008 2010 - operations Beam Energy e-: 9.0 e-: 8.0 e-: 7.0 (GeV) e+: 3.1 e+: 3.5 e+: 4.0 Int luminosity 550 fb-1 1 ab-1 50 ab-1

Michel H. Villanueva �→3�� Event Selection

• Tracks • �’s - pT > 0.1 GeV - E > 200 MeV - |dz| < 5 cm - nHits > 1.5 - |dr| < 1 cm - E9E25 > 0.9 - -0.8660 < cos(�) < 0.9565 - -0.8660 < cos(�) < 0.9565 - E/p < 0.8

• Event - 3 - 1 prong - �0 - veto in signal side. - thrustValue > 0.87 - �0 < 3 in tag side. - visibleEnergyCMS < 9.7 - N� ≤ 1 in signal side. - -0.8660 < cos(�) < 0.9565 - N� ≤ 5 in tag side. - E� signal at CMS < 5.29 - E� tag at CMS < 5.22 - We require data to fire the L1 CDC trigger.

Michel H. Villanueva 26 Vus from inclusive � decays

• At present, the total Vus error is strongly dominated by the uncertainties in the weighted flavor spectral integrals.

• Significantly reduced Vus errors should be possible through K , PDG 2016 l3 improvements of the strange 0.2237 ± 0.0010 mode branching fractions. Kl2, PDG 2016 0.2254 ± 0.0007 CKM unitarity, PDG 2016 0.2258 ± 0.0009 τ → s incl., HFLAV Spring 2017 0.2186 ± 0.0021 τ → Kν / τ → πν, HFLAV Spring 2017 0.2236 ± 0.0018 τ average, HFLAV Spring 2017 0.2216 ± 0.0015 0.22 0.225 HFLAV |V | us Spring 2017

Michel H. Villanueva 27 Measurement of �s(m�)

• Analyses of the τ hadronic decay width and spectral functions have been performed, leading to precise determinations of �s.

• They are based on different approaches to treat perturbative and non-perturbative contributions.

• The running of �s(m�2) to (mZ2) reduces the uncertainty by an order of magnitude.

Michel H. Villanueva 28 Sensitivity of R(D(*))

• Current measurements are dominated by statistical uncertainty.

• Dominant systematic: limited signal MC samples (Larger at Belle II).

Michel H. Villanueva 29