Heavy Flavor Physics at Belle II

Transcription

1 1 Heavy Flavor Physics at Belle II Toru Iijima Kobayashi-Maskawa Institute Nagoya University Dec , 2016 Lecture at Osaka University

2 Why do we want to study τ decays? Many physics involved in the production and decays. And, Goof probe for Electromagnetic interaction Weak interaction Strong interaction Small theory errors Experimental sensitivity not limited by theory errors Sensitive to New Physics τ = the heaviest lepton in the 3 rd gen. [ m= ±0.17 MeV/c 2, τ=(290.6±1.0)x10-15 s ] Lepton Flavor Violation CP violation, EDM, lepton universality χ! 0 τ τ! µ! ( e! ) µ ( e) (m ) 2 l! 23(13) γ τ µ h µ ( s) µ ( s ) 2

3 NP Search with τ 3 τ lepton The heaviest charged lepton High sensitivity to New Physics electron muon tau e µ τ Gen. I II III Mass [MeV] Life 2.20µs 0.291ps Unique laboratory to search for NP LFV (Lepton Flavor Violation) EDM, CPV, g-2 Tauonic decays of Ds, B, t, H Precision test of SM τ µγ!χ 0 τ µ also BNV (Baryon Number Violation) as m τ > m p, m Λ,!τ (m!l 2 ) 23(13) γ τ µµµ τ µ h µ µ

4 LFV in τ decays with NP τ à à µ γ χ! W γ τ µ ( e ) ν ν! νν! µ τ In SM, negligibly small even including neutrino oscillation. 2 3α * Δmli Br ( τ µγ ) = Uτ i U µ i < m U π i = 1, 2 : MNS neutrino mixing matrix Δ m = m m ij νi ν j :Neutrino mass square difference w 2 54 Example: SUSY + Seasaw (J.Hisano et. al., PRD60 (1999) ) 4 7 % tan β & % 1TeV & Br( τ µγ )! ) * ) * + 60, + msusy, Br(τà µγ)=o(10-7~9 ) Many extensions of the SM predict LFV decays. Their branching fractions are enhanced as high as current and near future experimental sensitivity. 2 LFV is a clear signature of NP, if observed. 4

5 Role of LFV in τ decays In case of SUSY, LFV processes are induced by offdiagonal elements of the slepton mass matrix. Sensitive to the SUSY breaking mechanism Diagonal elements by LHC & g-2, EDM Muon LFV (µ eγ, µ e conv.) Tau LFV processes τ eγ Slepton mass matrix 2 m 11 2 m 12 2 m 13 τ µγ ( 2 ) l! m = ij 2 m 21 2 m 31 2 m 22 2 m 32 2 m 23 2 m 33 qi q! i q! j (m ) 2 q! 23(13) & more decay modes. q j LFV in τ decays probes NP flavor mixing bet. 3 1, 3 2 generations. 5

6 Feature of LFV in τ decays Tau is the heaviest lepton Strong coupling to NP. Br(τ) = O( ) x Br(µ). Many (>40) decay modes are open, by which we can test New Physics models Next slide Classes τof à e conservation - /µ - + γ, l + l -, laws; h + h -, h 0, V 0 LF τ à e + /µ + + h - h - (h=π,k) L B τ à pγ, pπ 0, pk 0, Λπ, Λπ Q>0 for decays to hadronic final states. B-L violating Wide window to probe New Physics effects. 6

7 LFV τ Decays 7 Lepton flavor violated in the neutrino sector. Anomaly in (g-2) µ Δa µ = (286±60) Complementarity to LHC (ATLAS/ CMS) U(3) 5 flavor symmetry Complementarity to muon experiments µ eγ, eee (PSI) µ e conversion (Fermilab, J- PARC) Br(τ µγ) Br(µ eγ) G. Blankenburg, G. Isidori, J. Jones-Perez arxiv:

8 SUSY-GUT Goto, Okada, Shindou & Tanaka PRD77, (2008) SU(5)+ν R, non- degenerate ν R (I), normal Hierarchy MEG search region for µà eγ If MEG find µà eγ at ~10-13, good chance to see also τà µγ at 10-8à -10 Even if MEG does not, still important to search for τà µγ. 8

9 Comparison between NP models 9 Ratios of tau LFV decay BF allow to discriminate between new physics models. SUSY+GUT (SUSY+Seesaw) Higgs mediated Little Higgs non-universal Z boson ~ ~16 ~ ~ ~16 < 10-7 < < < 10-9 JHEP 0705, 013(2007) PLB (2002) Favorite modes τ µγ!χ 0 τ µ!τ (m!l 2 ) 23(13) γ τ µµµ τ µ h µ µ

10 Analysis Method at B-Factories Signal side : τ à decay of interest Tag side : τ à 1 trk w/ n γ + missing 1-prong decays occupy >80% of the τ decay Loose constraint on ν based on P miss, M 2 miss Signal evaluation : M inv ~ M τ ΔE = E rec E beam ~ 0 Signal extraction by ML fit or counting Signal region is open after analysis cuts are finalized (blind analysis) Estimate BG using side-band data and MC If no excess, set UL using ML fit or counting method Signal region Background Signal MC 10

11 LFV τ decays; Signal and Background e + e - à τ + τ 1 prong tau decay (BR~85%) ττ π π + π ν 2photon process f=leptons,quarks signal qq ν Both sides have neutrino(s). _ radiative Bhabha process e + e γ e e + e Only tag side has neutrino(s). many tracks e + 11

12 Resolution / BKG 12

13 τg µγ/eγ by Belle 535 fb τ + τ - τ g µ γ PLB666, 16(2008) µ mγ Blind region (3σ) Signal region (2σ) τ g e γ Yellow box: Expected signal dist. N data /N MC = 23/15.0±3.1 in 3σ = 94/88.4±7.4 in 5σ 5σ box for background examination N data /N MC = 13/8.1±1.6 in 3σ = 55/42.8±3.7 in 5σ Br<4.5x10-8 at 90%C.L. Br<1.2x10-7 at 90%C.L. Background: τà µνν/eνν + ISR (or beam background) Small amount of radiative µµ events in ΔE>0 µ ISR m ν ν Will be updated with the full data. 13

14 τg 3leptons by Belle Data: 782 fb -1 No event is found in the signal region. Almost BG free B < ( ) x 10-8 (90%C.L.) The most stringent upper limits among LFV τ decays Still a few background à Will be improved by 1/ L int Phys.Lett.B 687,139 (2010) Mode ε (%) N BG EXP σ syst (%) UL (x10-8 ) e e + e ± µ µ + µ ± e µ + µ ± µ e + e ± µ e + µ ± e µ + e ±

15 Status of τ LFV tau LFV modes have been searched for at B-factories. Note: need theorists help to relate them (other than lγ,lll) to NP Recently LHCb also made the search for τ g µ µ + µ, pµ + µ, pµ µ.

16 Statistics Much more at Super-KEKB! B- factory provides unprecedented large sample of τ leptons. B-factory is τ-factory! History of τà µγ Search Facility # τ CLEO 10 7 BES- III 10 8 B- factory 10 9 Super B factory (Super) B factories dominate the results 16

17 Future Prospect at Belle II 17 Sensitivity will be τg µγ : BG non-free : τ g µµµ : BG free z τà µγ τà µη τà lll Expected limit: B(µγ) ~ O(10-9 ) B(µµµ) ~ O(10-9 ) O(10-10 ) Belle II LHCb can also push down τ µµµ to O(10-9 ) with O(10fb -1 ) data

18 LFV τ Decays 18 SuperKEKB provides also large sample of τ decays (N τ ~NB) Lepton-Flavor-Violating decay is an unambiguous NP signal! τ µγ γ χ 0 τ τ!µ µ (m l 2 ) 23(13) τ µµµ τ µ h µ µ 48 tau LFV modes have been searched for at B-factories. Note: need theorists help to relate them (other than lγ,lll) to NP

19 ττγ BG events in τg µγ analysis If we can remove BG events caused by ISR completely 1.5 ab -1 generic ττ MC removed by generator info. 90% events removed! When we run an accelerator with lower energy than Υ(4S), Can we reduce these ISR BG events? 19

20 Low energy running Operation near ττ threshold for τà µγ search Advantage Larger cross section σ(ττ) ~3.6nb ( x 3.5 wrt 4S) max. at s = 4.25 GeV Dramatic reduction of ττγ background Eγ (CMS) from τà µγ and ISR(ττγ) Eγ from ττγ is low, and separated from the signal region. s =10.58GeV s = 5.0GeV Disadvantage Lower luminosity Acc. people say x 1/10! Higher µµγ background σ(µµ)~6.4nb ( x 6.2 wrt 4S) s = 4.25GeV s = 4.0GeV Want a machine running near threshold w/ L > cm -2 s -1! cf: Super Tau-Charm (BINP), INFN Super-B L = cm -2 s -1 near threshold. 20

21 SUSY-GUT Hisano, Nagai, Paradisi & Shimizu arxiv: SU(5)+ν R µà eγ depends strongly on U e3, but τà µγ does not at all. m M 2 2 (3 m + A ) 3 L ij π H u ( 2) 0 0 * ν3 m! " Ui3U j Close to the current experimental limit If U e3 is tiny, τà µγ still within the Super B factories reach, while µà eγ could be too small to be seen by MEG. 21

22 LFV in Higgs mediated model τ 3 l, lη h µ ( e) µ ( e ) or q q η, τ µ ( e) These decays become important when sleptons are much heavier than weak scale τà 3µ (A.Brignole, A.Rossi, PLB 566 (2003) 217) τà µη (M.Sher, PRD 66 (2002) ) τ µη may be enhanced. Br( τ µη) : Br( τ 3 µ ) : Br( τ µγ ) 8.4 :1:1.5 22

23 e + e - hadrons cross section Historically, one of the most important basic measurements in particle physics. - Discovery of new resonances - Quark charge R (e+ e! hadrons) (e + e! µ + µ ) Nowadays, still important for - Hadronic vacuum polarization muon g-2 - QCD tests - Hadron spectroscopy, baryon form factors,... 23

24 Muon (g-2) magnetic moment and spin ; ~µ = g e 2m ~s ~µ ; magnetic moment ~s ; spin g ; gyromagnetic ratio anomalous magnetic moment ; g =2(1+a) or a = g in Dirac, a =0(g = 2) - in QED (quantum effect), a(qed) = C 4 + C6 + C

25 Experiment BNL-E821 a exp µ = (63) ppm g = 2( + C 3 (α/π) 3 + C 4 (α/π) 4 + Had + Weak +? ) g = 2( + C 3 (α/π) 3 + Had) g = 2( + C 3 (α/π) 3 + Had) g = 2(1 + α/2π + C 3 (α/π) 3 ) g = 2(1 + α/2π) B. Lee Roberts 25

26 Comparison σ deviation a W eak µ = (153.6 ± 1.9) New Physics contribution??? 26

27 New experiments Fermilab J-PARC 27

28 SM model prediction a SM = a QED + a Had + a W eak Q.E.D. Hadronic Weak Recent review by T. Blum et al. ( ) 28

29 QED Aoyama, Hayakawa, Kinoshita, Nio PRL 109, (2012) diagrams a QED µ = (0.009)(0.019)(0.007)(0.077) lepton mass ratio 8-th order 10-th order α( 87 Rb) 29

30 Weak Electroweak contributions C. Gnedinger, D. Stockinger and H. Stockinger- Kim, PRD88, (2013) - two-loop a W eak µ = (153.6 ± 1.9)

31 Hadronic a Had µ = a Had:LO µ + a Had:NLO µ + a Had:lbyl µ Leading order (a) (6 949 ± 43) Next-to-leading order (b, c, d) a Had:NLO µ =( 98.4 ± 0.6 exp ± 0.4 rad. ) Light-by-light (e) a Had:LO µ = (6 923 ± 42) a Had:lbyl µ = (105 ± 26) Davier et al. (2011) Hagiwara et al. (2011) Hagiwara et al. (2011) J. Prades et al. (2009) 31

32 SM summary Recent review by T. Blum et al. ( ) [20] Davier et al. (2011) [21] Hagiwara et al. (2011) 32

33 Precision R and muon g-2 Dominant uncertainty in (g-2)sm comes from the lowest order VP, which cannot be calculated from perturbative QCD, but can be determined by experimental data on σ (e + e - hadron) D. Nomura 33

34 Present data Energy scan: Novosibirsk (CMD, CMD2, SND,...), BES, BES II, BES III,... Initial state radiation: BaBar, Frascati (KLOE/ KLOE-2), BES III σ(e + e - π + π - ) s 0 = s(1 2E / p s ) new KLOE data (not shown) agree with the earlier data 34

35 Breakdown of hadronic channels Dominated by π + π - (~80%) Want to reduce x 1/2 (al least) x 1/5 (ideal for 0.1ppm) 35

36 ISR method by BaBar Pros : Better control of systematic errors (within a single experiment) Cons : Reduced statistics (due to x 1/α reduction) Overcome by large luminosity at (super) B factories Davier (TAU2012) Updates at ICHEP2014 (I heard...) Belle could not measure π + π - due to trigger problems unfortunately. Result for π + π - π 0 available. 36

37 A little more details Cross section measured by the ππ/μμ ratio Result is independent of the BaBar luminosity meas. ISR photon efficiency cancels out. Vacuum polarization cancels out. The ISR luminosity is determined from e + e - μ + μ - (γ)γisr 37

38 BaBar: e + e - π + π - (γ) PRL 103, (2009) PRD 86, (2012) 232 fb -1 ( ),LO aµ = ( ± 2.22 ± 3.11) ± % rel. error 38

39 Systematic error 39

40 Prospect at Belle II 1-3 ab -1 data will do a good job in terms of statistics. - Good subject for early runs. PID, as well as Tracking and ECL, need be calibrated carefully. - Good subject for PID experts. Need some preparation - Trigger - MC generators ; AfkQed, PHOTOS, Phokhara Competition with other experiments - BES III, VEPP Cross checks by some experiments are always necessary. Timely for new g-2 results Fermilab 2018? J-PARC 2020? 40

41 Belle II and (g-2) μ BNL-E821 a exp µ = (63) ppm 3.4 σ diff. from SM pred.? T. Blum et al. ( ) [20] Davier et al. (2011) [21] Hagiwara et al. (2011) New experiments at J-PARC and Fermilab is aiming at 0.1ppm precision, requires reducing also the error for hadronic vacuum polarization effects. 41

42 Hadronic a Had µ = a Had:LO µ + a Had:NLO µ + a Had:lbyl µ Leading order (a) (6 949 ± 43) Next-to-leading order (b, c, d) a Had:NLO µ =( 98.4 ± 0.6 exp ± 0.4 rad. ) Light-by-light (e) a Had:LO µ = (6 923 ± 42) a Had:lbyl µ = (105 ± 26) Davier et al. (2011) Hagiwara et al. (2011) Hagiwara et al. (2011) J. Prades et al. (2009) 42

43 Precision R and muon g-2 Dominant uncertainty in (g-2)sm comes from the lowest order VP, which cannot be calculated from perturbative QCD, but can be determined by experimental data on σ (e + e - hadron) D. Nomura 43

44 Measurement w/ ISR BaBar, Frascati (KLOE/KLOE-2), BES III Pros : Better control of systematic errors (within a single experiment) Cons : Reduced statistics (1/α reduction) Overcome by large luminosity at Belle II. s 0 = s(1 2E / p s ) Belle could not measure π + π - due to trigger problems unfortunately. Result for π + π - π 0 available. 44