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Particle Identification of the ALICE TPC via dE/dx

Particle Identification of the ALICE TPC via dE/dx. Weilin Yu Goethe Universität Frankfurt for the ALICE Collaboration. The ALICE experiment at the LHC The ALICE Time Projection Chamber (TPC) PID performance at low p T PID performance at high p T Summary. ALICE Experiment.

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Particle Identification of the ALICE TPC via dE/dx

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  1. Weilin Yu -- Bari TRD Workshop Particle Identification of the ALICE TPC via dE/dx Weilin Yu Goethe Universität Frankfurt for the ALICE Collaboration The ALICE experiment at the LHC The ALICE Time Projection Chamber (TPC) PID performance at low pT PID performance at high pT Summary

  2. Weilin Yu -- Bari TRD Workshop ALICE Experiment Central Detectors: Inner Tracking System Time Projection Chamber Time-of-Flight Transition Radiation Detector Spectrometers: High Momentum PID (RICH) Photon Multiplicity Forward Multiplicity Muon Spectrometer Calorimeters: EM Calorimeter Photon Spectrometer (PHOS) Zero Degree Calorimeter Trigger: Trigger Detectors pp High-Level-Trigger

  3. Weilin Yu -- Bari TRD Workshop ALICE PID Detectors • ALICE has a unique capability on the particle identification Central PID Detectors: Inner Tracking System Time Projection Chamber Transition Radiation Detector Time-of-Flight High Momentum PID (RICH)

  4. Weilin Yu -- Bari TRD Workshop The ALICE TPC TPC: main tracking device in ALICE • TPC main features: • ~92 m3 active volume with gas mixture: Ne-CO2 (90-10)* • Low drift diffusion • Maximum drift time 94 ms • 72 (=18x2x2) MWPCs with pad readout • Excellent performance on momentum reconstruction and dE/dx • High readout rate capability: • 1 kHz pp collisions • 200 Hz central Pb-Pb collisions** *Was Ne-CO2-N2 before 2011 ** Requires hígh level data compression

  5. Weilin Yu -- Bari TRD Workshop TPC Readout Chamber Wire arrangement in readout chambers ALICE TPC end plate 50 cm 46 cm 28 cm 86 cm 114 cm • In total 557,568 pads • 3 different pad segments: • 63 rows with 4 x 7.5 mm2 (IROCs) • 64 rows with 6 x 10 mm2 (inner OROCs) • 32 rows with 6 x 15 mm2 (outer OROCs)

  6. Weilin Yu -- Bari TRD Workshop TPC Readout Chip (ALTRO) • 10 MHz sampling ADC, 2 baseline restoration circuits, tail cancellation, zero suppression and multievent buffer

  7. Weilin Yu -- Bari TRD Workshop Event Topologies in TPC Cosmic rays pp event Pb-Pb event

  8. Weilin Yu -- Bari TRD Workshop PID at High Multiplicity TPC PID in the environment of unprecedented densities of charged particles! Challenges at high track multiplicity • Cluster pileup • Distortion due to space charge • Baseline Fluctuation Corresponding treatments • High granularity (small pads) and low diffusion gas mixture • Drift gas mixture with low primary ionization (low Z) • ALTRO online ion tail correction (fully commissioned in 2011)

  9. Weilin Yu -- Bari TRD Workshop dE/dx Basics • Energy loss per unit path length is described by the Bethe-Bloch formula(depends only on charge and rest mass for a fixed momentum) • Truncated mean (≈70%) is used to remove fluctuations due to the tail towards higher deposits (“Landau-like”) • Small signals of 1-pad clusters are included in the calculation to improve the dE/dx resolution • Parameterization is fitted to the data (Aleph-Parameterization):

  10. Weilin Yu -- Bari TRD Workshop TPC Krypton Calibration • Prerequisite for precise PID: Gain of readout pads has to be equalized • Release of radioactive Krypton into gas system • Gain determination to within 1% C side Gain Map 41.6 keV Kr spectrum of all OROCs 29 keV 12.6 keV 9.4 keV

  11. Weilin Yu -- Bari TRD Workshop Results from Cosmic Rays • The calibration procedure has mostly been tested via the analysis of cosmic rays

  12. Weilin Yu -- Bari TRD Workshop dE/dx measurement in TPC • Up to 159 samples in Ne-CO2 gas mixture: σdE/dx ≈ 5% • Very large dynamic range • (up to 26x min. ionizing) • allows to identify light nuclei • and separate them by • their charge • PID can be extended • to higher momenta on • the relativistic rise using • statistical unfolding Separation of p to K, p becomes constant at large p

  13. Weilin Yu -- Bari TRD Workshop TPC PID at low pT • In the low pT region dE/dx is clearly separated, track-by-track PID is even possible, e.g. based on nσ-bands ALICE Collaboration Eur.Phys.J.C71:1655,2011 (pp collisions at 900 GeV)

  14. Weilin Yu -- Bari TRD Workshop TPC PID at low pT • Transverse momentum spectra of positive charged particles from TPC and other sub-detectors of ALICE. ALICE Collaboration Eur.Phys.J.C71:1655,2011

  15. Weilin Yu -- Bari TRD Workshop TPC PID on the Relativistic Rise Charged particle nuclear modification factorRAA Physics Letters B 696 (2011) 30–39

  16. Weilin Yu -- Bari TRD Workshop TPC PID on the Relativistic Rise Charged particle nuclear modification factorRAA Charged particle elliptic flow v2 in ALICE • Charged particle identification on the relativistic rise (pT > 3 GeV/c) holds very interesting physics in HI collisions Physics Letters B 696 (2011) 30–39

  17. Weilin Yu -- Bari TRD Workshop TPC PID on the Relativistic Rise • dE/dx - <dE/dx>p for different pT slices • Fitted with 4 Gaussians (p, K, p, e) • Statistical unfolding, means and widths constrained from Bethe-Bloch and MIP respectively

  18. Weilin Yu -- Bari TRD Workshop Charged Pion Fraction • No centrality dependence of pion-to-charged ratio for pT > 6 GeV/c

  19. Weilin Yu -- Bari TRD Workshop Charged Pion RAA • Agrees with charged particle RAA • - in peripheral events • - for pT > 6 GeV/c

  20. Weilin Yu -- Bari TRD Workshop Anti Alpha Observation • 4 candidates for anti-alpha (PID using TPC and TOF)

  21. Weilin Yu -- Bari TRD Workshop Summary • The ALICE TPC is a large 3-dimensional tracking device for ultra-high multiplicity events • It has been operated successfully with pp and Pb−Pb • collisions at the LHC • The TPC offers excellent PID with an energy resolution of 5% • Results on identified particles at large pT allow to explore very interesting physics in heavy ion collisions

  22. Weilin Yu -- Bari TRD Workshop Transverse Momentum Resolution • pT resolution obtained from combined tracking (TPC & ITS) of 2.76 TeV Pb-Pb collisions • σ(pT/pT) = 20% at 100 GeV/c • Requirement (pp collisions): • σ(pT/pT) = 5% at 100GeV/c • To be refined for 2011 data by improving the tracking code Pt resolution for TPC+ITS combine tracking

  23. Weilin Yu -- Bari TRD Workshop Drift Velocity Calibration • Drift velocity (vD) not saturated • (Ne - based gas, E=400 V/cm) • vD changes with p, T and gas • composition. Correction factors • extracted using • => laser data • => matching of tracks in TPC and ITS • => external vD monitor • need T homogenization within • 10-4 (achieved) • p and T updated constantly in • calibration, gas composition • every 15 min One input for drift velocity corrections: Photoelectrons extracted from central drift electrode by scattered laser light. This plot indicates a 1.5 ‰ variation of the drift velocity due to the vertical temperature gradient in the gas volume.

  24. Weilin Yu -- Bari TRD Workshop Space Point Resolution • Space point resolution depends on • the drift length • the track inclination angle a • the drift charge deposited on the anode wire Tan (a) = 0.92 Tan (a) = 0.92

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