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Preliminary Measurement of the BF( t - → K - p 0 n t ) using the B A B AR Detector

Preliminary Measurement of the BF( t - → K - p 0 n t ) using the B A B AR Detector. Fabrizio Salvatore Royal Holloway University of London. for the B A B AR Collaboration. Outline. Motivation PEP-II and the B A B AR Experiment. Event Selection: K - p 0 selection and efficiency

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Preliminary Measurement of the BF( t - → K - p 0 n t ) using the B A B AR Detector

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  1. Preliminary Measurement of the BF(t-→K-p0nt) using the BABAR Detector Fabrizio Salvatore Royal Holloway University of London for the BABAR Collaboration

  2. Outline • Motivation • PEP-II and the BABAR Experiment • Event Selection: • K-p0 selection and efficiency • Systematic errors • Preliminary Result • Conclusions and Outlook Fabrizio Salvatore, RHUL

  3. Motivation • Hadronic t decays provide a clean laboratory for studying the hadronic weak current • For decays with overall net strangeness, SU(3)f symmetry breaking can be used to determine the absolute value of the CKM matrix element Vus, the strong coupling constant as and ms • The uncertainty in the extraction of |Vus| and ms is dominated by the experimental measurement uncertainties • The high luminosity provided by PEP-II, coupled with stt=0.89 nb at BABAR energies, provides a high statistics sample to study t–→ K-p0nt decays Fabrizio Salvatore, RHUL

  4. PEP-II Performance PEP-II delivered ~254 fb-1 BaBar recorded ~244 fb-1 For analysis used 1999-2003 data: On peak 112.1 fb-1 Off peak 12.3 fb-1 Total Lumi: L = 124.4 fb-1 Total # of t decays: ~220M Fabrizio Salvatore, RHUL

  5. 1.5 T solenoid EMC 6580 CsI(Tl) crystals DIRC (PID) 144 quartz bars 11000 PMs e+ (3.1 GeV) e- (9.0 GeV) Drift Chamber 40 stereo layers Silicon Vertex Tracker 5 layers, double sided strips Instrumented Flux Return iron / RPCs (muon / neutral hadrons) The BABAR Detector Fabrizio Salvatore, RHUL

  6. e/m+ K- ne/m g t+ p0 g t- nt nt Event topology • Event divided in two hemispheres defined by the plane perpendicular to the thrust vector • Only events with one charged track in each hemisphere are selected: • “Signal” hemisphere: track identified as Kaon • “Tag” hemisphere: track identified as electron or muon Fabrizio Salvatore, RHUL

  7. Lepton identification and p mis-ID • Efficiency and mis-id measured from data: • efficiency: e+e-g and m+m-g sample • p mis-ID: D*+→D0p+, D0→K+p- Fabrizio Salvatore, RHUL

  8. Kaon efficiency and p mis-ID Good K/p separation using the information from the DIRC Efficiency and mis-ID computed using D*+ → D0p+ data sample K+ p- Fabrizio Salvatore, RHUL

  9. reject qq background Event selection • Thrust ≥ 0.9 • R2 = 2nd/0th Fox-Wolfram moment ≥ 0.5 • Missing momentum ≥ 0.5 GeV/c • Events where both charged tracks are identified as electron are rejected • Only 1 p0 in the event • Only p0s from two separated EMC energy deposits with Eg> 50 MeV; • 100 < mp0 < 160 MeV/c2 reject Bhabha events • Angle between K and p0 candidates qKp0 < 1.0 rad • Energy in the “signal” hemisphere not associated with the charged track or p0 candidate ≤ 50 MeV Fabrizio Salvatore, RHUL

  10. Selected events • After all cuts and selections: • 21678 events selected in 124.4 fb-1 of data • 12377 with e-tag • 9301 with m-tag • Signal Efficiency: • (1.61±0.01) % • (0.92±0.01) % e-tag • (0.69±0.01) % m-tag very small hadronic background signal MC scaled using the BF in the MC generator: 0.46% Fabrizio Salvatore, RHUL

  11. Background evaluation • Background evaluated using MC events • generic t, Bhabha, m+m-, qq and BB samples • Main contribution comes from non-signal t decays • for mKp0 ≤ 0.8 GeV/c2 mainly KK0p0, K0p- and Kp0p0 • In particular, BF has large measurement uncertainty for: sBF/BF(t→KK0p0) ~ 13% sBF/BF(t→Kp0p0) ~ 40% • for mKp0 ≥ 1.0 GeV/c2 mainly pp0 (sBF/BF ~ 0.5%) • Total background: • 6086 events (e+m tag) • 3524 in the e-tagged sample • 2562 in the m-tagged sample Fabrizio Salvatore, RHUL

  12. Systematic uncertainties All numbers are preliminary Fabrizio Salvatore, RHUL

  13. Branching fraction result Preliminary PDG: (0.45±0.03)% Fabrizio Salvatore, RHUL

  14. In agreement with PDG average Improved uncertainty Comparison with other results Fabrizio Salvatore, RHUL

  15. What next? Measure the branching fraction of thet-→ K0p-ntdecay and combine it with the presented result Study the mass spectrum oft-→ (Kp)-nt Use t-→ (Kp)-ntstudies as input into the total strange spectral function t-→ (Kp)-ntbranching fraction Fabrizio Salvatore, RHUL

  16. A preliminary measurement of the t-→ K-p0ntbranching fraction has been obtained using 124.4 fb-1 of data taken at BABAR: t-→(Kp)-ntis a clean laboratory for studying hadronic weak currents more results to follow ! Conclusions Measurement consistent with world average (PDG 2004) This analysis represents a significant improvement on previously published results Fabrizio Salvatore, RHUL

  17. Backup Slides Fabrizio Salvatore, RHUL

  18. t background systematic Systematic error due to the uncertainty in the BF of t background decays used in generated MC

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