Vertex reconstruction and selection strategies for D s detection in Pb-Pb collisions

1. Vertex reconstruction and selection strategies for Ds detection in Pb-Pb collisions Sergey Senyukov, Universita’ di Torino In collaboration with E. Bruna, M. Masera, F. Prino III Convegno sulla Fisica di ALICE. Frascati

2. Two kinds of events • Signal – 1000 PYTHIA events with full simulation each containing 9100 Ds generated in the barrel acceptance forced to decay into hadronic channels: • DsKK0* • Dsφπ • Background – 1000 HIJING events (out of 20000 we have) • Ideal PID III Convegno sulla Fisica di ALICE. Frascati

3. Values considered • Signal value • % kept Signal: S%=Number of selected Ds/Number of Ds reconstructed Ds reconstructed = 3 daughters reconstructed with 6 ITS points • Signal per event: Sevent=N(Ds)* BR(Ds)*RecF*AccF*S% • N(Ds) = Number of Ds in central PbPb event from NLO calculation • Signal per 107 events: S(107) =Sevent*107 • Background • Background per event: Bevent=Number of triplets selected/Number of combinatorial triplets • Background per 107 events: B(107)=Bevent*107 • Significance • Significance = III Convegno sulla Fisica di ALICE. Frascati

4. Analysis algorithm III Convegno sulla Fisica di ALICE. Frascati

5. B<107, kept Signal >7%, Significance maximized Single track cuts • Pt(K) – Transverse momentum of kaon • Pt(π) – Transverse momentum of pion • d0 – impact parameter • Case of 3 cut parameters is difficult to visualize • → for every combination of Pt cut maximize the value of significance, but limiting value of background and signal. • Chosen cuts: • Pt(K)>0.5 GeV • Pt(π)>0.5 GeV • d0>35 μm • Cut results: • Background – 107 (was 108) • kept Signal – 20% • Significance – 0.005 III Convegno sulla Fisica di ALICE. Frascati

6. Choice of vertexing algorithm Two possibilities from standard library Class AliVertexerTrack::VertexForSelectedTracks() • Finder algorithm only • Finder + Fitter Quality control via: • Average value of residual |xrec-xgen| • σ of Gaussian fit of residual distribution • Pulls III Convegno sulla Fisica di ALICE. Frascati

7. Quality of vertexing algorithms Finder + fitter appears to have systematically worse resolution III Convegno sulla Fisica di ALICE. Frascati

8. Quality of vertexing algorithms (2) Conclusion on vertexing algorithms: III Convegno sulla Fisica di ALICE. Frascati

9. Cuts on pairs • Cut results • Background - 4*105 • kept Signal - 5% • Significance - 0.007 • Chosen cuts: • Distance to KK cross point >500 μm • Distance to Kπ cross point >500 μm • Values considered: • Distance between primary vertex and cross point of K+ K- • Distance between primary vertex and cross point of K+π- (K-π+) III Convegno sulla Fisica di ALICE. Frascati

10. Cuts on pairs (2) Possible optimizations of cut on pairs • Take count on the error of vertex • Possibly will remove asymmetry between KK and Kπ case. • Possible cut on DCA. III Convegno sulla Fisica di ALICE. Frascati

11. Cut on the quality of secondary vertex • Cut value – 200 μm • Cut results: • Background – 3*104 • kept Signal – 3.3 % • Significance – 0.016 III Convegno sulla Fisica di ALICE. Frascati

12. Cut on the quality of secondary vertex (2) Invariant mass of the triplet considered: |mDs-mtripl|<0.012 GeV • Cut value – 200 μm • Cut results: • Background – 1.5*102 • kept Signal – 2.8 % • Significance – 0.18 III Convegno sulla Fisica di ALICE. Frascati

13. Cuts results We are going in the right direction Trends of the variables: • Significance: is going up • Background: is going down • Signal: is going down, but slower III Convegno sulla Fisica di ALICE. Frascati

14. Future plans • Cut on the quality of secondary vertex in different Pt regions • Study of other cuts: • DCA of pairs • Distance between primary and secondary vertex • Cosine of pointing angle • Sum of the impact parameters squared • Study of the cases of: • Real PID • No PID • Fine tuning of the cuts with a higher statistics III Convegno sulla Fisica di ALICE. Frascati