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Belle results on Lepton Flavor Violation in tau decays

Belle results on Lepton Flavor Violation in tau decays. K. Inami (Nagoya Univ.) for Belle collaboration. B-factory at KEK. KEKB: e + (3.5 GeV) e - (8GeV) s ( tt )~0.9nb, s (bb)~1.1nb A B-factory is also a t -factory!. Peak luminosity: 2.1x10 34 cm -2 s -1

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Belle results on Lepton Flavor Violation in tau decays

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  1. Belle results onLepton Flavor Violationin tau decays K. Inami (Nagoya Univ.) for Belle collaboration

  2. B-factory at KEK KEKB: e+(3.5 GeV) e-(8GeV) s(tt)~0.9nb, s(bb)~1.1nb A B-factory is also a t-factory! Peak luminosity: 2.1x1034 cm-2s-1 World highest luminosity! Belle Detector: Good track reconstruction and particle identifications Lepton efficiency:90% Fake rate : O(0.1) % for e O(1)% for μ

  3. Luminosity history Integrated luminosity: >1000 fb-1 ⇒~109 BB and t-pairs Belle is finished in 2010/6/30. Belle-II upgrade started.  Analysis with full data sample is on going.

  4. Lepton Flavor Violation in tau decay g SUSY is the most popular candidate among new physics models ~ ~ t ℓ t naturally induce LFV at one-loop due to slepton mixing e or m ~ c0 tlg mode has the largest branching fraction in SUSY-Seesaw (or SUSY-GUT) models When sleptons are much heavier than weak scale Higgs (h/H/A) m m s s LFV associated with a neutral Higgs boson (h/H/A) t e or m Higgs coupling is proportional to mass ⇒mm or ss (h, h’and so on) are favored and B.R. is enhanced more than that of tmg. To distinguish which model is favored, all of decay modes are important.

  5. t e e t generic t decay 1 prong + missing (tag side) mmm(signal side) 3s ellipse: blinded region Sideband region m Analysis procedure m m • e+e-gt+t- Br~85% Fully reconstructed Signalextraction:mmmm-DEplane Blind analysis ⇒Blind signal region Estimate number of BG in the signal region using sideband data and MC

  6. p- p+ p- signal n n _ qq e+ e- e- g • Neutrino(s) in tag side • Particle ID • Mass of mesons e+ e- • many • tracks e+ LFV t decays; Signal and Background 2photon process tt f=leptons,quarks • Neutrinos in both side • Missing energy in signal side radiative Bhabha process

  7. Simple Hard Analysis strategy • Rare decay searches • Need to understand background and reduce as much as possible • tℓℓℓ • tℓKs • tℓV0(hh’) • tℓP0(gg) • tℓhh’ • tℓg • Analyze the modes from simple selection to hard for background reduction • Provide feedback to next analysis of similar final state Difficulty of reducing the background

  8. Search for t3leptons e-e+e- m-m+m- e-m+m- m-e+m- e-m+e- m-e+e- Phys.Lett.B 687,139 (2010) • Data: 782fb-1 • No event is found in the signal region. • Almost BG free • Because of good lepton ID • Br<(1.5-2.7)x10-8 at 90% CL.

  9. p p q q Search for ℓV0(=r0,K*0,w,f) • Search with 854fb-1 data sample • Select one lepton and two hadrons • Require invariant mass to be a vector meson mass • The requirement reduces background rather easily. • Possible background • For ℓ=m, hadronic tau decay and qq with miss m-ID • For ℓ=e, 2photon process could be large BG. • It turns out that not only 2photon process but also ee+X process become large background.  Reduced using missing-momentum direction data r 2photon BG g* signal MC (er0) e- g* e+ Mpp g (missing) tt and qq MC

  10. Result for ℓV0(=r0,K*0,w,f) • After event selection • 1 eventmf, mK*0, mK*0 • 0 events others No signal compared to expected BG Expected number of background (0.1-1.5) events Br(tℓV0) < (1.2-8.4)x10-8 Phys.Lett.B 699,251 (2011)

  11. Search for ℓhh’ Updated this summer • Update with 854fb-1 data • BaBar; Br<(7-48)x10-8 at 221fb-1 • 14 modes are investigated (h,h’=p± and K±) • t-→ℓ-h+h’-: 8 modes (lepton flavor violation) • t-→ℓ+h-h’-: 6 modes (lepton number violation) miss-ID as K+ miss-ID as K+ e+ signal generic tt BG e- e- or m- p- p- p- p- n p+ n K+ g signal side g p0 t- t- t- e+ e+ e+ e- e- e- tag side t+ t+ t+ Also, qq events can be BG n n n Missing momentum can help to reject this kind of BGs since signal has n only on tag side.

  12. Result for ℓhh’ In the signal region 1event : in m+p-p- and m-p+K- no events: in other modes ⇒ no significant excess 予想される背景事象 (0.1-1.5)events Set upper limits at 90%CL: Br(tℓhh’)< (2.0-8.6)x10-8 (preliminary)

  13. Upper limits on LFV t decays Updated this summer Under studying with full data sample

  14. Effect to physics models • Experimental results have already ruled out some parts of the parameter space. • Exclude large tanb, small SUSY/Higgs mass • Accessing other models and other parameter space Undetectable

  15. Future prospects Belle-II will produce >1010 tau leptons. Sensitivity depends on BG level. Recent improvement of the analysis (BG understanding, intelligent selection)  Improve achievable sensitivity B(tmmm)~O(10-10) at 50ab-1 Improvement of BG reduction is important. Beam BG Signal resolution Old estimation New estimation

  16. Summary Search for LFV t decays using ~109t decays 48 modes have been investigated. No evidence is observed yet. Upper limits on branching ratio around O(10-8) B(tmmm)<2.1x10-8, Br(tmV0) < (1.2-8.4)x10-8 B(tmhh’) < (2.1-8.6)x10-8, Br(tLh) < (2.8-4.2)x10-8 etc. Exploring some new-physics parameters space. Update tmg/eg with full data sample (~1000fb-1) Optimization for BG reduction is important for future experiment Belle-II try to obtain the sensitivity of O(10-9~10).

  17. Result for tℓP0(=p0,h,h’) (2.1-4.4) times more stringent results than previous Belle result (401fb-1)

  18. eK*, eK*, er modes g-conversion generic tt MC data Other BG for eK*, eK* and er ⇒ Event with g conversion For example, eK*mode t-p-p0n with g conversion from p0 Mee with e-K+ n p- Fake K* e+ as miss KID P(K+) (GeV) g e- τ- g p0 Require large invariant mass of e+e- candidate, to reduce the BG overlap dE/dx region between e and K eK* MC

  19. Search for Lh • 4 modes are searched for. (h=pand K) • t-→Lh-: (L-B) conserving decay • t-→Lh-: (L-B) violating decay • Current upper limits • Belle Br<(7.2-14)x10-8 @ 154fb-1 • BaBar Br<(5.8-15)x10-8 @ 237fb-1 (no search for LK on Belle) updatewith 906fb-1 @Belle p+ p- p- p signal Generic tt BG p qqBG p- p- p- p- g signal side g L L t- t- Ks0 p0 e+ e+ e+ e- e- e- p0 misid as L tag side t+ t+ p g g n n

  20. BG rejection for Lh To reduce tt BG including KS0 ⇒ reconstruct KS0 and reject events that are likely to be KS0 qq MC A half of Ks0 BG events are rejected. To reduce qqBG including L ⇒ reject events with a proton in tag side (due to BN conservation, the events including a L tend to have baryon on tag side.) qq MC p A third of qq BG events are rejected.

  21. Results of Lh (preliminary) In the signal region no camdodate event are found ⇒ no significant excess 0 2.3 0 2.2 0 2.2 0 2.1 Set upper limits@90%CL: Br(t-gLp-)<2.8x10-8 Br(t-gLK-)<3.1x10-8 Br(t-gLp-)<3.0x10-8 Br(t-gLK-)<4.2x10-8 (L-V) cons. (L-V) viol. (preliminary) Around x(2-3) improvement from the previous BaBar results

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