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D + K - p + p + :reconstruction and perspectives for the measurement of the elliptic flow

D + K - p + p + :reconstruction and perspectives for the measurement of the elliptic flow. Elena Bruna University of Torino. Terzo Convegno Nazionale sulla Fisica di Alice, 12-14 Nov 2007. Outline. Exclusive reconstruction of D + → K - p + p + in Pb-Pb and pp collisions:

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D + K - p + p + :reconstruction and perspectives for the measurement of the elliptic flow

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  1. D+K-p+p+:reconstruction and perspectives for the measurement of the elliptic flow Elena Bruna University of Torino Terzo Convegno Nazionale sulla Fisica di Alice, 12-14 Nov 2007

  2. Outline • Exclusive reconstruction of D+ → K-p+p+in Pb-Pb and pp collisions: • Selection strategy • Results • pp: Results with more statistics w.r.t. previous analysis • pp: A look at the systematic errors due to the selection procedure • cut variables considered: distance primary-secondary vertices, cosqpoint • Perspectives for the measurement of D+ azimuthal distribution in Pb-Pb Elena Bruna

  3. primary vertex K p d Working point: d>700 mm d cut (mm) d (mm) Selection strategy (1/4) • Cuts on single tracks ( pT, d0rf ) • In Pb-Pb reduce the combinatorial background from 109 to 106 triplets per event keeping ≈7% of the signal • pT cut different for K and p (identified by the charge sign: D+K-p+p+) • Cuts on Kp candidate pairs • K and p have opposite charge sign • Cut on the distance d between the vertex of the 2 tracks and the primary vertex Elena Bruna

  4. Selection strategy (2/4) • Cuts on Kpp candidate triplets of tracks • selection based on the products of impact parameters of the two Kp pairs  25% of BKG triplets rejected, ≈ all signal kept • Cuts on the secondary vertex quality: BLACK: signal (Kpp from D+) RED: BKG (Kpp combinatorics) BLACK: signal (Kpp from D+) RED: BKG (Kpp combinatorics) σ (cm) Elena Bruna

  5. Selection strategy (3/4) • Four selection variables: • Distance between primary and secondary vertex (dPS) • cosqpoint • Sum of squared impact parameters s = d012+d022+d032 • Max. pT among the 3 tracks pM=Max{pT1,pT2,pT3} BKG cosqpoint SIG Elena Bruna d PS(mm)

  6. Selection strategy (4/4) • n-dimensional matrix procedure (n=4), i.e. one dimension for each cut variable (dPS, pM, cosqpoint, s): • Fill matrices Sijkl (signal) and Bijkl (background) • Each cell ijkl contains the number of triplets passing the set of cuts: [dPS > cuti AND pM > cutjANDcosqpoint >cutk AND s > cutl] cosqpointCUT Significance S/(S+B) for 2<pT<3 GeV/c (normalized to 107 PbPb events) • Advantages: • Global maximization • Easy to add new selection variables dCUT(mm) Elena Bruna

  7. Results: D+K-p+p+ in Pb-Pb (1) • Significance S/√S+B (normalized to 107 ev.) vs pT • Relative statistical error (sS/S =1/√S) on D± vs pT (in 107 ev.) • S/ev~10-3 , B/ev ~10-4 Elena Bruna

  8. Results: D+K-p+p+ in Pb-Pb (2) • Selection efficiency and pT spectra: • pT integrated ε (D+) ≈ 1.5% (Ideal PID), 0.6% (Real PID), 1% (no PID) Elena Bruna

  9. Results: D+K-p+p+ in pp (1) • Signal statistics improved by a factor 2 w.r.t. previous analysis • Significance and relative statistical error vs. D+ pT • S/ev~5 10-6 , B/ev ~5 10-6 • Significance and relative statistical error (=1/S) normalized to 109 pp Minimum Bias events Elena Bruna

  10. Results: D+K-p+p+ in pp (2) • Selection efficiency and pT spectra: • Selection efficiency (pT integrated) ε~ 4% (NO PID) Elena Bruna

  11. Results: Pb-Pb vs pp (PID) (no PID) • sS/S the same for Pb-Pb and pp for 2<pT<8 GeV/c (PID) (no PID) S/√S+B in one year Elena Bruna

  12. Studies on D+ Invariant Mass • Signal Inv. Mass distribution fitted with a Gaussian function, in different D+ pT ranges • Signal normalized to 109 pp randomly generated according to a Gaussian function • The same for the Bkg triplets, but a polinomial function was used Elena Bruna

  13. Invariant mass distributions Statistics enhanced in order to match 1 year of data taking 2<pT<3 GeV/c 0<pT<2 GeV/c pT>5 GeV/c 3<pT<5 GeV/c Elena Bruna

  14. Systematic errors on pp events • Two samples of events: • 1°) pp MB (85%) + pp with charm (14%) and beauty (1%) + events with cD+Kpp (divided in 4 subsamples of pThard) • extract the D+ number with Gaussian fit to the invariant mass distribution: Nsel • 2°) only events with cD+Kpp (divided in 4 subsamples of pThard) • extract the selection efficiency: ε • D+ reconstructed: Nrec=Nsel/e • Nrec calculated for different values of the cut variables Elena Bruna

  15. Nsel vs d (norm to 109) Cut on dprim-sec COUNT From FIT 2<pT<3 GeV/c Nrec vs d (norm to 109) COUNT From FIT e vs d Elena Bruna

  16. Nsel vs cosqpoint (norm to 109) Cut on cosqpoint COUNT From FIT 2<pT<3 GeV/c Nrec vs cosqpoint (norm to 109) COUNT From FIT e vs cosqpoint Elena Bruna

  17. Perspectives for D± elliptic flow • GOAL: Evaluate the statistical error bars for measurements of v2 for D± mesons reconstructed from their Kpp decay in Pb-Pb collisions • v2 vs. centrality (b=collision impact parameter) • v2 vs. pT in different centrality bins • HOW: fast simulation to generate: • ND+ (DpT,Db) with an angular distribution dND/dj = v2D,in cos [2(j-YRP)] • For each D+: an event made of Ntracks is superimposed dN/dj = v2ev,in cos [2(j-YRP)] • Inputs of the simulation: ND+ (DpT,Db) , v2D,in, Ntracks ,v2ev,in • Outputs: v2Dmeasured. Elena Bruna

  18. v2 vs. pT 6<b<9 fm • Large stat. errors on v2 of D± → Kpp in 2·107 MB events • How to increase the statistics? • Sum D0→Kp and D±→Kpp • Sufficient for v2 vs. centrality • Semi-peripheral trigger • v2 vs. pT that would be obtained from 2·107 semi-peripheral events (e.g. 6<b<9 fm) MB trigger Semi-peripheral trigger Elena Bruna

  19. Summary • D+ in PbPb and in pp: • The results show that the D+ → K-p+p+analysis in Pb-Pb and pp is feasible with a pretty good Significance. • The selection strategy based on n-dimensional matrices has been developed. • Recent updates: • Significance in pp improved with enhanced statistics • Invariant mass studies give the expected spectra in 1 year of data taking • No evident systematic error on the selection procedure from a first look: more statistics needed • D+ v2: • Large stat. errors on v2 of D± → Kpp in 2·107 MB events • How to increase the statistics? • Sum D0→Kp and D±→Kpp • Semi-peripheral trigger • Under study the methods for background subtraction Elena Bruna

  20. Backup slides Elena Bruna

  21. Initial Nsel statistics (vs dprim-sec) 2<pT<3 GeV/c 0<pT<2 GeV/c 3<pT<5 GeV/c pT>5 GeV/c Elena Bruna

  22. Initial Nsel statistics (vs cosqpoint) 0<pT<2 GeV/c 2<pT<3 GeV/c 3<pT<5 GeV/c pT>5 GeV/c Elena Bruna

  23. Initial Bkg statistics with |M-Minv|<1s (vs d) 2<pT<3 GeV/c 0<pT<2 GeV/c 3<pT<5 GeV/c pT>5 GeV/c Elena Bruna

  24. Results from RHIC • The Non-Photonic electrons RAA shows interesting results • Suppression is very large when compared to the expectation from radiative energy loss • Other possible mechanisms? • Collisional EL, resonances, in medium fragmentation… • First hint of strong charm v2 for pT<2 GeV/c • Very important, need to disentangle charm from bottom PHENIX nucl-ex/0611018 STAR nucl-ex/0607012 Elena Bruna

  25. Results scaled to a lower multiplicity scenario for Pb-Pb • Results presented so far for Pb-Pb based on dNch/dy=6’000 • Ntracks=7000 • BKG/ev=N(N-1)(N-2)/3! ~ 6x1010 • Extrapolations from RHIC results seem to favour a lower multiplicity scenario, dNch/dy=2’000 • Ntracks~7000/3 ~2300 • BKG/ev ~ 2x109 • Let’s consider the D+ pT interval: 0<pT<2 GeV/c: • The results in the highest multiplicity scenario (dNch/dy=6’000) are not satisfactory • BKG/ev is downscaled by a factor ~30; • SIG/ev not rescaled • S/√S+B (normalized to 107 ev.)~ 10 in case of Real PID: it is possible the study of the low-pT spectra Elena Bruna

  26. Systematic uncertainities on D± measurements • Acceptance, reconstruction and particle identification efficiencies (~10%) • Centrality selection (~7%) • Parameters of the Woods-Saxon profile and nuclear density (~5%) • Nucleon-nucleon inelastic cross section (~5%) • Branching ratio D+K-p+p+(~3.6%) • Feed-down from beauty: NcD± = ND± - NbB  D± • Contamination K= NbB  D±/ NcD± =4% Elena Bruna

  27. Feed-down from beauty • D+ from B are more displaced • The cut on distance between primary to secondary vertex increases the fraction of selected D+ coming from B decay Contamination K vs d d~1000mm  K =10% Histograms normalized to the same area Elena Bruna

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