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

Koji Ikado Beauty 2006  Leptonic decay proceeding through W boson annihilation in the Standard Model  Decay rate simply related to B meson decay constant f B and |V ub | Helicity suppressed by lepton mass :  is favored over e and   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 B  l Introduction

Koji Ikado Beauty 2006 d dl l l l B 0  l + l -  Neutral B mesons decay to l + l - via box or penguin annihilation  Branching fractions are suppressed by lepton mass  Flavor violating channel ( B 0  e +  –, etc.) are forbidden in SM  Some new-physics can enhance the branching fractions by orders of magnitude B(B 0  e + e – ) ~ B(B 0   +  – ) ~ B(B 0  ) ~ zero Introduction (cont.)

Koji Ikado Beauty 2006 B 0  l + l -   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(B 0  e + e –  ) ~ B(B 0   +  –  ) ~ B(B 0  –  ) ~ 9 × Introduction (cont.)

Koji Ikado Beauty 2006 

B   Analysis  B decays with missing neutrinos lack the kinematic constraints which are used to separate signal events from backgrounds (M bc and  E)  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   decay Tagging side : Fully reconstruct hadronic modes Signal side : Reconstruct particles from  decay  (4S) B-B- B+B+  e+e+  e B +  + ,  +  e + e  B-XB-X

Koji Ikado Beauty 2006 Fully Reconstructed B’s with 449 M Belle 7 modes 6 modes 2 modes  Beam constrained mass Signal region : GeV/c 2 ~10% for feed-across between B + and B 0 m ~ 5.28 GeV/c 2  ~ 3 MeV/c 2 due to  (E beam ) ~ 180 channels used N= 680k eff.= 0.29% purity = 57% N= 680k eff.= 0.29% purity = 57% N = 412 k eff.= 0.19% purity = 52% N = 412 k eff.= 0.19% purity = 52% Charged B Neutral B

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

Koji Ikado Beauty 2006 Signal Selection  Extra neutral energy in calorimeter E ECL - 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 E ECL arising only from beam background Higher E ECL due to additional neutral clusters B + B - is dominant in background

Koji Ikado Beauty 2006 Extra Calorimeter Energy in Data  Observed events compared with background expected After finalizing the signal selection criteria, the signal region is examined Observe excess in signal region Number of data in signal region 414 fb -1

Koji Ikado Beauty 2006  Unbinned Likelihood fit to the obtained E ECL distributions Fit Results Signal shape : Gauss + exponential Background shape : Gauss + second-order  : Significance with systematics Background yield is consistent with the expectation from the MC simulation Observe 17.2 events with a significance of 3.5  Signal + background B   Signal Background (peaking background is included)

Koji Ikado Beauty 2006 Branching Fraction  All  decay modes combined  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 B(    ) = (1.59  0.40)×10 -4 (SM prediction) (|V ub | = (4.38  0.33)×10 -3 & f B =  GeV ) f B =  GeV (HPQCD) Phys. Rev. Lett. 95, (2005)

Koji Ikado Beauty 2006 Constraints on Physics Parameters 95.5%C.L. exclusion boundaries 22 rHrH  Constraint on Charged Higgs Phys. Rev. D48, 2342 (1993)

Koji Ikado Beauty 2006 B   Babar  Babar searches for in a sample of 324x10 6 BB events Reconstruct one B in a semileptonic final state B  Dl X D  K , K   , K  , K s   (X= ,  from D*0 is not explicitly reconstructed) Require lepton CM momentum > 0.8 GeV Require that -2 < cos  B-D0l < 1 Parent B energy and momentum are determined from the beam energy Tagged B reconstruction efficiency ~0.7%  Discriminate signal from background using E extra   lepton is identified in the 4 decay modes

Koji Ikado Beauty 2006  Results Observe a result consistent with zero signal at 1.3 , set a limit on the branching fraction and quote a central value Calculate the product of f B and |V ub | B   Babar (cont.) Babar preliminary

Koji Ikado Beauty 2006    e +     

Koji Ikado Beauty 2006  Search for B  l in a sample of 229x10 6 BB events Reconstruct one B in a fully hadronic final state Reconstruct B  D (*)0 X    e +     Identify a monoenergetic electron or muon recoiling against B tag

Koji Ikado Beauty 2006    e +      fb   Observe 0 (0) events in the signal box in electron (muon) events  Set upper limits on the branching fractions Babar preliminary

Koji Ikado Beauty 2006  Highly energetic lepton  Companion B reconstructed with the remaining particles Update is coming soon P l B rest (GeV)    e +     Belle preliminary

Koji Ikado Beauty 2006 B 0  l +  l –

Koji Ikado Beauty 2006 B 0  l +  l – (e + e –,  +  –, e +  –  Very straightforward : reconstruct m ES and  E Extremely clean monochromatic kinematics Strategy: define selection for sideband and blind signal box in m ES,  E  Selection variables | cos  T |:  T is angle between thrust axes of l + l – candidate and rest of event m ROE : invariant mass of rest of event R 2 : norm ’ d 2 nd Fox-Wolfram moment N trk + ½ N  : measure of multiplicity E EMC < 11 GeV, rejects QED Signal MC (blue) vs. sideband data accept

Koji Ikado Beauty 2006 B(B 0  e + e – ) < 6.1 × (90%CL) B(B 0   +  – ) < 8.3 × (90%CL) B(B 0  e +  – ) < 18 × (90%CL) B 0  l +  l – (e + e –,  +  –, e +  – ) results e+e–e+e– +–+– e-+e-+ Signal regions Events observed Limits : Phys. Rev. Lett. 94, (2005) B(B 0  e + e – ) < 1.9 × (90%CL) B(B 0   +  – ) < 1.6 × (90%CL) B(B 0  e +  – ) < 1.7 × (90%CL) B(B 0 d      ) < 2.3×10 -8 (90% CL) Phys. Rev. D 68, (2003) 78 fb  780 pb  111 fb  BB pairs used: (122.5±1.0)×10 6

Koji Ikado Beauty 2006 B 0   +   First limit on this channel Experimentally very challenging : 2-4 neutrinos in signal Constrain in particular leptoquark coupling and tan  in SUSY  Analysis Reconstruct one B in a fully hadronic final state B  D (*) X =>280k events In the event remainder, look for two  decays (   l, ,  ) Kinematics of charged partilce momenta and residual energy are fed into a neutral network to separate signal and BG Observed events : 263  19 Expected events : 281  48 B(B 0   +  – ) < 4.1 × (90%CL) Phys. Rev. Lett. 96, (2006) 210 fb   Data Control sample

Koji Ikado Beauty 2006 B 0    Semileptonic tags : B 0  D ( * )- l + (D* -  D 0   )  Require nothing in recoil: no charged tracks, limited neutrals  Signal obtained from ML fit to E extra Signal: 17 ± 9, background  Upper limits Systematics Additive: 7.4 events Multiplicative: 10.9% Frequentist limit-setting procedure – 8 B pairs used: (88.5±1.0)×10 6 B(B 0  invisible) < 22 × (90%CL) Phys. Rev. Lett. 93, (2004) 

Koji Ikado Beauty 2006 B 0  l + l - 

Koji Ikado Beauty 2006  Search for B  ll  in a sample of 324x10 6 BB events  Reconstruct B candidates from two leptons and a photon Leptons required to be 0.3 < m ll < 4.9 (4.7) GeV for electrons (muons) Reject backgrounds from J/ ,  (2S) decay (leptons) or  0 decay(photon) Reject qq background using signal B kinematics and event shape in a Fisher discriminant  Background is determined by extrapolating the m ES sideband into the signal box B 0  l + l -  e signal = 6.07  0.14% &   signal = 4.93  0.12% N e bkg = 1.28  0.80 & N  bkg = 1.40  0.42 e+e-e+e-+-+-

Koji Ikado Beauty 2006  Observe 0 (3) events in the signal box in electron (muon) events Set upper limits on the branching fractions B 0  l + l - Babar preliminary e + e -   +  -  B(B 0    ) < 4.7 × (90%CL) B(B 0  e + e -  ) < 0.7 × (90%CL) B(B 0       ) < 3.4 × (90%CL) Phys. Rev. Lett. 93, (2004)

Koji Ikado Beauty 2006 Summary  Performed searches for rare leptonic B decays B  l, ll, ll   Belle found first evidence for B   - First direct measurement of the B meson decay constant f B 4

Koji Ikado Beauty 2006 Backup Slides

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

Koji Ikado Beauty 2006 Features with Fully Reconstructed B Tag  Merit : Offline B meson Beam  B momentum is available -Resolution of M miss 2 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 w/o B momentum with B momentum M miss 2 for B -  D 0    (MC) low middle high poor mod. good S/N Eff. trad. recon. Advanced recon. D (*) l tag Lum. Full recon.

Koji Ikado Beauty 2006 Signal Selection  Extra neutral energy E ECL Validation -Double tagged sample, B tag is fully reconstructed and B sig is semileptonic mode Validate with double tagged events B +  D (*)0 X + (fully reconstruction) B -  D *0 B -  D *0 l  D 0  D 0  0 K -  + K -  + K -  +  -  + K -  +  -  + B+B-B+B- 494  18 B0B0B0B0 7.9  2.2 Total 502  18 Data458 Purity ～ 90%

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

Koji Ikado Beauty 2006 B   Candidate Event B +  D 0  + K +  -  +  - K +  -  +  - B -   -  e - e -

Koji Ikado Beauty 2006 B   and Decay Constant f B  Expected branching fraction  Currently, our best knowledge of f B comes from lattice QCD calculations with uncertainty of 10% |V ub | = (4.38  0.33)×10-3 from HFAG (hep-ex/ ) f B =  GeV from lattice QCD HPQCD result, Phys. Rev. Lett. 95, (2005)

Koji Ikado Beauty 2006 Systematic Uncertainty  Signal selection efficiencies  Tag reconstruction efficiency : 10.5% B -  D *0 l Difference of yields between data and MC in the B -  D *0 l - control 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% % +25.5% %

Koji Ikado Beauty 2006 Constraints on Physics Parameters  CKM parameters -Constraint in the ( ,  ) plane from the    branching fraction and  m d

Koji Ikado Beauty 2006  M bc and P mis distributions Fit Results

Koji Ikado Beauty 2006 Constraints at Super-B  Br(B   ) 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  |V ub | measurement: < 5% in future is an realistic goal  f B from theory ~10% now  5% (?) Lum.  B(B   ) exp  |V ub | 414 fb -1 36%7.5% 5 ab -1 10%5.8% 50 ab -1 3%4.4% Assumption in the following plots

Koji Ikado Beauty 2006 Constraints at Super-B (cont.)  f B (LQCD) = 5% 95.5%C.L. exclusion boundaries rHrH 22 rHrH 5ab -1 50ab -1 If  |V ub | = 0 &  f B = 0