1. 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

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

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

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

5. 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

6. 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

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

8. 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

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

10. 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

11. 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

12. 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

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)
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Weilin Yu -- Bari TRD Workshop

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

15. 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

16. 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

17. 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

18. 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

19. 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

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

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

23. Transverse Momentum Resolution
pT resolution obtained from combined tracking (TPC & ITS) of 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

24. 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

25. 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