IOP March 2005

1. Opposite Side Flavour Tagging At DØ Philip Lewis, Imperial College London • B mixing at DØ • Flavour tagging • Studying the opposite side flavour tagger • Towards BS mixing measurement IOP March 2005

2. B mixing at the Tevatron Measurement of Δms / Δmd allows a stringent test of the CKM formalism of the standard model and a precise determination of Vtd Δmd = 0.502±0.007 ps-1 (world average) Δms > 14.9 ps-1 (combined limit at 95% CL) Δms = 18.3 ps-1 S.M. (15.6 - 22.2 at 95% CL) At present the Tevatron is the only place where Bs mixing can be measured Phil Lewis IOP March 2005

3. The DØ Detector Run II upgrade: • Silicon Vertex Detector (SMT) • Central Fibre Tracker (CFT) • 2T Solenoid • Forward Muon system Coverage • Tracking | η | < 3 • Muon | η | < 2 Phil Lewis IOP March 2005

4. μ± π Opposite side tag π* B B μ± K Same side tag Flavour Tagging Reconstructed B Mixing measurement: - B lifetime - mixed/unmixed i.e. initial state & final state Final State: from reconstruction Initial state: same side tag - charge correlation of fragmentation pions opposite side tag - from other b in bb pair production Phil Lewis IOP March 2005

5. μ± π Opposite side tag π* B B μ± K Opposite Side Flavour Tags O.S. Tags - Lepton tag: b→Xl-, b→Xl+ branching ratio 10% into e, μ - Partial reconstruction of secondary vertex (SV tag) - Jet charge tagging O.S. Tags independent of reconstructed side → studies on B+, B0 (Bu, Bd) applicable to Bs O.S. B meson can mix (B0, Bs) 17.4% B0, 50% Bs opposite side mesons will be mixed Phil Lewis IOP March 2005

6. Tagging Study: Samples 460 pb-1 data M(J/ψ K+) GeV/c2 M(Kπ) GeV/c2 M(D0π)-M(D0) GeV/c2 Phil Lewis IOP March 2005

7. Tagging Study: O. S. Muon Tag Efficiencyε = Ntagged events / Ntotal DilutionD = (Ncorrect tags – Nwrong tags) / (Ncorrect tags + Nwrong tags) Tagging power~εD2 Simple O.S. muon tag: charge of muon gives b flavour - hits in SMT and CFT, >1 muon detector segment - PT muon > 2.5 GeV/c2 - not in reconstructed B jet B+→J/ψ K+ M(J/ψ K+) GeV/c2 Phil Lewis IOP March 2005

8. Combined Flavour Tag To improve ε, D we can make more sophisticated taggers, and combine different types of tagger: Calculate dilution dependant on properties of O.S. muon e.g. - number of segments of muon detector - PT muon relative to jet - charge of muon jet Currently use muon + SV combined tag: Phil Lewis IOP March 2005

9. Fitting Procedure for B0 Sample For tagging studies of B+→D0μ+, B0→D*-μ+ we need to account for sample composition: To fit B0d component need to measure oscillations. • split into VPDL bins and measure asymmetry in each bin • compare with calculated asymmetry. - correct for K factor and detector resolution Binned fit determines Δmdand dilution for each sample Phil Lewis IOP March 2005

10. Dilution of Combined Tagger Asymmetry = (Nunmixed -Nmixed ) / (Nunmixed +Nmixed) Dilution (B+) = 0.47 ± 0.03 Dilution (B0) = 0.45 ± 0.04 Δm = 0.558 ± 0.048 Phil Lewis IOP March 2005

11. Towards Bs Measurement at DØ World Average Δms • Preliminary mixing result to be presented soon. • Bs→DsμνX • Bs→Ds→φπ • Bs→Ds→φ→KK • Improving flavour tag is vital to making the best possible measurement. • increase efficiency of muon tagger • addition of electron tag • Move to unbinned fit method • - can use event by event dilution • - need to make sure we can calculate this reliably Phil Lewis IOP March 2005

12. Summary • Observation of Bs oscillations is a major goal of the Tevatron • To do this need good performing, well understood flavour tagging • Combined muon and SV opposite side flavor tagger studied at DØ • Dilution of tagger consistent for Bd and Bu mesons; suitable for application to Bs decays • Several improvements to the tagger in development • Preliminary Bs mixing measurement to be presented soon Phil Lewis IOP March 2005