Flow measurements from CMS Julia Velkovska for the CMS Collaboration CMS flow measurements: LHC flow WG, August 2011
Publically available documents • CMS PAS HIN-10-002 – preliminary v 2 results • CMS PAS HIN-11-005 – preliminary v n results from cumulants and LYZ • Not covered in this talk are vn from di-hadron correlations • arXiv:1105.2438 ; CMS-HIN-11-001 ; CERN-PH-EP-2011-056 ; to be published in: J. High Energy Phys – di-hadron correlations v 1 -v 5 from central events • CMS PAS HIN-11-006 – preliminary di-hadron correlations v 1 -v 5 : centrality dependence 2 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
The CMS detectors used in the analyses 3.23 < | η | < 4.65 2.9 < | η | < 5.2 | η | < 2.4 • Additional EP defined in HF to be included in the final results 3 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
Event selection and centrality determination Trigger efficiency : 97+/- 3% Vertex cut |z vtx |< 10 cm Additional centrality bins in the final analysis 4 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
Track reconstruction Mid-rapidity Forward rapidity HYDJET BASS Efficiency and fake rate also estimated from sim pions embedded in real events 5 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
Track selection Preliminary results: 0.3 < p T < 12 GeV/c • Improved “standard cuts” for final analysis 6 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
v n preliminary measurements from CMS n = 2 n = 3 Add EP in final results Add EP in final results n = 4 Add EP in final results n = 5 Add EP in final results n = 6 7 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
EP v 2 • HF EP in final results : larger rapidity gap ∆η ≥ 2 8 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
EP efficiency corrections • No efficiency correction for the differential v2(pt) • To obtain integrated v2 at mid rapidity and v2( η ), first the differential v2(pt ) is measured in bins of ∆η = 0.4 – then integrated in the range 0.3 < pt < 3 GeV/c • No fake track v2 correction in preliminary v2(pt) results – a correction is being implemented for the final results to reduce the low-pt systematic uncertainty 9 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
Cumulants (I) • Cumulants are obtained using generating functions Differential gen function are evaluated using several interpolation points in the complex plane 10 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
Cumulants details of analysis (II) • Results obtained in 5% centrality bins : averaged to get 40-50%,etc • Fixed multiplicity: M = <M> -2 σ : σ is r.m.s. of the multiplicity distribution for that centrality interval • 21 Interpolation points in the complex plane 3 values for r0 and 7 for the phase, p: r0 – radius determined from the multiplicity and the number of events in each centrality class. Varied for systematic studies • To avoid auto-correlations , the particles used for determining differential flow are excluded from the integrated flow calculation • Track-by-track efficiency correction applied as a weight in the gen function 11 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
Cumulants (III) 12 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
Lee-Yang Zeros • Flow vector Q projected onto an arbitrary direction wrt x axis : 5 values of θ used • weight used in gen function to correct for efficiency • Differential v 2 , v 4 and v 6 reconstructed from V 2 13 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
Systematic studies 14 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
Summary of systematic uncertainties: EP 15 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
Systematics EP : v2( η ) 16 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
17 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
18 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
19 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
20 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
Summary of changes for final results • Vn from EP • Using HF EP with large rapidity gap • Rapidity gaps in cumulant analysis • Improved track selections + corrections for fake track v2 at low-pt • Improved statistics ( ~ factor of 10) • Reduced systematic uncertainties 21 Julia Velkovska (Vanderbilt) CMS flow measurements: LHC flow WG, August 2011
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