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Rare B Decays : B l n , ll, ll g

Rare B Decays : B l n , ll, ll g. Nagoya University Koji Ikado The 11 th International Conference on B-Physics at Hadron Machines (Beauty 2006) Sep. 28, 2006. Introduction. B l n. Leptonic decay proceeding through W boson annihilation in the Standard Model

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Rare B Decays : B l n , ll, ll g

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  1. Rare B Decays : Bln, ll,llg Nagoya University Koji Ikado The 11th International Conference on B-Physics at Hadron Machines (Beauty 2006) Sep. 28, 2006 Koji Ikado Beauty 2006

  2. Introduction Bln • Leptonic decay proceeding through W boson annihilation in the Standard Model • Decay rate simply related to B meson decay constant fB and |Vub| Helicity suppressed by lepton mass : tn is favored over en and mn • Clean experimental method of measuring B meson decay constant fB • Physics beyond the SM could enhance the branching fraction through the introduction of a charged Higgs boson Koji Ikado Beauty 2006

  3. Introduction (cont.) B0l+ l- l l l d l d • Neutral B mesons decay to l+l- via box or penguin annihilation • Branching fractions are suppressed by lepton mass • Flavor violating channel (B0e+ m–, etc.) are forbidden in SM • Some new-physics can enhance the branching fractions by orders of magnitude B(B0 e+e–) ~ 10-15B(B0m+m–) ~ 10-10B(B0nn) ~ zero Koji Ikado Beauty 2006

  4. Introduction (cont.) B0l+ l-g • Radiative dilepton decays • Photon emission from the initial state relaxes the helicity suppression • SM predictions • SM branching fraction much below experimental sensitivity  Search for new physics B(B0 e+e–g) ~ 10-10B(B0m+m–g) ~ 10-10 B(B0nn–g) ~ 9 ×10-10 Koji Ikado Beauty 2006

  5. Btn Koji Ikado Beauty 2006

  6. Btn Analysis Concepts@Belle e+ (4S) B- B+ ne n n B++, +e+e B-X • B decays with missing neutrinos lack the kinematic constraints which are used to separate signal events from backgrounds (Mbc and DE) • Reconstruct the decay of the non-signal B (tagging), then look for the signal decay in whatever is left over More than 2 neutrinos appear in B tn decay Tagging side : Fully reconstruct hadronic modes Signal side : Reconstruct particles from t decay Koji Ikado Beauty 2006

  7. Fully Reconstructed B’s with 449 M BB @ Belle ~ 180 channels used 7 modes 6 modes 2 modes • Beam constrained mass N= 680k eff.= 0.29% purity = 57% N = 412 k eff.= 0.19% purity = 52% m ~ 5.28 GeV/c2 s~ 3 MeV/c2 due to s(Ebeam) ~10% for feed-across between B+ and B0 Neutral B Charged B Signal region : -0.08 < DE < 0.06 GeV, Mbc > 5.27 GeV/c2 Koji Ikado Beauty 2006

  8. Signal Selection • t lepton is identified in the 5 decay modes 81% of all t decay modes Total efficiency with t decay branching fraction : 15.81 0.05% All the selection criteria have been optimized to achieve the highest sensitivity Koji Ikado Beauty 2006

  9. Signal Selection B+B- is dominant in background • Extra neutral energy in calorimeter EECL - Most powerful variable for separating signal and background -Total calorimeter(ECL) 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 EECL arising only from beam background Higher EECL due to additional neutral clusters Koji Ikado Beauty 2006

  10. 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 Koji Ikado Beauty 2006

  11. Fit Results • Unbinned Likelihood fit to the obtained EECL distributions Signal + background S : Significance with systematics Background Background yield is consistent with the expectation from the MC simulation (peaking background is included) Btn Signal Observe 17.2 events with a significance of 3.5s +5.3 - 4.7 Signal shape : Gauss + exponential Background shape : Gauss + second-order Koji Ikado Beauty 2006

  12. Branching Fraction • All t decay modes combined • Product of B meson decay constant fB and CKM matrix element |Vub| • Using |Vub| = (4.38  0.33)×10-3 from HFAG B(Btn) = (1.59  0.40)×10-4 (SM prediction) (|Vub| = (4.38  0.33)×10-3 & fB = 0.216  0.022 GeV ) fB = 0.216  0.022 GeV (HPQCD) Phys. Rev. Lett. 95, 212001 (2005) Koji Ikado Beauty 2006

  13. Constraints on Physics Parameters • Constraint on Charged Higgs Phys. Rev. D48, 2342 (1993) rH 2s 95.5%C.L. exclusion boundaries Koji Ikado Beauty 2006

  14. BtnSearch @ Babar • Babar searches for in a sample of 324x106 BB events • Reconstruct one B in a semileptonic final state BDlnX DK p, K ppp, K pp, Kspp (X=g, p from D*0 is not explicitly reconstructed) • Require lepton CM momentum > 0.8 GeV • Require that -2 < cosqB-D0l < 1 • Parent B energy and momentum are determined from the beam energy • Tagged B reconstruction efficiency ~0.7% • Discriminate signal from background using Eextra • t lepton is identified in the 4 decay modes Koji Ikado Beauty 2006

  15. BtnSearch @ Babar (cont.) • Results Observe a result consistent with zero signal at 1.3s, set a limit on the branching fraction and quote a central value • Calculate the product of fB and |Vub| Babar preliminary Koji Ikado Beauty 2006

  16. B+e+n & B+m+n Koji Ikado Beauty 2006

  17. B+e+n & B+m+n@Babar • Search for Bln in a sample of 229x106 BB events • Reconstruct one B in a fully hadronic final state Reconstruct B D(*)0 X Identify a monoenergetic electron or muon recoiling against Btag Koji Ikado Beauty 2006

  18. B+e+n & B+m+n@Babar • Observe 0 (0) events in the signal box in electron (muon) events • Set upper limits on the branching fractions 208.7fb-1 Babar preliminary Koji Ikado Beauty 2006

  19. B+e+n & B+m+n@Belle • Highly energetic lepton • Companion B reconstructed with the remaining particles Belle preliminary PlB rest (GeV) Belle preliminary Update is coming soon Koji Ikado Beauty 2006

  20. B0 l+l– Koji Ikado Beauty 2006

  21. B0 l+l– (e+e–,m+m–,e+m–) @Babar • Very straightforward : reconstruct mES and DE • Extremely clean monochromatic kinematics • Strategy: define selection for sideband and blind signal box in mES, DE • Selection variables • | cos qT |: qT is angle between thrust axes of l+l– candidate and rest of event • mROE: invariant mass of rest of event • R2: norm’d 2nd Fox-Wolfram moment • Ntrk + ½Ng: measure of multiplicity • EEMC < 11 GeV, rejects QED accept accept accept accept Signal MC (blue)vs. sideband data Koji Ikado Beauty 2006

  22. B0 l+l– (e+e–,m+m–,e+m–) results BB pairs used: (122.5±1.0)×106 Signal regions Events observed e+e– m+m– • Limits: • B(B0e+e–) < 6.1 × 10-8 (90%CL) • B(B0m+m–) < 8.3 × 10-8 (90%CL) • B(B0e+ m–) < 18 × 10-8 (90%CL) e-m+ 111 fb-1 Phys. Rev. Lett. 94, 221803 (2005) • B(B0e+e–) < 1.9 × 10-7 (90%CL) • B(B0m+m–) < 1.6 × 10-7 (90%CL) • B(B0e+ m–) < 1.7 × 10-7 (90%CL) 78 fb-1 Phys. Rev. D 68, 111101 (2003) B(B0dm+m-) < 2.3×10-8 (90% CL) 780 pb-1 Koji Ikado Beauty 2006

  23. B0t+t– @Babar • First limit on this channel • Experimentally very challenging : 2-4 neutrinos in signal • Constrain in particular leptoquark coupling and tanb in SUSY • Analysis • Reconstruct one B in a fully hadronic final state B D(*) X =>280k events • In the event remainder, look for two t decays (tlnn, pn, rn) • Kinematics of charged partilce momenta and residual energy are fed into a neutral network to separate signal and BG Control sample Data • B(B0t+t–) < 4.1 × 10-3 (90%CL) Observed events : 26319 Expected events : 28148 Phys. Rev. Lett. 96, 241802 (2006) 210 fb-1 Koji Ikado Beauty 2006

  24. B0 n n (invisible) @Babar B pairs used: (88.5±1.0)×106 • Semileptonic tags : B0D(*)-l+n (D*-D0p-) • Require nothing in recoil: no charged tracks, limited neutrals • Signal obtained from ML fit to Eextra • Signal: 17 ± 9, • background 19 + 10 • Upper limits • Systematics • Additive: 7.4 events • Multiplicative: 10.9% • Frequentist limit-setting procedure nn – 8 nng B(B0 invisible) < 22 × 10-5 (90%CL) Phys. Rev. Lett. 93, 091802 (2004) Koji Ikado Beauty 2006

  25. B0l+ l-g Koji Ikado Beauty 2006

  26. B0l+ l-g @Babar • Search for Bllg in a sample of 324x106 BB events • Reconstruct B candidates from two leptons and a photon • Leptons required to be 0.3 < mll < 4.9 (4.7) GeV for electrons (muons) • Reject backgrounds from J/y, y (2S) decay (leptons) or p0 decay(photon) • Reject qq background using signal B kinematics and event shape in a Fisher discriminant • Background is determined by extrapolating the mES sideband into the signal box eesignal = 6.07 0.14% & emsignal = 4.93 0.12% Nebkg = 1.280.80 & Nmbkg = 1.400.42 e+e-g m+m-g Koji Ikado Beauty 2006

  27. B0l+ l-g @Babar • Observe 0 (3) events in the signal box in electron (muon) events • Set upper limits on the branching fractions e+ e-g m+ m-g B(B0 e+e-g) < 0.7 × 10-7 (90%CL)B(B0 m+m-g ) < 3.4 × 10-7 (90%CL) Babar preliminary B(B0 n ng ) < 4.7 × 10-5 (90%CL) Phys. Rev. Lett. 93, 091802 (2004) Koji Ikado Beauty 2006

  28. Summary • Performed searches for rare leptonic B decays Bln, ll, llg • Belle found first evidence for Btn - First direct measurement of the B meson decay constant fB 4 Koji Ikado Beauty 2006

  29. Backup Slides Koji Ikado Beauty 2006

  30. KEKB & Belle 500 fb-1 • Asymmetric-energy e+e- collider • 8GeV – 3.5 GeV • High Luminosity • L ~ 1.5 x 1034 Collected ~560 fb-1 Koji Ikado Beauty 2006

  31. Features with Fully Reconstructed B Tag good mod. poor low high middle Mmiss2 for B-D0 m- n (MC) • Merit : Offline B meson Beam • B momentum is available -Resolution of Mmiss2 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 : 0.2 - 0.3% • However, we can rely on KEKB, providing World record luminosity w/o B momentum with B momentum S/N Full recon. D(*) l n tag Advanced n recon. Lum. trad. n recon. Eff. Koji Ikado Beauty 2006

  32. Signal Selection • Extra neutral energy EECL Validation -Double tagged sample, Btag is fully reconstructed and Bsig is semileptonic mode B+g D(*)0 X+ (fully reconstruction) B-g D*0 l-n D0p0 K-p+ K-p+ p-p+ Purity ～ 90% Validate with double tagged events Koji Ikado Beauty 2006

  33. Background Estimation MC : 41.3  6.2 Data : 43 MC : 89.6  8.0 Data : 93 MC : 94.2  8.0 Data : 96 MC : 18.5  4.1 Data : 21 Sideband Total MC : 267  14 Data : 274 MC : 23.3  4.7 Data : 21 Large MC samples for e+e- BB, qq, Xuln, Xu tn,t+ t- , and rare B decays are used (including beam-background) Good agreement between data and MC in sideband region  Validity of background MC simulation Koji Ikado Beauty 2006

  34. Btn Candidate Event B+g D0 p+ K+p- p+p- B-gt - n e-nn Koji Ikado Beauty 2006

  35. Btn and Decay Constant fB • Expected branching fraction • Currently, our best knowledge of fB comes from lattice QCD calculations with uncertainty of 10% |Vub| = (4.38  0.33)×10-3 from HFAG (hep-ex/0603003) fB = 0.216  0.022 GeV from lattice QCD HPQCD result, Phys. Rev. Lett. 95, 212001 (2005) Koji Ikado Beauty 2006

  36. Systematic Uncertainty • Signal selection efficiencies • Tag reconstruction efficiency : 10.5% Difference of yields between data and MC in the B- D*0l-ncontrol sample • Number of BB : 1% • Signal yield : signal shape ambiguity estimated by varying the signal PDF parameters BG shape : changing PDF • Total systematic uncertainty +22.5% - 25.7% +25.5% - 28.4% Koji Ikado Beauty 2006

  37. Constraints on Physics Parameters • CKM parameters -Constraint in the (r,h) plane from the Btnbranching fraction and Dmd Koji Ikado Beauty 2006

  38. Fit Results • Mbc and Pmis distributions Koji Ikado Beauty 2006

  39. Constraints at Super-B Br(Bt n) measurement : Further accumulation of luminosity helps to reduce both statistical and systematic errors - Some of the major systematic errors come from limited statistics of the control sample |Vub| measurement: < 5% in future is an realistic goal fB from theory ~10% now  5% (?) Assumption in the following plots Koji Ikado Beauty 2006

  40. Constraints at Super-B (cont.) If D|Vub| = 0 & DfB = 0 95.5%C.L. exclusion boundaries DfB(LQCD) = 5% 5ab -1 rH 2s 50ab -1 rH Koji Ikado Beauty 2006

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