B s 0  D s +  -  events for B s 0 oscillation measurements

1. Thursday Meeting 2 Mar 2000 Bs0Ds+- events for Bs0 oscillation measurements Andrea Sciaba` With the collaboration of D. Abbaneo, S. Armstrong, G. Boix, A. Halley and F. Palla

2. Outline • Event selection, efficiency, purity • Decay length, momentum resolution, proper time • Initial state tag, mistag • Data sample • Systematic uncertainties • Conclusions

3. Event selection • Semileptonic Bs decays: Ds - • Hadronic Ds decays: +, K*0K+, K+ K0, +,++-, K*0K*+ • Semileptonic Ds decays: e+, + • plepton> 2.5 GeV/c

4. Efficiency • Improved by the reprocessing by 10-20%

5. Relevant backgrounds • Combinatorial background estimated from data • Physical background: BDsDX, D  X • Discriminated using (p, pT) of the lepton and no. of tracks forming a good vertex with the lepton • Large experimental uncertainty • Reflections from D+ decays

6. Physical background • Background branching ratio (from PDG): • P(bDsD0(X)) = (9.1  1.9  1.3  2.5) % • P(bDsD+(X)) = (4.0  1.6  0.7  1.2) % • P(bDsD(X),D X) = (1.3  0.3  0.3) % • Signal branching ratio (from PDG): • P(bBs) = (10.5  1.7) % • P(BsDs X) = (8.1  2.5) % •  P(bBs Ds X) = (0.85  0.3) % • B/S ratio: 1.5  0.7

7. Physical background (cont’d) • In the last published analysis a different (and wrong) estimate of the background was given • P(bDs(*)D(*)+) = 5.0 % (in 1994 PDG) (only 2-body decays!) • B/S 2.6 lower! • In fact, the estimated ratio of background to signal in selected events was  0.1; now it is  0.3 (with the same selection)

8. Physical background (cont’d) Signal Background Lepton transverse momentum Lepton momentum No. of tracks vertexing with the lepton

9. Physical background (cont’d) • If lepton PT > 2 GeV/c   > 0.9 • else, if lepton P> 15 GeV/c   0.85 - 0.95 • else,  0.65 - 0.80 (if Ntracks<4) • or   0.35 - 0.65 (if Ntracks4) • B0 : Bs : B+ = 0.42 : 0.19 : 0.39 (from MC) • relative fractions varied by 25% for systematics

10. Other backgrounds • Combinatorial background: estimated from the number of candidates and events fitted in the data (error used for systematics) • Reflection background: estimated from the Monte Carlo • Overall average purity  40%

11. Decay length • Average bias from the Monte Carlo (30 m) • Correction factors to the error are evaluated from the pull (3-20% depending on the channel) • Mean error  250 m

12. Momentum • E estimated from energy flow • Bias corrected as function of pBs • Error as a function of pBs(p  3 GeV/c)

13. Proper time • The proper time error is  0.25 ps

14. Proper time (cont’d) • Proper time distributions for background taken from Monte Carlo BDsD background

15. Initial state tagging • Data and Monte Carlo are in good agreement • The average mistag on signal is (26.4  0.5) %(from MC)

16. Data sample • 297 candidates are selected in the data

17. Systematics • Physical background (varied by 50%) • Combinatorial background (varied by its statistical uncertainty) • Decay length resolution (varied by stat. uncert. on the error correction) • Momentum resolution (varied by 10%) • Mistag (varied by 10%)

18. Conclusions • Good selection efficiency • Purity as a function of the lepton properties (necessary, given the “unexpectedly” high amount of BDsDX background) • Reasonable proper time resolution • The rest in the following presentation...