Belle results on Lepton Flavor Violation in tau decays K. Inami (Nagoya Univ.) for Belle collaboration

2 B-factory at KEK KEKB: e + (3.5 GeV) e - (8GeV)  (  )~0.9nb,  (bb)~1.1nb A B-factory is also a  -factory! Good track reconstruction and particle identifications Belle Detector: Lepton efficiency:90% Fake rate : O(0.1) % for e O(1)% for μ Peak luminosity: 2.1x10 34 cm -2 s -1 World highest luminosity!

3 Luminosity history Belle is finished in 2010/6/30. Belle-II upgrade started.  Analysis with full data sample is on going. Integrated luminosity: >1000 fb -1 ⇒ ~10 9 BB and  -pairs

4 Lepton Flavor Violation in tau decay SUSY is the most popular candidate among new physics models naturally induce LFV at one-loop due to slepton mixing   l  mode has the largest branching fraction in SUSY-Seesaw (or SUSY-GUT) models  ℓ ~  e or   ~ ~  LFV associated with a neutral Higgs boson (h/H/A) Higgs coupling is proportional to mass ⇒  or ss ( ,  ’ and so on) are favored and B.R. is enhanced more than that of   . When sleptons are much heavier than weak scale  e or  Higgs (h/H/A)  ssss To distinguish which model is favored, all of decay modes are important.

5 Analysis procedure e + e    +    (signal side) 1 prong + missing (tag side) Blind analysis ⇒ Blind signal region Estimate number of BG in the signal region using sideband data and MC Signal extraction : m   E plane e e   generic  decay  3  ellipse: blinded region Sideband region Br~85% Fully reconstructed  

6 LFV  decays; Signal and Background    2photon process  f=leptons,quarks Neutrinos in both side Missing energy in signal side radiative Bhabha process e+e+ ee ee ee ee ee  qq _ many tracks signal Neutrino(s) in tag side Particle ID Mass of mesons

7 Analysis strategy Rare decay searches –Need to understand background and reduce as much as possible   ℓℓℓ   ℓK s   ℓV 0 (  hh’)   ℓP 0 (   )   ℓhh’   ℓ  –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 Hard Simple

8 Search for   3leptons Data: 782fb -1 No event is found in the signal region. Almost BG free –Because of good lepton ID Br<( )x10 -8 at 90% CL. Phys.Lett.B 687,139 (2010) Mode  (%) N BG EXP  syst (%) UL (x10 -8 ) eeeeee  ee eeee ee eeee ee+eee+e ++ e+e+ e+ee+e e+e+ e+ee+e

9 Search for ℓV       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= , hadronic tau decay and qq with miss  -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  e+ e-  M  data signal MC (e   )  and qq MC q q     (  missing) 2photon BG

10 Result for ℓV        Eff.N BG exp N obs. UL x10 -8  Eff.N BG exp N obs. UL x10 -8 ee 7.6%0.29± e  K *0 4.4%0.39±  7.1%1.48±   K *0 3.4%0.53± ee 4.2%0.47± e  K *0 4.4%0.08±  3.2%0.06±   K *0 3.6%0.45± ee 2.9%0.30±  2.4%0.72± After event selection 1 event ,  K* 0,  K* 0 0 events others No signal compared to expected BG Br(   ℓV 0 ) < ( )x10 -8 Expected number of background ( ) events Phys.Lett.B 699,251 (2011)

11 Search for ℓhh’ Update with 854fb -1 data –BaBar; Br<(7-48)x10 -8 at 221fb modes are investigated (h,h’=   and   ) –   →ℓ - h + h’ - : 8 modes (lepton flavor violation) –   →ℓ + h - h’ - : 6 modes (lepton number violation)  e  or      ee ee      ee ee signal generic  BG miss-ID as        ee ee  ee ee miss-ID as   Missing momentum can help to reject this kind of BGs since signal has only on tag side. Also, qq events can be BG signal side tag side Updated this summer

12 Result for ℓhh’ 予想される背景事象 ( )events Set upper limits at 90%CL: Br(   ℓhh’)< ( )x10 -8 (preliminary) 1event : in       and       no events: in other modes ⇒ no significant excess In the signal region

13 Upper limits on LFV  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 tan , small SUSY/Higgs mass –Accessing other models and other parameter space reference      SM+ mixing PRD45(1980)1908, EPJ C8(1999) SM + heavy Maj R PRD 66(2002) Non-universal Z’ PLB 547(2002) SUSY SO(10) PRD 68(2003) mSUGRA+seesaw PRD 66(2002) SUSY Higgs PLB 566(2003) Undetectable

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

16 Summary Search for LFV  decays using ~10 9  decays –48 modes have been investigated. No evidence is observed yet. Upper limits on branching ratio around O(10 -8 ) –B(    )<2.1x10 -8, Br(    V 0 ) < ( )x10 -8 –B(    hh’) < ( )x10 -8, Br(    h) < ( )x10 -8 etc. –Exploring some new-physics parameters space. Update    /e  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 ).

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18 Result for   ℓP 0 (=    ’)  Eff. % N BG exp N obs. UL x10 -8    ±      ±  comb  2.3 e    ± e      ± e  comb  4.4  ’    8.1%  ’      6.2%0.59±  ’  comb  3.8 e  ’    7.3%0.63± e  ’      7.5%0.29± e  ’  comb  3.6      4.2%0.64± e      4.7%0.89±  ( ) times more stringent results than previous Belle result (401fb -1 )

19 eK*, eK*, e  modes Other BG for eK*, eK* and e  ⇒ Event with  conversion For example, eK*mode       0 with  conversion from  0 τ- -- 00   e- Fake K* e+ as miss KID overlap dE/dx region between e and K data P(K+) (GeV) Mee with e-K+  -conversion generic  MC eK* MC Require large invariant mass of e + e - candidate, to reduce the BG

20 Search for  h 4 modes are searched for. (h=  and K)  - →  h - : (L-B) conserving decay  - →  h - : (L-B) violating decay Current upper limits Belle Br<(7.2-14)x10 154fb -1 BaBar Br<(5.8-15)x10 237fb -1 update with 906fb (no search for  K on Belle)    ee ee signal side tag side  p  signal    ee ee Ks 0   misid as  ee ee   p     p    Generic  BG qqBG

21 BG rejection for  h To reduce  BG including K S 0 ⇒ reconstruct K S 0 and reject events that are likely to be K S 0 qq MC A half of Ks 0 BG events are rejected. To reduce qq  BG including  ⇒ reject events with a proton in tag side (due to BN conservation, the events including a  tend to have baryon on tag side.) qq MC p A third of qq BG events are rejected.

22 Results of  h no camdodate event are found ⇒ no significant excess In the signal region Set upper Br(      )<2.8x10 -8 Br(      )<3.1x10 -8 Br(      )<3.0x10 -8 Br(      )<4.2x10 -8 (L-V) cons. (L-V) viol. Around x(2-3) improvement from the previous BaBar results (preliminary)