Experimental Review on Lepton Flavor Violating Tau decays

1. International workshop e+e- collisions from phi to psi PHIPSI08 Experimental Review onLepton Flavor ViolatingTau decays 2008/4 K.Inami Nagoya university

2. Lepton Flavor Violation (LFV) • Standard Model (SM) exactly forbids LFV. • Extended SM including neutrino mixing term includes LFV. • Charged Lepton Flavor Violation can occur through loop diagram. But, extremely tiny branching ratio below experimental sensitivity • Br(tmg)<O(10-40) (X.-Y. Pham, EPJ C8(1999)513) • Many new models include LFV vertex naturally. • Observation of charged LFV •  Clear signal of new physics! • Tau lepton has many possible LFV decay modes. •  Wide window to probe New Physics effect.

3. LFV tau decays • Predicted by many new physics models • Normal NP models enhance t  mg. • For some NP parameters, other modes may be enhanced instead. • Many models allow measurable BR of O(10-8) within the parameter space.

4. SUSY • Most famous SM extension • LFV are generated through slepton mixing. • Independent parameter for meg • SUSY seesaw (J.Hisano et. al.,PRD 60 (1999) 055008) • Achievable BR of O(10-7~-8) if tanb~60 and mSUSY~1TeV/c2

5. SUSY Higgs • When sleptons are heavy (>weak scale), Br(tmg) is suppressed. • Higgs in SUSY enhances other LFV modes, such as t3l, l+hadrons, ... • Higgs-mediated MSSM • t3m(A.Brignole, A.Rossi, PLB 566 (2003) 217) • Enhanced if tanb is large and Higgs mass is small.

6. SUSY Higgs • Higgs-mediated MSSM (M.Sher, PRD 66 (2002) 057301) • Br(tmh) : Br(tmmm) = 8.4 : 1 • Phase space, color factor, mass • MSSM seesaw(E.Arganda, arXiv:0803.2039v1) • Large BR of O(10-7) for tmh, mh’, mK+K- • Need to search for all possible LFV modes • To probe unknown physics and discriminate models

7. Experiment • Recent results obtained by B-factories • High intensity e+e- collider on U(4S) • KEKB/Belle; World highest luminosity of 1.7x1034/cm2/s • Similar cross-section with BB (~0.9nb)  t-factory • Detectors; Good PID ability and vertex resolution Lepton ID eff. : ~90% m fake rate : ~2% e fake rate : ~0.3 %

8. Status of LFV search at spring 2007 • Current accumulated luminosity; >700fb-1 at Belle and >500fb-1 at BaBar. • Searches for 3l, lhh modes should be updated.

9. LFV analysis • Signal extraction • Calc. Minv and DE • DE=Erec-Ebeam • Blinded signal region •  Event selection study • Estimate background using sideband data • Open blind and estimate signal yield • Estimate upper limits by Likelihood fit or number of events in signal region • BG reduction is important to improve the sensitivity. • Find effective ways to use all information Signal MC Signal region Background Minv

10. _ qq LFV t decays; Signal and Background • e+e- t+ t- • 1 prong tau decay (BR~85%) 2photon process tt p- p+ f=leptons,quarks p- n signal • Both sides have neutrino(s). n radiative Bhabha process e+ e- e- g e+ e- • many • tracks • Only tag side • has neutrino(s). e+

11. t3 leptons; BG reduction • Optimized event selection for each mode separately because the BG processes are different. BG events eemm(eeee) Bhabha(mm) Signal topology • g conversion veto • (e-e+e- and m-e+e-) • Electron-veto on the tag-side • (e-e+e- and e-m+m-)

12. data signal t-ge-e+e- signal region t-gm-m+m- tlll search; Belle • Data; 535fb-1 • Efficiency; (6.0-12.5)% • NBG; 0.0-0.4 events • Nobs; 0 events for all modes • Br<(2.0-4.1)×10-8 at 90%C.L. (Y.Miyazaki et. al., PLB 660 (2008) 154)

13. tlll search; BaBar • Data; 376fb-1 • Efficiency; (5.5-12.4)% • NBG; 0.3-1.3 events • Total BG; 4.2±0.8 • Nobs; 6 events in total • Br<(3.7-8.0)×10-8at 90%C.L. (B.Aubert et. al.,PRL 99 (2007) 251803) =Minv-Mt

14. tlV0 search data signal • Belle; 543fb-1 data • Ten modes are investigated, l=e,m and V0=r,K*0,K*0,f,w • Main BG: SM t decay;tpwn,tpppn • Signal Efficiency; 2.5 - 4.9 % • Expected NBG;0.0 - 1.0 events • Observed number of events; 1 event for tmf, ew, eK*0 0 events for others • Br<(5.9-18)×10-8 (Y.Nishio et. al., arXiv:0801.2475) • BaBar; 384fb-1 data • Searched for mw, ew Br<(10-11)×10-8 (B.Aubert et. al., PRL 100;071802 2008)

15. mf0 ef0 ● data signal MC tlf0 search • Scalar Higgs mediation • No search yet. • Belle; 671fb-1 • Use f0p+p- (4s) • No event is observed. • Preliminary result Br(t→lf0)xBr(f0→p+p-) • mf0 : <3.3X10-8, ef0 : <3.4X10-8 Br(t→mf0):Br(t→mmm):Br (t→mh) =1.3: 0.54 :1 (C.H.Chen et.al, PRD74:035010,2006 )

16. Current status of LFV search • Sensitivity is improved by factors of 3-10. It comes from the luminosity increase and successful BG reduction. • Search for t3leptons sets most stringent upper limit.

17. Current status of LFV search • 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

18. Future prospect • Int. Luminosity at B-factory; >1300fb-1 • ~1.2x109t-pairs • At super B-factory >10 times more data • BR sensitivity • It depends on background. • tlg; scale as ~1/L • e+e-t+t-g is irreducible BG. • ~10-8 level at super B-factory • tlll, lX0; scale as ~1/L • BG can be reduced by lepton-ID or mass requirement for X0 • O(10-9) level at super B-factory

19. Summary • A data sample of >109t-pairs has been successfully obtained by the B-factories. • New searches for 3-prong final state • t3l, lV0, lf0 using 400~700 fb-1 of data • Better sensitivity than previous analysis • Applied event selection optimized for each mode separately • LFV sensitivities are Br~O(10-8). • Exploring some parameter space of New Physics • Many models predict possible BR of O(10-7~-8). • Super B-factory is planned to produce >1010t-pairs.  LFV sensitivity will reach Br~O(10-9).

20. LFV t decays; where we are. • Sensitivity of Br~O(10-8)

21. tLp, LK 237 fb-1 BaBar data used for this analysis. (hep-ex: 0607040)

22. t3 leptons; Results • No signal events • No BG events in sideband except for e-e+e-