Searches for Leptonic Decays of the B-meson at BaBar

Transcription

1 Searches for Leptonic Decays of the B-meson at BaBar Stephen Jacob Sekula (MIT) on behalf of the BaBar collaboration Presented at Frontiers in Contemporary Physics III Vanderbilt University May 22-28, 2005 Page 1

2 Rare Leptonic B Decays B + l +, B + ( K + /p + ) Å, B 0 Å(g)... proceed through W-boson annihilation or Electroweak penguin and box diagrams in the Standard Model (SM) + B + l + B 0 Z 0 Å(g) Å are theoretically clean are suppressed in the SM by CKM matrix elements, helicity suppression, or by penguin/loop domination. probe for new physics B - K - Å Page 2

3 Event Reconstruction Method Reconstruct ( tag ) one B in either a hadronic or semileptonic final state. B D X hadronic X hadronic represents a combination of K +,p +,K s0,p 0 or B D l Constrain the recoil to determine its consistency with a signal hypothesis Statistically independent samples D 0 K D 0 K s 0 D 0 K 0 D 0 K Page 3

4 B + l + Overview Standard Model Predictions BR SM B l f B 2 V ub 2 m l BR SM (B ) = BR SM (B ) = (4.2±1.7) 10-7 BR SM (B e ) = (9.3±3.9) 10-5 Largest theoretical uncertainties due to... f B = V ub = (9.8±4.1) (0.196±0.032) GeV (3.67±0.47) m B 2 Known only from theory (i.e. lattice QCD) Challenging measurements Mode Belle: Existing Experimental Limits Branching Fraction Limit (90% Confidence Level) Reference (Experiment) B e Phys. Rev. Lett. 75, 785 (1995) (CLEO) Phys. Rev. Lett. 92, (2004) (BaBar) Phys. Lett. B 396, 327 (1997) (L3) BR(B ) < Preliminary (2004), 152M BB pairs (hep-ex/ ) BaBar B Analysis 232 Million BB pairs used. Events are currently tagged using only the semileptonic method. Result is combined with a hadronic tagged result using 89 Million BB pairs Page 4

5 B + + Method Semileptonic Tag Reconstruction tag = ( (stat.) 0.053(syst.) ) 10-3 purity: ~64% ~200k B in the recoil Reconstruct taus in several modes: (~80% of the tau branching fraction) e Select single-prong modes using particle ID and track momentum. 0 Select multi-prong modes using intermediate resonances Suppress e + e - qq, + - backgrounds with event-shape variables Final signal selection done using cut on energy remaining in the calorimeter (E extra ) signal simulation background simulation Data e Å Selection Page 5

6 BaBar Preliminary B + + Results Mode Signal-side Expected Bkg. Observed Events in e p Failed PID pp 0 ppp Efficiencies ( i ) % 7.5±0.4± ±0.2± ±0.4± ±0.4± ±0.2± ±0.16±0.10 Events (N exp ) ± On-peak Data (N obs ) Dominant systematic uncertainties: background estimate E extra modeling particle identification and tracking Total Systematic Error on Efficiency: 2% 90% C.L. Upper Limits: BR B Semileptonic BR B Hadronic (89M BB events, hep-ex/ ) BR B COMBINED Page 6

7 Constraints on the Standard Model BR B τν Δm d and Beyond from B + + V ub 2 V td = 1 2 [1 λ 2 /2 ] 2 ρ 2 η 2 Type II Two-Higgs Doublet 1 ρ 2 η 2 Model BR=BRSM 1 tan 2 β m 2 B ± 2 m H ± 2 excluded Page 7 tan /m H ± 0.34 GeV /c 2 1

8 B + (K + /p + ) Å Overview Standard Model Predictions Flavor-changing neutral current process, allowed via EW box/penguin diagrams. BR SM (B K Å) = b d Å is suppressed over b s Å by V td 2 V ts 2 Existing Experimental Limits Best published limit (CLEO): BR B + K Phys. Rev. Lett. 86, 2950 (2001) at the 90% confidence level. There is no existing limit for p + Å. BaBar Analysis 89 Million BB pairs used. Events are tagged using both the hadronic and semileptonic methods. Page 8

9 Hadronic B tags selected using the variables m ES and ΔE B + (K + /p + ) Å Method (Hadronic B tags) Signal Selection Signal B must contain only a single track Track identified as a kaon, or as neither a kaon, electron, or muon (selects pions). Event shape variables used to suppress e + e - qq, + - background Events containing p0 s are vetoed Yield: ~180k B ± in the recoil Signal selected using E extra and the K/p track momentum Page 9

10 B + K + Å Results (Hadronic B tags) Data Background Simulation Signal Simulation Data Background Simulation Signal Simulation Phys.Rev.Lett. 94 (2005) Results: Background expectation: 3.8±1.1 Data Yield: 3 events Dominant systematic uncertainties: background estimate MC Statistics E extra modeling Total Systematic Error on Efficiency: ~44% 90% C.L. Upper Limits: BR B K BR B K BR B K Page 10 Hadronic Semileptonic COMBINED

11 Data Background Simulation Signal Simulation B + p + Å Results (Hadronic B tags) Data Background Simulation Signal Simulation Phys.Rev.Lett. 94 (2005) Results: Background expectation: 24.1±3.6 Data Yield: 21 events Dominant systematic uncertainties: background estimate MC Statistics E extra modeling Total Systematic Error on Efficiency: ~44% 90% C.L. Upper Limit: BR B Hadronic Only Page 11

12 B 0 Å(g Motivation Standard Model Predictions B 0 Å negligible due to helicity-suppression......whereas for such decays with a photon in the final state... BR SM (B 0 Åg) Both are well below the sensitivity of existing experiments. m 2 2 m 0 B Existing Experimental Limits No direct searches. BaBar Analysis 89 Million BB pairs used. Events are tagged using only the semileptonic method: B 0 D l Clear observation would be a sign of new physics. Page 12

13 B 0 Å(g Method Signal B selection: No tracks remaining B 0 νå < 3 K 0 L candidates < 3 photon candidates B 0 νåg at least one photon candidate with p * >1.2 GeV/c Background dominated by B decays with lost or unreconstructed particles. B 0 B 0 Data Page 13

14 B 0 Å(g Results Phys.Rev.Lett. 93 (2004) Mode Total Signal Fitted Background Fitted Signal ĄÅ ĄÅg Efficiencies ( 10-3 ) (16.7±1.0±1.8) (14.1±1.0±1.6) Events Events 17± Dominant systematic uncertainties: Simulation statistics (PDF modeling) Neutral cluster/e extra modeling particle identification and tracking Total Systematic Error on Efficiency: ~7% ( Å) ~4%( Åg) Cross-check of results on B tagbased control samples: B ± yields results consistent with zero signal events. 90% C.L. Upper Limits: BR B BR B Page 14

15 Conclusions and Outlook YELLOW = Standard Model Range B B K B B B Branching Fraction Page 15