Search for tau LFV decays at Belle. K.Hayasaka (KMI, Nagoya University)

Transcription

1 Search for tau LFV decays at Belle K.Hayasaka (KMI, Nagoya University) 1

2 Introduction Leton flavor violation (LFV) in charged letons negligibly small robability in the Standard Model (SM) even including neutrino oscillations: W - B(tgmg) <O(10-54 ) t m g n t n m Observation of LFV is a clear signature of New Physics (NP) Many extensions of the SM redict LFV decays. These branching fractions could be enhanced as high as current exerimental sensitivity. (~10-8 ) Tau leton = The heaviest charged leton Exected strong couling to NP Many ossible LFV decay modes 2

3 New Physics and t LFV We like to find LFV and New Physics (NP) as well as to know what NP induces LFV. NUHM JHEP06(2008)079 Various LFV searches are imortant because they can distinguish NP models even if one LFV decay has been observed. It is a strong advantage that t has many kinds of LFV decays. Using Belle t data, we search for t LFV decays. 3

4 KEKB/Belle 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 World highest luminosity! Belle finalized data taking on Jun.30,2010. total:>1ab -1 U(4S):711fb -1 U(5S):121fb -1 U(3S):3.0fb -1 U(2S):24fb -1 U(1S):5.7fb -1 Off-resonance: 87fb -1 U(4S) off-resonance Belle Detector: Good track reconstruction and article identifications Leton efficiency:90% Fake rate : O(0.1) % for e O(1)% for μ tt 4

5 Analysis rocedure e + e - gt + t - 2 m ( E - m 1 rong + missing (tag side) m (signal side) m CM E Em - Br~85% Fully reconstructed Signal extraction m m -E lane E CM beam 2 m Blind analysis Blind signal region Estimate number of BG in the signal region using sideband data and MC ) e E ~ 0 t m e t generic t decay m m ~ m t 3s ellise: blinded region Sideband region m m 5

6 signal LFV t decays; Signal and Background m tt n 2hoton rocess f=letons,quarks n Neutrinos in both side Missing energy in signal side _ qq radiative Bhabha rocess Neutrino(s) in tag side Particle ID Mass of mesons many tracks and hotons e + e - g e - e + e - e + 6

7 Search for tglhh NUHM favors t - gl while doubly charged higgs induces t - gl Udate with 854fb -1 data BaBar; Br<(7-48)x10-8 at 221fb modes are investigated (h,h = ± and K ± ) t - l - h + h - : 8 modes (leton flavor violation) signal t - l + h - h - : 6 modes (leton number violation) e - or m - t - - K + e - e + generic tt BG n t - - e - e + - miss-id as K + + arxiv: [he-ex] n t - miss-id - as K + 0 g e + e - g e - e + t + t + t + n n Missing momentum can hel to reject this kind of BGs since signal has n only on tag side. n Also, qq events can be BG 7

8 BG rejection for t lhh To reduce tt and qq BG mk mode m 2 miss -- miss correlation ehh, m and mkk modes m miss2 selection 2d selection 75% of eff. is ket while 75% of BG is rejected. 90% of eff. is ket while 50% of BG is rejected. To reduce tt BG mk mode Dominant BG is from t n with a combination misidentified as Km. M mk is shifted into the t-mass signal region while the original M will be below the t-mass due to the missing neutrino. Assign mass for selected events M >1.52 GeV/c 2 65% of eff. is ket while 65% of BG is rejected. data qq MC tt MC mk MC 8

9 Result for tglhh In the signal region 1event : in m and m - + K - no events: in other modes no significant excess/exected # of BG: 予想される背景事象 ( )events Set uer limits at 90%CL: Br(tlhh )< ( )x10-8 most sensitive results (reliminary) 9

10 Search for tglh/lh GUT allows t - Lh - while more comlicated model is required for t - Lh -. Search with 904fb -1 data samle Select three hadrons Require Lambda vertex 4 modes are searched for. (h= and K) t - Lh - : (B-L) conserving decay t - Lh - : (B-L) violating decay - - signal Generic tt BG qqbg - - n t - L e - e + t + n t - Ks 0 e - e + t + misid as L g L g e - e g g 10

11 BG rejection for tglh/lh To reduce tt BG including K S 0 reconstruct K S 0 and reject events that are likely to be K S 0 85% of eff. is ket while 75% of Ks 0 BG events is rejected. qq MC To reduce qq BG including L reject events with a roton in tag side (due to BN conservation, the events including a L tend to have baryon on tag side.) A third of qq BG events are rejected while a loss of eff. Is negligibly small. 11

12 Results for tglh/lh In the signal region: no candidate event are found no significant excess Exected # of BG: ( ) reliminary) Set uer limits@90%cl: Br(t - gl - )<2.8x10-8 Br(t - glk - )<3.1x10-8 Br(t - gl - )<3.0x10-8 Br(t - glk - )<4.2x10-8 (B-L) cons. (B-L) viol. (reliminary) most sensitive results 12

13 Previous result for tgmg search 545 fb -1 Belle data samle 94 events are found while (88.4±7.4) BG events are exected in 5s region and the detection eff. is 6.1%. Main BG comes from tgmnn + ISR g. Uer Limits are evaluated by 2d UEML fit on M-E lane. Exected UL: Obtained Now, we are udating the result with Belle full data samle (980fb -1 )! UEML=Unbinned Extended Maximum Likelihood fit 13

14 Tag side missing mass (tag-side missing mass) 2 Tag side missing mass. e t m t g This can be reconstructed from another t (tag side missing)= (reconstructed tag-side t) -(visible tracks and gammas belonging to tag side) e Hadronic tags letonic tags signal MC + Data Cyan: Total MC Red: tt MC 14

15 eegmmg rejection Mainly, eegmmg events are rejected by muon-veto for tagside charged track. But, due to the inefficiency of the muon ID, some eegmmg events survive. e g m m e signal MC + Data Cyan: Total MC Red: tt MC Blue: mm MC In the e-e-g rest frame, m-m direction should be oosite. e g m m e cosa = oening angle between signaland tag-side tracks in e-e-g rest frame. 15

16 Remaining events and eff. After the selections, 105 events are found and the detection efficiency is 6.5%. For the blinded region, 10.2±2.2 BG events are exected. Blind region Signal MC In total, 115.2±11.4 events are exected. BG level reduced by 33% while efficiency is similar to the revious analysis. As a result, the evaluated 90% CL exected UL for BF is 5.3 x Previous New analysis Lum. 545fb fb -1 Eff. 6.1% 6.5% About x1.5 gain is obtained for exected UL while the sqrt. of increase for lum. is 1.3. #BG Final result will Exected UL 7.8x x10-8 be shown soon! 16

17 Reach uer limits around 10-8 ~100x more sensitive than CLEO 17

18 The remaining mode are tgmg and eg! Previously, a 545 fb -1 data subsamle was analyzed. 18

19 Summary Belle comleted oeration with a 1000fb -1 data samle, which contains ~10 9 tau-airs. This is the world s largest t data samle. There are many kinds of t LFV including leton number (L) violation or violation of (B-L), i.e., baryon number leton number. By adding more data and studying the dominant BGs and otimizing the analyses to suress these BGs, we have significantly imroved t LFV uer limits. Br(t lhh )< ( )x10-8 Almost all t LFV modes have been investigated with the 1000fb -1 data samle. The last remaining ones are tgeg/mg. The most sensitive results for these modes. Search for tgeg/mg with the full data samle is on-going and the final result will be shown soon! (reliminary) 19

20 tgmg vs mgeg SUSY GUT SU(5)+νR,non-- degenerate νr(i),normal Hierarchy Leton sector constraints in an SU(3)-flavored MSSM Calibbi, Jones Perez, Masiero, Park, Porod & Vives arxiv: v2 SUSY seesaw model JHEP11(2006)090 mgeg exected Goto, Okada, Shindou & Tanaka PRD77, (2008) These make us to exect Br(tgmg)~O( ). Even if mgeg has very small BF, some model redicts that tgmg can have sufficiently large BF to observe it exerimentally. ex.) SU(3) 3 symmetry in SUSY model: arxiv: v1 [he-h] 20

21 21 In the CM frame, tau direction can be calculated from the charged tracks. This direction is comared w/ reconstructed one from m and g. Exected t direction ) ( 2 1, ) ( 1 cos cos, ) ( 1 cos cos, B A B A A B A B B A B B A A B A B A B A q q uv v u w q q q q v q q q q u w vq uq k Here, A ( B ) means 3-vector for m (hadron) in CM frame and q A (q B ) is a unit vector arallel to A ( B ). Cosine of the angle btw exected and reconstructed tau directions