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THE PHYSICS OF THE ALICE INNER TRACKING SYSTEM

THE PHYSICS OF THE ALICE INNER TRACKING SYSTEM. Elena Bruna, for the ALICE Collaboration Yale University. 24 th Winter Workshop on Nuclear Dynamics, South Padre Island 5-12 April 2008. OUTLINE. The ALICE Inner Tracking System (ITS) Performance of the ITS Tracking

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THE PHYSICS OF THE ALICE INNER TRACKING SYSTEM

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  1. THE PHYSICS OF THE ALICE INNER TRACKING SYSTEM Elena Bruna, for the ALICE Collaboration Yale University 24th Winter Workshop on Nuclear Dynamics, South Padre Island 5-12 April 2008

  2. OUTLINE • The ALICE Inner Tracking System (ITS) • Performance of the ITS • Tracking • Primary vertex reconstruction • Secondary vertex reconstruction (from heavy flavor decays) • Particle identification • Physics analyses with the ITS • Hadronic and semi-leptonic decays of heavy-flavor particles • Multiplicity studies • Conclusions Elena Bruna, Yale University

  3. THE INNER TRACKING SYSTEM (ITS) • 6 layers of Silicon detectors: • Pixel Chambers (SPD): 2 innermost layers • Drift Chambers (SDD): 2 intermediate layers • Double-sided Strip Chambers (SSD): 2 outermost layers SSD SDD SPD Elena Bruna, Yale University SSD Lout=97.6 cm SPD Rout=43.6 cm SDD

  4. ALICE @ LHCsetup HMPID B = 0.5 T TRD TPC MUON SPECTR.. TOF Elena Bruna, Yale University PHOS Inner Tracking System (ITS): 6 SILICON layers (pixel, drift, strip) Vertices reconstruction, PID (dE/dx) -0.9<<0.9 Size: 16 x 26 m Weight: ~10,000 tons

  5. STATUS OF THE ITS • The ITS was put inside the TPC in March-April 2007 SPD+SDD Elena Bruna, Yale University • First cosmics seen in February! • Tracking worked in the SPD • Aligned clusters seen in SPD and SDD • The ITS is ready to collect the first pp collisions!

  6. ITS PERFORMANCE: TRACKING (1 of 3) RICH TRD ITS TPC TOF PHOS • Tracking is the major challenge in ALICE: ~7000 tracks in a central HIJING Pb-Pb event at 5.5 TeV in the ITS + TPC acceptance Elena Bruna, Yale University • Tracking strategy: • from TPC ‘seeds’, tracks are extrapolated towards the ITS with the Kalman filter technique and then backpropagated to the outer detectors

  7. ITS PERFORMANCE: TRACKING (2 OF 3) • Tracking Stand-alone in the ITS: • used in the reconstruction software • tracks not reconstructed by the TPC • first day physics (track multiplicity and PID), both in pp and Pb-Pb • useful in case of initial alignment problems with the TPC Elena Bruna, Yale University

  8. ITS PERFORMANCE: TRACKING (3 OF 3) impact parameter resolution (on the bending plane) vs pT Elena Bruna, Yale University

  9. ITS PERFORMANCE: PRIMARY VERTEX • Primary vertex reconstruction: more problematic in pp than in Pb-Pb • Pb-Pb: primary vertex resolution dominated by the mis-alignment • pp: 2 steps for primary vertex finding: • Before tracking: using ITS pixels (“tracklets”) • After tracking: using tracksbetter Tracks from ITS+TPC Pixels from ITS Elena Bruna, Yale University (w. beam line constraint) # tracklets # tracklets

  10. ITS PERFORMANCE: SECONDARY VERTEX • Charmed mesons: ct~ 100-300 mm • A good tracking system is required to separate primary and secondary vertex • Good resolution on primary and secondary vertices • RMS ~ 120 mm: good to measure vertices displaced of 300 mm D+K-π+π+ (my thesis work) Xfound-Xtrue Elena Bruna, Yale University D+K-π+π+ D+K-π+π+ Zfound-Ztrue Yfound-Ytrue

  11. SECONDARY VERTEX FINDER y y π+ x’ y’ π+ K- bending plane rotated D+ x Elena Bruna, Yale University Along Pt D+ coord Orthog Pt D+ coord z coord Elena Bruna

  12. ITS PERFORMANCE: PID • Based on specific ionization (dE/dx) in the SDD and SSD (4 Silicon layers) • Add information to the PID given by the TPC (combined-Bayesian PID) • Identify tracks not reconstructed by the TPC: • Low momentum • Out of TPC acceptance • Dead zones of TPC (between sectors) dE/dx in the ITS, full tracking Protons Kaons Pions electrons Elena Bruna, Yale University 0.45<p<0.48 GeV/c 12

  13. PHYSICS ANALYSES RELATED WITH THE ITS • Measurement of the charged particle multiplicity with the silicon pixel detector of the ITS • Exclusive reconstruction of open-heavy-flavor particles: • D0Kπ (B.R. 3.8%) • D+Kππ (B.R. 9%) • D+sKKπ (B.R. 5.2%, through ϕ or K0*) • ΛcKpπ(B.R. 5%) • Semi-leptonic decays of beauty mesons: • Be+X(B.R. 10%) Elena Bruna, Yale University

  14. CHARGED PARTICLE MULTIPLICITY WITH THE SPD • Why multiplicity: • first measurement in pp 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 Elena Bruna, Yale University

  15. statistical. systematic. D0K-Π+ • S/B ≈ 10% • Significance for 1 month Pb-Pb run: S/√(S+B) ≈ 40 Elena Bruna, Yale University

  16. primary vertex K p D+K-Π+Π+(1) • Analysis strategy (1) • Cuts on single tracks (pT, transverse impact parameter) • Cuts on pairs: • distance primary vertex-Kπ vertex:δ • Product of impact parameters: d Signal Background Elena Bruna, Yale University d0K X d0π2 d0K X d0π2 d0K X d0π1 d0K X d0π1 • selection based on the products of impact parameters of the two Kp pairs: 25% of BKG triplets rejected When (d0K x d0p1)<0 & (d0K x d0p2)<0: empty region kinematically not allowed

  17. D+K-Π+Π+ (2) • Cuts on the triplets Kππ : • Quality of the secondary vertices • Global optimization on a hyper-surface of: • Distance between prim and sec vertices • Maximum transverse momentum among the 3 tracks pM=Max{pT1,pT2,pT3} • cosϑpoint • s=d012+d022+d032 qpoint pT D+ 17 Results for Pb-Pb Elena Bruna, Yale University D+K-π+π+ produced in 4π 3 dau in acceptance 3 dau reconstructed D+ sel (Id. PID) D+ sel (Real PID) D+ sel (No PID)

  18. D+K-Π+Π+ (3) Results for pp (No PID) Elena Bruna, Yale University 0<pT<2 GeV/c

  19. DS+K-K+Π+ Significance Significance • WHY? • To measure charm yield more precisely we need to measure as many channels as we can • Study of different ways of hadronization: • String fragmentation:Ds+ (cs) / D+ (cd) ~ 1/3 • Recombination: Ds+ (cs) / D+ (cd) ~ N(s)/N(d) (~ 1 at LHC?) • Analysis: • Resonance separation: • DsϕπKKπ (2.16%) • DsK0*KKKπ (2.5%) • Variables considered: • cosϑpoint • cosφopening • Distance between primary and secondary vertex • Sum of impact parameters squared • Dispersion of secondary vertex • Results: • Analysis feasible in Pb-Pb down to pT=3GeV/c DsϕπKKπ Elena Bruna, Yale University DsK0*KKKΠ pT(GeV/c) pT(GeV/c)

  20. BMESONS VIA B e e X • Inclusive measurement of electrons coming from semi-electronic decay of beauty hadrons • need good electron identification: combined PID in TPC (dE/dx) + TRD (+EMCal in future) • good measurement of the track impact parameter Elena Bruna, Yale University

  21. SUMMARY • Interesting analyses will be possible with the ITS, thanks to its excellent vertexing and tracking capabilities and PID: • Heavy flavor physics: • Hadronic and semi-leptonic decays of charm and beauty particles • Charged multiplicity, the “day one” measurement • ALICE is looking forward to collecting wonderful data. Elena Bruna, Yale University Thank you

  22. BACKUP SLIDES Elena Bruna, Yale University

  23. ITS ALIGNMENT xy Elena Bruna, Yale University 250 mm

  24. RESULTS FOR 3< PT(D+)<5 GEV/C cosqpoint Significance normalized to 109 pp MB events Elena Bruna d s=d02 cosqpoint

  25. ITS DETECTOR RESOLUTIONS

  26. ITS DIMENSIONS Elena Bruna, Yale University

  27. SECONDARY VERTEX FINDER • Tracks (helices) approximated with Straight Lines: analytic method • Vertex coordinates (x0,y0,z0) from minimization of: • xi, yi, zi are the errors on the track parameters • Quality of the vertexer (not weighted): track 3 track 1 d3 d1 d2 Secondary Vertex (x0,y0,z0) track 2 E.Bruna where:

  28. primary vertex K p COMBINING KP PAIRS • K and p have opposite charge sign • Cut on the distanced between the vertex of the 2 tracks and the primary vertex d • Working point:d>700 mm • Selected SIG=67% • Selected BKG=5% d (mm)

  29. SECONDARY VERTEX FINDER ON THE TRIPLETS • Secondary vertex resolution: ~120 mm • Cut on the quality of the Vertex: BLACK: signal RED: BKG Kpp Triplets • Working point: s < 200 mm • (optimized in pT ranges of D+) • Selected SIG=50% • Selected BKG=1% • S/B~3 x 10-4: still too small s (cm)

  30. RESULTS: D+K-P+P+ IN PB-PB • Selection efficiency and pT spectra: • pT integrated ε (D+) ≈ 1.5% (Ideal PID), 0.6% (Real PID), 1% (no PID) E.Bruna

  31. 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 Histograms normalized to the same area E.Bruna d~1000mm  K =10%

  32. Hadronic 3-charge-body decays of D+ D±I(JP) = ½ (0-) m = 1869.4 MeV/c2 c = 311.8 m (PDG ’04) D+K-++ BR = 9.2 % Elena Bruna

  33. Kinematics (1) K PT distributions of the generated particles (ONLY PYTHIA generation, NO propagation and reconstruction in the detector) (nonresonant events) Mean = 0.87 GeV/c D Mean = 1.66 GeV/c  Elena Bruna Mean = 0.67 GeV/c Knowledge of the PT shapes of the decay products important at the level of the selection strategy

  34. Kinematics (2) p Comparing with Pb-Pb central events (ONLY HIJING generation, NO propagation and reconstruction in the detector): PT distributions: Mean = 0.67 GeV/c Mean = 0.50 GeV/c nonresonant D+ decay K Elena Bruna HIJING central(normalized) Mean = 0.87 GeV/c Mean = 0.65 GeV/c Kand p from D+ are harder than K and p produced in a Pb-Pb event

  35. Dalitz Plots: Kinematics (3) Sharp borders due to PYTHIA cut off on the tails of distributions Non resonant Resonant Elena Bruna

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