Belle Hot Topics. Nagoya University Koji Ikado (for the Belle Collaboration) Flavor Physics and CP Violation Conference (FPCP2006) Apr.

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1 Belle Hot Topics Nagoya University Koji Ikado (for the Belle Collaboration) Flavor Physics and CP Violation Conference (FPCP2006) Apr. 9, 2006 Outline Search for the Purely Leptonic Decay Β τν Results from the Y(5S) Engineering Run Summary

2 KEKB & Belle Low Energy Ring (LER) for Positron TSUKUBA Area (Belle) HER LER Interaction Region High Energy Ring (HER) for Electron WIGGLER RF RF NIKKO Area WIGGLER RF RF OHO Area (TRISTAN Accumulation Ring) HER Electron Positron e + /e - LER RF RF FUJI Area Linac Asymmetric-energy e + e - collider 8GeV 3.5 GeV High Luminosity 500 fb -1 Collected ~560 fb -1 L ~ 1.5 x 10 34

3 Search for the Purely Leptonic Decay Β τν

4 B τν and B Meson Decay Constant f B Leptonic decay proceeding through W boson annihilation in the Standard Model Decay rate simply related to B meson decay constant f B Helicity suppressed by light lepton mass : τν is favored over eν and µν Clean experimental method of measuring B meson decay constant f B Physics beyond the SM could enhance the branching fraction through the introduction of a charged Higgs boson Apr

5 B τν and B Meson Decay Constant f B (cont.) Search for B τν is important for both of SM and physics beyond the SM Experimentally challenging due to the presence of additional neutrinos from τ decay B τν search history B factories Experimental limit on B(B τν ) is getting close to the prediction LEP The story has a happy ending?

6 B τν Analysis Concepts B decays with missing neutrinos lack the kinematic constraints which are used to separate signal events from backgrounds (M bc and E) ν ν B - Y(4S) B + More than 2 neutrinos appear in B τν decay π + Reconstruct the decay of the non-signal B (tagging), then look for the signal decay in whatever is left over Tagging side : Fully reconstruct hadronic modes Signal side : Reconstruct particles from τ decay Apr

7 Features with Fully Reconstructed B Tag Merit : Offline B meson Beam B momentum is available -Resolution of M miss2 can be significantly improved. -separate similar semileptonic decays reduce background significantly B-flavor is also available -We can treat charged & neutral B separately Demerit : Low statistics Efficiency : % However, we can rely on KEKB, providing World record luminosity M miss2 for B - D 0 µ ν (MC) w/o B momentum with B momentum S/N good mod. Full recon. D (*) l ν tag Advanced ν recon. Lum. trad. ν recon. poor low middle high Eff.

8 Fully Reconstructed B s with 447 M BB B D + π / ρ / a / DS + (*) (*) D B D + π / ρ / a / DS 0 (*) (*) 1 D π / D γ D + s γ π / D π 0 0 Beam constrained mass D + s γ 0 D D 7 modes 6 modes D + s 2 modes ~ 180 channels used N= 680k eff.= 0.29% purity = 57% Charged B N = 412 k eff.= 0.19% purity = 52% Neutral B m ~ 5.28 GeV/c 2 σ~ 3 MeV/c 2 due to σ(e beam ) ~10% for feed-across between B + and B 0 Signal region : < E < 0.06 GeV, M bc > 5.27 GeV/c 2

9 Signal Selection τ lepton is identified in the 5 decay modes 81% of all τ decay modes Total efficiency with τ decay branching fraction : ±0.12% All the selection criteria have been optimized to achieve the highest sensitivity

10 Signal Selection Extra neutral energy in calorimeter E ECL - Most powerful variable for separating signal and background -Total calorimeter energy from the neutral clusters which are not associated with the tag B Minimum energy threshold Barrel : 50 MeV Forward(Backward) endcap : 100(150) MeV Zero or small value of E ECL arising only from beam background Higher E ECL due to additional neutral clusters B + B - is dominant in background

11 Signal Selection Extra neutral energy E ECL Validation -Double tagged sample, B tag is fully reconstructed and B sig is semileptonic mode B + D (*)0 X + (fully reconstruction) B - D *0 l ν D 0 π 0 K - π + K - π + π - π + B + B ± 18 B 0 B ± 2.2 Total 502 ± 18 Data 458 Purity 90% Validate with double tagged events

12 Background Estimation MC : 94.2 ± 8.0 Data : 96 MC : 89.6 ± 8.0 Data : 93 MC : 41.3 ± 6.2 Data : 43 MC : 23.3 ± 4.7 Data : 21 MC : 18.5 ± 4.1 Data : 21 Sideband Total MC : 267 ± 14 Data : 274 Large MC samples for e + e - BB, qq, X u lν, X u τν, τ + τ -, and rare B decays are used (including beam-background) Good agreement between data and MC in sideband region Validity of background MC simulation

13 Extra Calorimeter Energy in Data Observed events compared with background expected 414 fb -1 After finalizing the signal selection criteria, the signal region is examined Number of data in signal region Observe excess in signal region

14 Fit Results Unbinned Likelihood fit to the obtained E ECL distributions Signal + background Σ : Significance with systematics B τν Signal Background Background yield is consistent with the expectation from the MC simulation Signal shape : Gauss + exponential Background shape : second-order polynomial +6.7 Observe 21.2 events with -5.7 a significance of 4.2σ

15 Systematic Uncertainty Signal selection efficiencies Tag reconstruction efficiency : 10.5% Difference of yields between data and MC in the B - D *0 l - ν control sample Number of BB : 1% Signal yield : +12% -10% signal shape ambiguity estimated by varying the signal PDF parameters BG shape : changing PDF Total systematic uncertainty +17% -15%

16 B τν Branching Fraction Branching fractions are calculated by Extracted branching fraction for each τ decay mode All τ decay modes combined SM : B(Β τν)=(1.59 ± 0.40) 10-4 Result is consistent with SM prediction within error

17 f B Extraction Product of B meson decay constant f B and CKM matrix element V ub Using V ub = (4.38 ± 0.33) 10-3 from HFAG f B = ± GeV (HPQCD) Phys. Rev. Lett. 95, (2005)

18 B τν Candidate Event BELLE B + D 0 π + B - τ - ν K + π - π + π - e - νν Missing K + π + π e π π + 20 cm

19 Constraints on Physics Parameters CKM parameters -Constraint in the (ρ,η) plane from the Β τν branching fraction and m d

20 Constraints on Physics Parameters Constraint on Charged Higgs 2σ 95.5%C.L. exclusion boundaries

21 B τν Search History Branching fraction upper limits obtained from various experiments First evidence of the purely leptonic decay Β τν

22 Results from the Y(5S) Engineering Run

23 B Factory Running at Y(5S) Provide the possibility to study Bs decays -e + e - collider has advantages over hadronic machines -CLEO has observed B s signals from 0.42fb -1 Y(5S) data Y(5S) Engineering Run performed at KEKB/Belle - Achieved high luminosity as Y(4S) run L peak =13.9/nb/sec -L int =1.86 fb -1 has been taken during 3 days runs (June 21-23, 2005) e + e - Y(5S) BB, B * B, B * B *,BBπ,BBππ, B s B s, B s* B s, B s* B s * e + and e - beam energies were increased by 2.7% (same Lorentz boost = 0.425) to move from Y(4S) to Y(5S) [PRL (2006)] ϒ(4S) ϒ(5S) CLEO PRL 54, 381 (1985) No modifications for Belle detector, trigger system or software

24 Reconstructed B s Candidates B s signals are identified with M bc and E B s B s, B s *B s, B s *B s * B s D s+ π - B s D s * + π - 9 events in B s * B s * B s D s( * )+ ρ - 4 events in B s * B s * B s J/ψ φ/η M bc vs E B s B s, B s* B s, B s* B s * signals can be separated well < E< MeV 7 events in B s * B s * 3 events in B s * B s * B s * B s * Clear B s signals seen in B s* B s* region M (B s ) = 5370 ± 1 ± 3 MeV/c 2

25 Search for Rare B s Decays B s γ γ B s K + K - B s φ γ 0 candidates Exp. BG: candidates Exp. BG: candidates Exp. BG: 0.15 B(B s γγ )<0.56x10-4 (90% CL) PDG :<1.48x10-4 SM: x10-4 B(B s K + K - )<3.4x10-4 (90% CL) PDG :<0.59x10-4 B(B s φγ )<4.1x10-4 (90% CL) PDG :<1.2x10-4

26 Other Topics by Belle Collaborators φ 1 /β from B Charm and Charmoniumm - T. Hara (14:00-14:30 Sunday, Apr. 9) φ 2 /α - C. C. Wang (16:00-16:30 Sunday, Apr. 9) V ub from exclusive semileptonic decays - K. Varvell (11:30-12:00 Tuesday, Apr. 11) Inclusive semileptonic decays - E. Barberio (14:00-14:30 Tuesday, Apr. 11) New physics searches at B factories - H. Kakuno (17:00-17:30 Tuesday, Apr. 11) Direct CP violation in Charmless B decays - Y. Chao (14:00-14:30 Wednesday, Apr. 12) Super B factories - N. Katayama (16:00-16:25 Wednesday, Apr. 12)

27 Summary Performed search for the rare leptonic B decay B τν with 414 fb -1 - Detect the signal with a significance of 4.2σ - First evidence of the purely leptonic B decay - First direct measurement of the B meson decay constant f B Result is submitted to PRL Engineering run at Y(5S) at KEKB - See A. Drutskoy s talk at Moriond EW for more details

28 Backup Slides

29 B τν and Decay Constant f B Expected branching fraction V ub = (4.38 ± 0.33) 10-3 from HFAG (hep-ex/ ) f B = ± GeV from lattice QCD Currently, our best knowledge of f B comes from lattice QCD calculations with uncertainty of 10% HPQCD result, Phys. Rev. Lett. 95, (2005) Apr

30 Fit Results Likelihood fit to the obtained E ECL distributions Signal + background Background Signal

31 Fit Results Likelihood distributions for each τ decay

32 Fit Results M bc and P mis distributions

33 Search for B s D s (*) D s (*) CP even eigenstates lead to DG/G Γ s Bf(B s ->D ( s * ) + D s( * ) - ) ~ Γ s 1- Bf(B s ->D s( * ) + D s( * ) - ) / 2 Can be compared with the direct measurement to test SM Expected :~ 0.5 events in each mode. B(B s D s + Ds - ) < 7.1% at 90% CL B(B s D s * + D s - ) < 12.7% at 90% CL B(B s D s * + D s * - ) < 27.3% at 90% CL 50fb -1 expectation: B(B s ->D s( * ) + Ds( * ) - ) = (12 3 )%