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

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 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

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 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

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, Introduction Acceptance coverage: SPD layers: -2.0 < < 2.0 (inner) -1.5 < < 1.5 (outer)

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 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 of the cluster on the inner layer is used Introduction Fiducial window

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

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, Corrections: SPD acceptance (I) MC data samples: Pb-Pb (HijingParam) 5.5 TeV: 20,000 tracks/evt, 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: within [-3,3] nEtabins = 120 d = 0.05 vtx-Z within [-20,20] cmnVtxzbinx = 40 dVtx-Z = 1 cm

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 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: 10 4 resulting error on the acceptance: Corrections: SPD acceptance (II)

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

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, Correction applied: Corrections: acceptance & efficiency (I)

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, Corrections: acceptance & efficiency (II) Correction applied:

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 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

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, Clusters (inner layer): Corrections: background from secondaries (II)

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

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

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

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, Corrections: vertex reconstruction (I) Generated dN ch /d N.B. The correction is integrated, but it should be a function of multiplicity and vertex position

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

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, All corrections applied: inner layer clusters Final dN/d distributions

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, All corrections applied: tracklets Final dN/d distributions

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, Multiplicity distributions (uncorrected)

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, Multiplicity distributions Background correction: 16% 7% Background fractions for each event

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, Multiplicity distributions All corrections applied:

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

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

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, 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

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Maria Nicassio Convegno Nazionale sulla Fisica di ALICE Frascati - November 12, Backup slides

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

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

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