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Alberto Pulvirenti (INFN and University of Catania) 21st Winter Workshop on Nuclear Dynamics

A first study of the feasibility of HBT analysis in the ALICE Inner Tracking System stand-alone. Alberto Pulvirenti (INFN and University of Catania) 21st Winter Workshop on Nuclear Dynamics Breckenridge, 8 February 2005. Outline. HBT utility and forecasts for LHC

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Alberto Pulvirenti (INFN and University of Catania) 21st Winter Workshop on Nuclear Dynamics

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  1. A first study of the feasibility of HBT analysis in the ALICE Inner Tracking System stand-alone Alberto Pulvirenti (INFN and University of Catania) 21st Winter Workshop on Nuclear Dynamics Breckenridge, 8 February 2005

  2. Outline • HBT utility and forecasts for LHC • LHC, ALICE and the Inner Tracking System • Tracking in ALICE • HBT simulation in the off-line ALICE framework • Tracking in the ITS stand-alone • Study of the detectability of HBT signals in the ITS stand-alone • Conclusions and outlook

  3. Intensity interferometry and QGP • HBT  source characterization in heavy ion collisions • Information about phase-space density S(x,k) • Sketch of the source state at freeze-out • Study of source final state for comparison with theoretical models • Evidence of collective effects which can be explained with QGP formation and NOT without it • R “out” / R “side” ratio greater than 1 • Long the emission duration at freeze-out due to a permanence of the fireball in the mixed phase while QGP hadronizes • Rischke-Gyulassy Nucl.Phys. A608 (1996), 479

  4. Using the value for the freeze-out temperature = 120 MeV: Average expansion duration (tf) = 6-8 fm / c Transverse expansion velocity ~0.5c Transverse source radius:7-9 fm (R2out – R2side) w.r. to KT: emission duration: ~ 2-3 fm/c  anomalously small “HBT puzzle” From recent experiments (SPS – RHIC):KT dependence and collective dynamics of fireball NA49  from QM2004 proceedings  RHIC Rlong Rout,side

  5. Expectation for ALICE (from ALICE PPR) • Source size (≈ RoRsRl) should grow with dN/dy • the increment of the three radii can be different. • Initial expectation (based also on the recent results from RHIC): 8-12 fm • but the estimation cannot be extremely definite. • Observed increment in the average phase space density: [Bertsch, Phys. Rev. Lett. 72, 2349 – 77, 789] • influence on the HBT radii expectation • Possibility of smaller radii. J. Cramer, INT/RHIC workshop, Dec 15, 2002 Bertsch, Phys. Rev. Lett. 72, 2349 – 77, 789

  6. http://www.cern.ch LHC ~9 km SPS CERN Large Hadron Collider

  7. 5.5 A TeV Pb-Pb Expected multiplicity (dNch/dy)y=0: Major uncertainties not completely resolved Still no safe way to extrapolate Simple scaling form RHIC  ~2500 Safe guess  ~1500 – 6000 Worst case  ~8000 Baseline in the project Luminosity for Pb-Pb: Lmax = 11027 cm-2s-1 Some numbers related to ALICE and LHC worst (confusion) seen up to now: STAR …and the worst case for ALICE

  8. HMPID PID (RICH) at very high pt TOF PID at high pt TRD Electron identification PMD g multiplicity TPC Tracking, dE/dx ITS Small pt tracking, parameter refinement at vertex Vertexing MUON-Arm m-pairs PHOS g,p0 The ALICE detector

  9. SSD SDD SPD Inner Tracking System (ITS) Lout=97.6 cm Rout=43.6 cm • 6 Layer, 3 technologies (occupancy ~2% at max multiplicity) • Silicon Pixels (0.2 m2, 9.8 Mchannels. Single chip size = 50x425 μm) • Silicon Drift (1.3 m2, 133 kchannels) • Double-sided Strip (4.9 m2, 2.6 Mchannels)

  10. ITS performances

  11. ALICE PPR CERN/LHCC 2003-049 Dp/p (%) 50 30 9% 10 50 pt (GeV/c) 100 10 ITS purposes • Track propagation to the closest distance from the interaction point • Best resolution for all track parameters • Tracking of low transverse momentum particles • Primary vertex estimation before tracking (and used as a constraint for primary particles tracking) • Secondary vertices detection • D, B • strangeness • Event characterization in a “high-rate acquisition phase” • TPC powered off  ITS unique tracking device • Neural Tracking in the ITS stand-alone • Can HBT be done with these data?

  12. Tracking in ALICE TIME PROJECTION CHAMBER Up to 180 pts / track [starting point] INNER TRACKING SYSTEM Up to 6 pt / track [resolution improvement] • “Standard TPC + ITS” tracking • Seed determined in the external pad-row of TPC • Seed propagation through TPC via a Kalman Filter algorithm • Track propagation in the ITS still with the Kalman Filter [rif.: A. Badalà et al.: NIM A 485 (2002) 15] • Back-propagation to outermost TPC  TRD  TOF • Final back-propagation again to the interaction point

  13. Why an ITS stand-alone tracking? • “high-rate acquisition mode”: • how: getting data from the “fast” modules only (ITS is one) • target: physics analysis requesting huge statistics • requirements: good efficiency in the high transverse momentum range (pt >1 GeV/c). • Algorithm: Denby-Peterson neural network

  14. correlation  increases activation competition  decreases activation Fully connected ANN How the neural network works Activation variation becomes smaller after a number of cycles Final stabilization 

  15. How the neural network works Map of real-valued activations between 0 and 1 Comparison with a threshold S Binary activation map (“on” / “off”)

  16. Implementation ITS rec-points  Neurons = segments between points in consecutive layers Competition  negative constant weight No relations among non connected segments Chain  positive variable weight depending on the alignment (NB: we look for HIGH pt particles) BAD alignment: wE 0 GOOD alignment: wE A

  17. good “fake” Stand-alone tracking efficiency & resolution Efficiency calculation: • “Found” track: a track where at least 5 of 6 points share the same GEANT label • “Fake” track: otherwise • “Findable” track: a track with generates at least 5 points in the ITS • EFFICIENCY = “found tracks” / “findable tracks” Track reconstruction: Kalman Filter • Seed: from a Riemann Sphere global fit • Propagation: vertex  layer 6  vertex

  18. HBT analysis package • Developed by the ALICE group from the Warsaw University of Technology (J. Pluta. P. Skowronski & C.)

  19. HBT simulation in ALICE • HBT processor (after-burner) • shifts the vector momentum of generated particles, to generate the correlations • Correlations are “fixed” for each event. • Weight method [R. Lednicky, Heavy Ion Physics 3 (1993), 93] • A weight is defined for each particle pair according to: • HBT correlation • FSI interactions • Space-time coordinates of the pair • 4-momenta of single particles • The correlation function is determined by normalizing the “weighted” pair distribution (w.r. to q) to the “un-weighted” one. • Advantages: • It is possible to modify the source parameters or the particle pair to study without having to re-generate the event • Much CPU time • It is possible to make a systematic study of HBT signals in different condition with the same statistical sample.

  20. Ingredients of the used simulation • HBT effects simulation in AliRoot: • Weight method • Generator: parameterized HIJING • 160 events at multiplicity (dNch/dy)y=0 = 4000 • Magnetic field: 0.4 T • “Perfect” PID: • Realistic PID for high-momentum particles in the ITS requires matching with outer ALICE modules (TRD, TOF) • Under study • Gaussian 3-dimensional source simulation (only one R parameter to fix) • R = 6, 8, 10, 12 fm • λ = 0.5, 0.75, 1 • Coulomb effect included • 1-D correlation function studied (Qinv)

  21. Reconstruction effects and evidence of HBT signal HBT enhancement • Double-Track efficiency • Cluster finding • Tracking

  22. Comparison:HBT generator turned on vs. HBT generator turned off R = 6 fm R = 8 fm R = 10 fm R = 12 fm

  23. Compatibility between CF and flat (=1) distributions(outside the fall-down due to double track efficiency)

  24. Correction for double track efficiency Normalization of CF with HBT effect with respect to the CF without the HBT effect, in order to remove double track efficiency

  25. Reconstruction of R Comparison of reconstructed R vs. simulated R Straight line = “optimum” line (reconstructed = generated)

  26. Reconstruction of λ Comparison of reconstructed λ versus simulated λ Straight line = “optimum” line (reconstructed = generated)

  27. Conclusions and outlook • HBT enhancement signal: • Generally visible, for the examined radii & lambda • Reconstructed parameters are underestimated due to Coulomb FSI • Mapping of rec vs. sim • More suitable fit being developed within the framework package • Fundamental requirement: high statistics • Statistics problems affect the study at large radii • Anyway, an encouraging result for further study • TODO: • Increment of statistics • Improve the results for larger radii • Check feasibility for 3-D HBT • Improvement of low-momentum particles tracking • Study of influence of “realistic” PID

  28. Correlation Functions • Clear merging effect in Qside and Qlong • Very good resolution and PID • We will anyway correct it but first we have get rid of the merging effect

  29. Fits to Correl. Functions: 3D • 3D fit, (range 0-50MeV): • Qout=7.92 ± 0.03 fm • Qside=7.84 ± 0.02 fm • Qlong=8.16 ± 0.02 fm • l=0.87 ± 0.01 • 2/NDF=1.48 • 2 depends strongly on the maximum range • If wide range is used 2 is good, of course • Fitted values does not depend on it • Fitted values depend on minimum of the range • Already mentioned merging effect

  30. Coverage in [η - pt] in ALICE

  31. p/K TPC + ITS (dE/dx) K/p e /p p/K TOF e /p K/p HMPID (RICH) p/K K/p 0 1 2 3 4 5 (GeV/c) TRDe /p PHOS g /p0 1 10 100 GeV/c PID capability of ALICE • hadrons (, K, p): 60 MeV/c < p < 5 GeV/c • dE/dx in silicon (ITS) and gas (TPC) +(TOF) +Cherenkov(HMPID) • leptons (e, ), photons, 0 : TRD:p > 1 GeV/c • muons: (Muon Spectrometer) • p > 5 GeV/c • 0 inPHOS: 1 < p < 80 GeV/c

  32. ALICE: overwiew Peso: 10.000t Diametro esterno: 16,00m Lunghezza totale: 25m Campo magnetico: 0.2-0-5Tesla ALICE On-line System multi-level trigger per filtrare il fondo e ridurre la quantità di dati registrati 8 kHz (160 GB/sec) level 0 – custom hardware 200 Hz (4 GB/sec) level 1 – embedded processors 30 Hz (2.5 GB/sec) level 2 – PC’s 30 Hz (1.25 GB/sec) La Collaborazione ALICE include più di 1200 persone da oltre 90 istituzioni in 29 paesi data storage & off-line analysis

  33. Event generator: HIJING • Main standard of the collaboration • Parameterized version for generation of a “signal-free” event • Variable multiplicity, HIJING-like shaped • Useful for study of tracking efficiency and event reconstruction performances. Distrib. In η Distrib. In pt

  34. Framework di simulazione e analisi • AliROOT  ROOT-based • Object Oriented • Modularità • Riutilizzabilità del codice • Implementazione completa di tutte le fasi di simulazione e analisi • Simulazione dettagliata di tutte le parti (sensibili e non) del rivelatore ALICE

  35. Multiplicity expectations • The major uncertainties in the energy dependence are still there (only some improvement with the RHIC data!). • Still no safe way to extrapolate • shadowing/saturation (might decrease Nch) • jet quenching (might increase it) • A-scaling (soft vs. hard) • Simple scaling form RHIC (log-log plot) ~2500 • safe guess dNch/dη ~ 1500 – 6000 • ALICE conservative design: all hardware able to handle ~ 8000

  36. Source-averaged phase space density Bertsch, Phys. Rev. Lett. 72, 2349 – 77, 789 This is compatible with a not so huge increment of HBT radii going to higher energy. Important issue for forecasts on HBT radii which will be measured in ALICE

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