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Vertex Reconstruction and Selection Strategies for Ds Detection in Pb-Pb Collisions

This paper presents vertex reconstruction and selection strategies for the detection of Ds mesons in Pb-Pb collisions. It discusses two types of events (signal and background), ideal PID values, signal significance analysis, and the impact of single track cuts and vertexing algorithms on the detection process. The study evaluates the effects of various cut parameters on background, kept signal, and significance, highlighting the importance of quality control in vertexing algorithms. Several cut optimizations are suggested for enhanced detection efficiency. Moreover, the article outlines future plans for refining vertex quality cuts and investigating additional parameters to improve Ds detection in different scenarios.

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Vertex Reconstruction and Selection Strategies for Ds Detection in Pb-Pb Collisions

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  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

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