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 Weilin Yu -- Bari TRD Workshop

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 4/28/2017 Weilin Yu -- Bari TRD Workshop

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) 4/28/2017 Weilin Yu -- Bari TRD Workshop

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 4/28/2017 Weilin Yu -- Bari TRD Workshop

Weilin Yu -- Bari TRD Workshop TPC Readout Chamber Wire arrangement in readout chambers ALICE TPC end plate 46 cm 50 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) 4/28/2017 Weilin Yu -- Bari TRD Workshop

TPC Readout Chip (ALTRO) 10 MHz sampling ADC, 2 baseline restoration circuits, tail cancellation, zero suppression and multievent buffer 4/28/2017 Weilin Yu -- Bari TRD Workshop

Event Topologies in TPC Cosmic rays pp event Pb-Pb event 4/28/2017 Weilin Yu -- Bari TRD Workshop

PID at High Multiplicity TPC PID in the environment of unprecedented densities of charged particles! Cluster pileup Distortion due to space charge Baseline Fluctuation Challenges at high track multiplicity 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) 4/28/2017 Weilin Yu -- Bari TRD Workshop

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): 4/28/2017 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 4/28/2017 Weilin Yu -- Bari TRD Workshop

Results from Cosmic Rays The calibration procedure has mostly been tested via the analysis of cosmic rays 4/28/2017 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 4/28/2017 Weilin Yu -- Bari TRD Workshop

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) 4/28/2017 Weilin Yu -- Bari TRD Workshop

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 4/28/2017 Weilin Yu -- Bari TRD Workshop

TPC PID on the Relativistic Rise Charged particle nuclear modification factor RAA Physics Letters B 696 (2011) 30–39 4/28/2017 Weilin Yu -- Bari TRD Workshop

TPC PID on the Relativistic Rise Charged particle nuclear modification factor RAA 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 4/28/2017 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 4/28/2017 Weilin Yu -- Bari TRD Workshop

Weilin Yu -- Bari TRD Workshop Charged Pion Fraction No centrality dependence of pion-to-charged ratio for pT > 6 GeV/c 4/28/2017 Weilin Yu -- Bari TRD Workshop

Weilin Yu -- Bari TRD Workshop Charged Pion RAA Agrees with charged particle RAA - in peripheral events - for pT > 6 GeV/c 4/28/2017 Weilin Yu -- Bari TRD Workshop

Anti Alpha Observation 4 candidates for anti-alpha (PID using TPC and TOF) 4/28/2017 Weilin Yu -- Bari TRD Workshop

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 4/28/2017 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 4/28/2017 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. 4/28/2017 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 4/28/2017 Weilin Yu -- Bari TRD Workshop