Belle results on Lepton Flavor Violation in tau decays

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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 - PowerPoint PPT Presentation

Transcript of Belle results on Lepton Flavor Violation in tau decays

  • Belle results onLepton Flavor Violationin tau decaysK. Inami (Nagoya Univ.)for Belle collaboration

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

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

  • Lepton Flavor Violation in tau decaySUSY is the most popular candidate among new physics models naturally induce LFV at one-loopdue to slepton mixingtlg mode has the largest branching fractionin SUSY-Seesaw (or SUSY-GUT) modelsc0 ~te or mt~~gLFV associated with a neutral Higgs boson (h/H/A)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 tmg.When sleptons are much heavier than weak scalete or mHiggs (h/H/A)mmssTo distinguish which model is favored, all of decay modes are important.

  • Analysis proceduree+e-gt+t-

    Blind analysis Blind signal region Estimate number of BG in the signal regionusing sideband data and MCSignal extraction: mmmm-DE planemBr~85%Fully reconstructedmm

  • LFV t decays; Signal and Background2photon process tt f=leptons,quarksNeutrinos in both sideMissing energy in signal sideradiative Bhabha process

  • Analysis strategyRare decay searchesNeed to understand background and reduce as much as possible

    t tKs tV0(hh) tP0(gg) thh tg

    Analyze the modes from simple selection to hard for background reductionProvide feedback to next analysis of similar final stateDifficulty ofreducing the background

  • Search for t3leptonsData: 782fb-1

    No event is found in the signal region.

    Almost BG freeBecause of good lepton ID


  • Search for V0(=r0,K*0,w,f) Search with 854fb-1 data sample Select one lepton and two hadronsRequire invariant mass to be a vector meson massThe requirement reduces background rather easily. Possible backgroundFor =m, hadronic tau decay and qq with miss m-IDFor =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

    g*e+e-rMppdatasignalMC (er0)g*g (missing)2photon BG

  • Result for V0(=r0,K*0,w,f)After event selection 1 eventmf, mK*0, mK*0 0 events othersNo signal compared to expected BGBr(tV0) < (1.2-8.4)x10-8Expected number of background(0.1-1.5) events Phys.Lett.B 699,251 (2011)


  • Search for hhUpdate with 854fb-1 dataBaBar; Br
  • Result for hh(0.1-1.5)eventsSet upper limits at 90%CL: Br(thh)< (2.0-8.6)x10-8 (preliminary)1event : in m+p-p- and m-p+K- no events: in other modes no significant excess In the signal region

  • Upper limits on LFV t decaysUpdated this summerUnder studying with full data sample

  • Effect to physics modelsExperimental results have already ruled out some parts of the parameter space.Exclude large tanb, small SUSY/Higgs mass

    Accessing other models and other parameter spaceUndetectable

    referencetmgtmmmSM+ n mixingPRD45(1980)1908, EPJ C8(1999)51310-4010-14SM + heavy Maj nRPRD 66(2002)03400810-910-10Non-universal ZPLB 547(2002)25210-910-8SUSY SO(10)PRD 68(2003)03301210-810-10mSUGRA+seesawPRD 66(2002)11501310-710-9SUSY HiggsPLB 566(2003)21710-1010-7

  • Future prospectsOld estimationNew estimationBelle-II will produce >1010 tau leptons.Sensitivity depends on BG level.Recent improvement of the analysis(BG understanding, intelligent selection) Improve achievable sensitivity B(tmmm)~O(10-10) at 50ab-1Improvement of BG reduction is important.Beam BGSignal resolution

  • SummarySearch for LFV t decays using ~109 t decays48 modes have been investigated.No evidence is observed yet.Upper limits on branching ratio around O(10-8)B(tmmm)
  • Result for tP0(=p0,h,h)(2.1-4.4) times more stringent results than previous Belle result (401fb-1)

    tgEff. %NBGexpNobs.ULx10-8mh(ggg)8.20.630.3703.6mh(gppp0)

    mh(gpph)8.1%0.00+0.16-0.00010.0mh (gr0g)6.2%0.590.4106.6mh (comb.)3.8eh (gpph)7.3%0.630.4509.4eh (gr0g)7.5%0.290.2906.8eh (comb.)3.6mp0(ggg)4.2%0.640.3202.7ep0(ggg)4.7%0.890.4002.2

  • eK*, eK*, er modesOther BG for eK*, eK* and er Event with g conversion For example, eK*mode t-p-p0n with g conversion from p0overlap dE/dx regionbetween e and Kdata P(K+) (GeV)Mee with e-K+ g-conversiongeneric tt MCeK* MCRequire large invariant mass of e+e- candidate, to reduce the BG

  • Search for Lh4 modes are searched for. (h=p and K)t-Lh-: (L-B) conserving decayt-Lh-: (L-B) violating decay Current upper limits Belle Br
  • BG rejection for LhTo reduce tt BG including KS0 reconstruct KS0 and reject eventsthat are likely to be KS0 qq MCA half of Ks0 BG events are rejected.To reduce qq BG including L reject events with a proton in tag side(due to BN conservation, the eventsincluding a L tend to have baryon on tag side.)qq MCpA third of qq BG events are rejected.

  • Results of Lhno camdodate event are found no significant excess In the signal region00002. upper limits@90%CL:Br(t-gLp-)