Measurement of D 0 cross section from Kπππ decay channel in pp collisions at s = 7 TeV with ALICE experiment

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1 Measurement of D 0 cross section from Kπππ decay channel in pp collisions at s = 7 TeV with ALICE experiment Fabio Colamaria Università degli Studi Aldo Moro BARI INFN Section of BARI ISSP Erice - 30/06/2012 Fabio Colamaria 1

2 Outline of the talk ALICE experiment Goal and features of the analysis Construction and selection of candidates Results: Invariant mass spectra Efficiency & feed-down from B corrections Systematic uncertainties Cross section and comparisons Conclusions ISSP Erice - 30/06/2012 Fabio Colamaria 2

3 ALICE experiment All-purpose experiment 14 detectors Two main regions: Central barrel ( η < 0.9) Muon spectrometer ( 4 < η < 2.5) Optimized for working in environment with huge tracks density (dn ch /dη = 4000) Excellent tracking and vertexing capabilities, expecially at low p T ISSP Erice - 30/06/2012 Fabio Colamaria 3

4 Charm analysis at LHC Charm quarks go through the whole evolution of QGP excellent probes for studying its properties α s << 1 for charm production pqcd can be used Analysis in pp features: Reference for comparison with PbPb results Test of pqcd predictions Probes nucleon PDF for values of x 10-4 at y = 0 D 0 meson chosen for analysis, from 4-prong decay channel (Kπππ) Not the main channel for reconstruction (Kπ), but Grants higher BR (8.09% vs 3.89%), higher precision in secondary vertex reconstruction, K π + ρ 0 (relative BR = 66%) resonant channel Drawback: combinatorial background extremely high Analysis has been performed on the full 2010 pp statistics (L int = 5 nb -1 ) ISSP Erice - 30/06/2012 Fabio Colamaria 4

5 Candidates selection D 0 candidates constructed combining all possible (+,,+, ) quadruplets of tracks in the event Selection of candidates via topological and PID cuts Topological selection Main cut variables: distance of closest approach btw tracks, D 0 decay length, cosine of pointing angle, ρ 0 compatibility cut PID selection TOF and TPC detectors PID capabilities exploited The two detector responses are combined to accept/reject tracks. If all 4 tracks for a mass hypotheses accepted -> D 0 selected ISSP Erice - 30/06/2012 Fabio Colamaria 5

6 Reflections issue PID is not perfect: wrong mass hypotheses could still be accepted (K-π swaps). These reflections can give a fake contribution to signal value, having a different shape as combinatorial background Reflection hidden by the true signal data-driven approach is not possible 3 < p T < 5 GeV/c 5 < p T < 6 GeV/c 6 < p T < 8 GeV/c 8 < p T < 12 GeV/c Solution: fit of reflections distribution (on MC) with a particular function (polynomial + gaussian) FuncRefl Fit on data forced to include also this reflection distribution (FuncRefl), with a proper normalization factor N The global fit function is: Pol2 + gaussian + N*FuncRefl ISSP Erice - 30/06/2012 Fabio Colamaria 6

7 Invariant mass spectra Signal peaks spotted in 6 p T bins for D 0, from 3 to 25 GeV/c. Significance > 3 in all peaks, > 6 in central bins; low statistics for last bin Sigma and mean of peaks in nice agreement with MC prediction (always inside 2σ) ISSP Erice - 30/06/2012 Fabio Colamaria 7 7

8 Efficiency*Acceptance Correction Evaluated from MC c & b enriched sample (PYTHIA generator [1]) p T dependence very smooth Higher Eff*Acc for harder D 0 Tighter cuts at low p T, where background is larger Ratio B->D/c->D > 1; decreases for increasing p T Distance-to-vertex cuts tend to favour feed-down D 0 [1] T. Sjostrand, S. Mrenna and P.Z. Skands, PYTHIA 6.4 physics and manual, JHEP 05 (2006) 026 ISSP Erice - 30/06/2012 Fabio Colamaria 8 8 8

9 B feed-down Correction Only prompt D 0 have to be considered B feed-down rejection Two approaches for feed-down evaluation: N b taken as reference (uses only B->D prediction, found to be more accurate than c->d) Their difference, including errors, taken as systematic uncertainty ISSP Erice - 30/06/2012 Fabio Colamaria 9 9 9

10 Systematic uncertainties Bin1 Bin2 Bin3 Bin4 Bin5 Bin6 Yield extraction 20% 15% 15 % 20% 20% 20% Cuts efficiency 20% 20% 20 % 20% 10% 10 % PID efficiency 15% 15% 15 % 10 % 10 % 10 % MC p T shape 10% 5% 5% 5% 5% 5% + Feed-down (from N b and f C ) Other sources of uncertainties (non bin-dependent): All Bins Tracking eff. 16% Branch. Ratio 2.5% Normalization 3.5% ISSP Erice - 30/06/2012 Fabio Colamaria 10 10

11 Cross section evaluation Ingredients for cross section evaluation Integrated luminosity: L = N MB,pp /σ MB,pp Branching ratio of Kπππ Correction for acceptance and efficiency Value of signal, extracted from spectra Feed-down correction (N b method) This is for particle + antiparticle For D 0 multiply for 1/2 11 PWG3 14/06/2011 Fabio Colamaria 11

12 Cross section of D 0 Evaluation of cross section and comparison with Kπ results ISSP Erice - 30/06/2012 Fabio Colamaria 12 12

13 Cross section of D 0 Ratio with rebinned Kπ results Compatibility within error in all p T bins Very good agreement also with pqcd theoretical predictions (FONLL [1] and GM-VNFS [2]) Overall fit of ratio compatible with 1 within 1σ [1] M. Cacciari et al., Theoretical predictions for charm and bottom production at the LHC, CERN-PH-TH [2] B.A. Kniehl, G. Kramer, I. Schienbein and H. Spiesberger, in preparation ISSP Erice - 30/06/2012 Fabio Colamaria 13 13

14 Conclusions HF quarks are excellent probes for studying QGP properties in PbPb; pp analysis mandatory for reference, useful also for testing pqcd at low values of Bjorken x D 0 has been reconstructed from Kπππ decay channel with ALICE, in pp collisions at 7 TeV, spotting signal peaks in 6 p T bins, from 3 to 25 GeV/c After the evaluation of corrections and uncertainties, cross section for D 0 direct production has been determined Results are in very good agreement with cross section from Kπ decay channel. FONLL and GM-VNFS pqcd calculations also describe reasonably the results Thanks for your kind attention! ISSP Erice - 30/06/2012 Fabio Colamaria 14 14

15 BACKUP SLIDES ISSP Erice - 30/06/2012 Fabio Colamaria 15

16 Bjorken x at LHC Squared invariant mass of QQ pair: Rapidity of QQ pair in lab system: Combining the previous relationships, x1 and x2 can be derived; For symmetrical systems, with A1 = A2, Z1 = Z2: ISSP Erice - 30/06/2012 Fabio Colamaria 16

17 Detector performances ITS TPC TOF ISSP Erice - 30/06/2012 Fabio Colamaria 17

18 Topological cuts (1) List of topological cuts for candidate selections DCA between opposite charge tracks Applied for all the 4 different couples of opposite charge of the 4 prongs: All of them must pass the cut to accept the candidate Cosine of pointing angle Cosine of angle between the line-of-flight connecting primary and secondary reconstructed vertex and the direction of the momentum of recostructed D 0 This for 2 Prong, but for cosθ POINT the idea is the same ISSP Erice - 30/06/2012 Fabio Colamaria 18

19 Topological cuts (2) List of topological cuts for candidate selections Distance btw. primary vertex and vertex of couple Distance btw. primary vertex and vertex of trip Distance btw. primary vertex and vertex of quad Candidates are obtained starting from a couple of opposite charge tracks; then a third track is added, and a fourth (of opposite charge form the third). The vertexes of: couple, trip and quad are the reconstructed vertexes of: the first two tracks, those two with the third; those three with the fourth Inv. mass π + π - - Mass of ρ 66% of 4Prong decays goes through the resonant intermediate state Kπρ. Candidate is accepted if there is at least a couple of opposite sign π with invariant mass inside 100 MeV/c 2 from the ρ mass ISSP Erice - 30/06/2012 Fabio Colamaria 19

20 Topological cuts - Optimization Starting point: cut values studied on MC (thanks to Rossella) For each cut variable, an analysis run produced a set of scatter plots (TH2F), one for each p T bin. In the analysis all the other cuts were applied except the one under exmamination (kept very loose) In these scatter plots, the invariant mass versus the value of the variable under study are shown For each cut variable & p T bin, analysis of relative TH2F plot Projection on X axis (invariant mass) from different values of the cut onwards/backwards and calculation of significance Choosen value for the cut: the one whose projection has the maximum values of significance and S/B In case of contrast, significance maximization has been preferred ISSP Erice - 30/06/2012 Fabio Colamaria 20

21 PID selection Single track PID Indipendent checks by TPC and TOF Each detector gives an output of -1, 0, 1 if track is: excluded, compatible, or identified as a given specie, basing on the table below. As global output of the PID for the track, sum of TPC ad TOF outputs is taken Output of detector TPC TOF 1 (hypotesis accepted) < 2 σ if p T < 0.6 GeV/c; < 1 σ if 0.6 < p T < 0.8 GeV/c < 2 σ if p T < 1.5 GeV 0 (hypotesis compatible) < 3 σ for every p T (if not 1) < 2 σ if p T > 1.5 GeV -1 (hypotesis excluded) > 3 σ for every p T > 2 σ for every p T D 0 candidate PID selection Four mass hypotheses for each candidate (depending on which track is the K) A mass hypothesis is accepted if global output is >= 0 for each of the four tracks Each track must be compatible with the corresponding specie The ρ invariant mass cut helps in hardening the selection. ISSP Erice - 30/06/2012 Fabio Colamaria 21

22 Reflections issue When running the analysis on MC production, study of MCtruth for selected candidates reveals that a considerable fraction is made by wrong hypothesis of true D0 These reflections account for S value, while they are discarded when efficiency corrections are calculated. So, they bias the analysis and must be removed Reflection distributions cover a wide invariant mass interval, but show a sort of peak close to the D 0 invariant mass, hidden by the true signal, so a data-driven approach is not possible Fit of reflections with a particular function (polynomial + gaussian, bindependent) FuncRefl Distribution of reflections only for bins 2,3,4,5 Bin 2 Bin 3 Bin 4 Bin 5 ISSP Erice - 30/06/2012 Fabio Colamaria 22