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Multiplicity analysis and dN/d h reconstruction with the silicon pixel detector

Terzo Convegno Nazionale sulla Fisica di ALICE Frascati (Italy) – November 12-14, 2007. Multiplicity analysis and dN/d h reconstruction with the silicon pixel detector. Maria Nicassio (Univ. and INFN Bari) in collaboration with D. Elia, B. Ghidini (Univ. and INFN Bari)

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Multiplicity analysis and dN/d h reconstruction with the silicon pixel detector

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  1. Terzo Convegno Nazionale sulla Fisica di ALICE Frascati (Italy) – November 12-14, 2007 Multiplicity analysis and dN/dh reconstruction with the silicon pixel detector Maria Nicassio (Univ. and INFN Bari) in collaboration with D. Elia, B. Ghidini (Univ. and INFN Bari) T. Virgili (Univ. Salerno)

  2. Contents • Introduction: • physics motivation • tracklet reconstruction algorithm • Status of the analysis: • study of the corrections: • geometrical acceptance • detector efficiency • background from secondaries • vertex reconstruction efficiency • minimum bias trigger acceptance • Summary and outlook Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  3. Introduction • Why multiplicity: • first measurement in p-p collisions for ALICE • global observable characterizing the event • comparison with results obtained at lower energies • Why multiplicity with pixels: • available in a short time • advantages over reconstructed tracks (ITS+TPC) • larger acceptance coverage • only alignment of the two pixel layers required Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  4. Introduction • Acceptance coverage: SPD layers: -2.0 < < 2.0 (inner) -1.5 < < 1.5 (outer) Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  5. q Fiducial window Df Introduction • Multiplicity reconstruction: • (a) counting clusters on the inner layer (|| < 2.0) • no detector alignment required • reliable at high multiplicity • (b) counting tracklets (|| < 1.5) • alignment, vertex needed • more reliable (e.g. good noise rejection) • Pseudorapidity reconstruction: • vertex needed for both methods • the angle q of the cluster on the inner layer is used Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  6. dN/dh distributions (uncorrected) Tracklets Inner layer clusters asymmetry due to the detector efficiency losses in PDC06 Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  7. Corrections: SPD acceptance (I) • MC data samples: • Pb-Pb (HijingParam) collisions @ 5.5 TeV: • 20,000 tracks/evt, h within [-4,4] • event vertex-Z within [-20,20] cm • fully efficient SPD  2,500 evts pure geom acceptance • standard PDC06 SPD dead chip map  2,500 evts convoluted acc+eff • Correction matrix: • binning and range: • h within [-3,3] nEtabins = 120  dh = 0.05 • vtx-Z within [-20,20] cm nVtxzbinx = 40  dVtx-Z = 1 cm Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  8. Corrections: SPD acceptance (II) • Calculation method: • detectable_tracks (fDenAcc): • primary charged • no decay, no secondary interactions up to the sensitive layer • detected_tracks (fNumAcc): • detectable tracks with associated (label) cluster on the sensitive layer • if there are 2 clusters on adjacent modules: track is counted twice  this takes into account the overlapping regions ( 2%) • compute acceptance and error in each bin (fAcc,fErrAcc) • statistics in each bin: • detectable tracks:  104 • resulting error on the acceptance:  10-3 Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  9. Inner layer Outer layer Tracklets Corrections: SPD acceptance (III) • Results: • convoluted acceptance & efficiency: Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  10. Corrections: acceptance & efficiency (I) • Correction applied: Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  11. Corrections: acceptance & efficiency (II) • Correction applied: Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  12. Corrections: background from secondaries (I) • Studied using the SPD cluster labels • Definition of background: • for clusters on the inner layer: • clusters having secondary track labels only • for tracklets: • at least one of the two clusters in the tracklet having secondary track labels only Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  13. Corrections: background from secondaries (II) • Clusters (inner layer): Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  14. Corrections: background from secondaries (III) • Clusters (inner layer): correction (to be multiplied by) Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  15. Tr(S+S) + Tr(P+S) Tr(P+P)+Tr(P+P’) Tr(P+P) Tr(S+S) Tr(P+P’) Tr(P+S) Corrections: background from secondaries (IV) • Tracklets: to be subtracted (total bkg fraction: 7.5%) Tracklets from primaries Tracklets from secondaries P, P’ = cluster with a label of a primary track S = cluster with all labels of secondary tracks Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  16. Corrections: background from secondaries (V) • Tracklets: correction (to be multiplied by) Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  17. Corrections: vertex reconstruction (I) • Generated dNch/dh N.B. The correction is integrated, but it should be a function of multiplicity and vertex position Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  18. Corrections: vertex reconstruction (III) • Correction (to be multiplied by) The correction depends both on h and on multiplicity at low multiplicity To be checked as a function of Z-vtx Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  19. Final dN/dh distributions • All corrections applied: inner layer clusters Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  20. Final dN/dh distributions • All corrections applied: tracklets Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  21. Multiplicity distributions (uncorrected) Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  22. Multiplicity distributions • Background correction: 7% 16% Background fractions for each event Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  23. Multiplicity distributions • All corrections applied: Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  24. Effect of trigger selection: first look (I) • Trigger correction: MB2 =(GFO.and.V0OR).and.notBG Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  25. All events No trigger No vertex Effect of trigger selection: first look (II) • Generated dN/dh: MB2 =(GFO.and.V0OR).and.notBG Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  26. Summary and outlook • Multiplicity and pseudorapidity density in p-p: • first measurement in ALICE • reconstruction with the Silicon Pixel Detector only • Status of the analysis: • raw reconstructed distributions with PDC06 data • study of the main corrections: • acceptance, efficiency, background from secondaries, vertex, trigger • What next: • check correction dependence on multiplicity, Z-vtx • estimate of the systematics • tests with PDC07 data Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  27. Backup slides Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  28. Tracklet algorithm efficiency in p-p • Couples of clusters associated with the same track Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

  29. Tracklet algorithm efficiency in p-p • Couples of clusters associated with the same track Using the default cuts the algorithm efficiency is 99% Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 2007

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