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Progress on CP Violation Measurements from DØ Collaboration

This talk highlights the progress on CP violation measurements in B-physics at the Tevatron, focusing on the rich B physics menu, performance of the DØ detector, tracker feats, baseline lifetime measurement, and decays for CKM angles. The presentation emphasizes the need for sophisticated triggering and event selection methods to enhance data accuracy for precise CP violation measurements.

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Progress on CP Violation Measurements from DØ Collaboration

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  1. Towards CP violation results from DØ • This talk: • Progress on CP violation measurements • Expectations Paul Balm, NIKHEF (for the DØ collaboration) EPS meeting July 2003, Aachen Paul Balm - EPS 2003 - July 17, 2003

  2. Why do B-physics at the Tevatron? • Producing large numbers of bb pairs: • s(pp  bb) = 1.5 105 nb @ 2TeV • s(e+e- Z0 bb) = 7 nb • s(e+e-(4S) bb) = 1 nb • Expect 1010 bb pairs/year at 4 1031 cm-2s-1 • fragmenting into all B-species: Bd, Bu, Bs, Lb, … • But: much higher backgrounds, need more sophisticated triggering/event selection Paul Balm - EPS 2003 - July 17, 2003

  3. Rich B-physics menu • cross-sections: improve precision and extend pT–range over Run I • B hadron spectroscopy: study and discovery of new particles: Bc, Xb, Wb, … • Many CP violation measurements possible • CKM angles from decays like • Bd J/y KS • Bs  J/y f • Mixing measurements: Bs, Bd Paul Balm - EPS 2003 - July 17, 2003

  4. Performance of DØ • Silicon Microstrip Tracker (SMT): 91% operational • Central Fiber Tracker (CFT): 99% operational • Calorimeter: 99.9% operational • Muon system 99.5% operational • over entire rapidity range • Over past year, running efficiency greatly improved • taking data at 50 Hz • June ’03 average 88% Paul Balm - EPS 2003 - July 17, 2003

  5. Tracker performance • Distance of closest approach (DCA): resolution closing in on MC • At pT=1 GeV, s(DCA)=53 mm • Better for higher pT Paul Balm - EPS 2003 - July 17, 2003

  6. Tracker feats • photon conversions: p • X- decay: 10 cm L0 p- X- p- XLp Paul Balm - EPS 2003 - July 17, 2003

  7. Today’s B sample • 114 pb-1 taken with dimuon trigger • requires muon scintillator coincidence at L1 • At the Tevatron, a J/y trigger yields a lot of B’s: 15-20% • Current baseline Tevatron plan: 2 fb-1 at the end of 2005 • All J/y modes suffer from (implicit) |p| cut in L1 muon trigger • Awaiting trigger completion • (see later slides) m(m+m-) 114 pb-1 Paul Balm - EPS 2003 - July 17, 2003

  8. Baseline lifetime measurement:B J/y K 47 pb-1 114 pb-1 123535 B J/y K • Combining J/y and one track • we have only limited K/p separation using dE/dx from the SMT • Lifetime fit to ct distribution, use • Gaussian for combinatorics • exp. convoluted with Gaussian for B bkg • exp. convoluted with Gaussian for B+ signal • Result: <t> = 1.761  0.24 (stat) ps • Compare PDG: <t> = 1.671  0.018 ps Paul Balm - EPS 2003 - July 17, 2003

  9. Decays for CKM angles:Bs  J/y f b c J/y c Bs s f s s Bs  J/y f N(Bs) = 133 17 • CP violation in this channel allows measurement of CKM angle bs • Effect expected to be small (few %), but directly proportional to h (Wolfenstein param.) • Lifetime fits are in progress • see LeptonPhoton03 ! Paul Balm - EPS 2003 - July 17, 2003

  10. sin(2b) = Accurate measurement requires: initial-flavour tagging trigger completion Decays for CKM angles: Bd J/y KS Bd J/y KS N(Bd)=15420 • Bd mass formed from mass-constrained KS and J/y A(Bd) - A(Bd) A(Bd) + A(Bd) KSp+p- Paul Balm - EPS 2003 - July 17, 2003

  11. Initial Flavour Tagging • Jet charge tag: sum of charge of tracks in opposite-side jet • Soft muon tag: use opposite-side muon charge • Using exclusively reconstructed B+ J/y K+ sample Same-side tag being worked on signal + bkg for B signal (errors are statistical) Efficiency: e = (Nright + Nwrong) / Nall Dilution: D = (Nright - Nwrong) / (Nright + Nwrong) Paul Balm - EPS 2003 - July 17, 2003

  12. Bs mixing: Dms significance Dms [ps-1] • Dms reach in hadronic channel: BsDs(*)p • used soft muon tag • proper time res. used: 75 fs, reco. effiency 15% • in 2 fb-1, expect 1300-1900 events (single-m trigger) With 2 fb-1, can make 3s measurement to Dms of ~23 ps-1 - also looking into semileptonic channel Reach determined by vertexing resolution and trigger efficiencies 2 fb-1 Excluded PDG 95% Paul Balm - EPS 2003 - July 17, 2003

  13. Trigger completion • CTT: L1 Central Track Trigger • Track finding at L1, allows match with muon system • Effectively allows lowering pT threshold on muon triggers • Coming online now! (installed, running tests) • STT: L2 Silicon Track Trigger • Displaced track trigger allowing b-tagging at L2 • Expected after the summer • Expect large increase in J/y yield Paul Balm - EPS 2003 - July 17, 2003

  14. Conclusions • B’s in 114 pb-1 sample have been studied • Results promising: compatible with expectations • With more data and completed trigger, DØ will make accurate CP violation measurements Stay tuned! Paul Balm - EPS 2003 - July 17, 2003

  15. Back-up slides Paul Balm - EPS 2003 - July 17, 2003

  16. RunI dataset Paul Balm - EPS 2003 - July 17, 2003

  17. Algorithm: Remove daughter tracks from the reconstructed B Remove tracks w. 2D impact parameter greater than 0.2 cm Remove tracks with | z(DCA) – z(PV) | > 2.0 cm Use remaining tracks in j window opp. B to calculate the jet charge Q (weighing tracks with pT) Events with |Q|>0.2 are “tagged” Efficiency = (Nright + Nwrong)/ Nall Dilution = (Nright - Nwrong)/ Nall Flavour tagging: Jet charge (using 47 pb-1 ) • Errors only statistical • Errors in fractions of signal, bgd events in mass window (from fit) are ignored Paul Balm - EPS 2003 - July 17, 2003

  18. Flavour tagging: Soft muon tag (using 47 pb-1 ) • Algorithm: • must have ΔR > 2.0 separation from reconstructed B • must have pT > 1.9 GeV • charge of highest-pT muon in event gives (opposite-side) B-tag • Errors only statistical • Errors in fractions of signal, bgd events in mass window (from fit) are ignored Paul Balm - EPS 2003 - July 17, 2003

  19. Mass spectrum used for lifetime fit: mass 5.254 GeV  8 MeV 179 signal events Mass spectrum with add’l lifetime cut: 96 signal events B+  J/y K+ mass plots 47 pb-1 47 pb-1 Paul Balm - EPS 2003 - July 17, 2003

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