Future Upgrade and Physics Perspectives of the ALICE TPC

A Large Ion Collider Experiment

Future Upgrade and Physics Perspectives of the ALICE TPC

Taku GunjiOn behalf of the ALICE Collaboration

Center for Nuclear Study, The University of Tokyo

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A Large Ion Collider Experiment

Outline

• ALICE upgrade after Long Shutdown 2 (LS2)• ALICE TPC upgrade with micro-pattern gaseous detectors• Status of R&D activities• Summary and Outlook

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http://cds.cern.ch/record/1622286

ALICE TPC Upgrade Technical Design Report

(submitted in 2013)

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ALICE Physics Program in Run3• Detailed characterization of the QGP at the highest LHC energy• Main Physics topics. Uniquely accessible with ALICE after LHC

luminosity and detector upgrade.– Heavy-flavors (charm, beauty):

• Diffusion coefficient – azimuthal anisotropy and RAA

• In-medium thermalization and hadronization – meson-baryon– Low-mass and low–pt di-leptons:

• Chiral symmetry restoration – vector meson spectral function• Space-time evolution and thermodynamical properties – radial and

elliptic flow of emitted radiation – Quarkonia (J/y, y’, U) :

• Charm and bottom thermalization, regeneration – RAA, flow

– Jet quenching and fragmentation:• Energy loss, transport properties vs. Q2 – RAA, flow

– Heavy-nuclei, exotic hadrons:• Confinement, Coalescence, quasi-state in QGP – RAA, flow

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ALICE Upgrade LoI:http://cds.cern.ch/record/1475243

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ALICE Upgrade Strategy• Operate ALICE at high rate, record all MB events

– Goal: 50kHz in Pb-Pb (~10nb-1 in Run3 and Run4)• Upgrade detectors and electronics during Long

Shutdown 2 (2018)– New Inner Tracking Systems

• Improved vertexing, tracking at low pT, and improved rate capability

– GEM TPC with continuous readout• High rate capability, preserve PID and tracking

performance– Muon Forward Tracker– Electronics, Trigger, online-offline upgrade

Talk by S. Siddhanta (172)

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Posters by L. V. Palomo(M-29), A. Uras(F-56)

Posters by R. Romita(M-23), C. Terrevoli(M-27), J. Stiller(M-26)

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Example: Low Mass Di-electrons • High statistics + Dalitz, conversion and charm rejection in

new ITS, TPC+TOF for eID• Reduced systematic uncertainties from charm decay

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ALICE SimulationTPC Current rate

New ITSB= 0.2T

ALICE SimulationTPC High rate

New ITSB=0.2T

dedicated low-field rundedicated low-field run

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ALICE TPC

114cm

50cm

5m

5m

E E

Readout chamber

Central Electrode (-100kV)

• Diameter: 5 m, length: 5 m• Acceptance: |h|<0.9, Df=2p• Readout Chambers: total = 72

– Outer (OROC): 18 x 2– Inner (IROC): 18 x 2– Pad size

• Inner: 4×7.5 mm2, Outer: 6×10&15 mm2

– Pad channel number = 557,568

• Gas: Ne-CO2 (90-10) (in Run1)at drift field = 400V/cm

– sT~sL ~0.2mm /√cm, vd~2.7cm/ms

• Total drift time: 92ms• MWPC + Gating Grid Operation

– Rate limitation < 3.5kHz

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OROC

IROC

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GEM TPC upgrade• Operation of MWPC w/o Gating Grid in 50 kHz Pb-Pb

would lead to massive space-charge distortion due to back-drifting ions.

• Continuous readout with GEMs– GEM has advantages in:

• Reduction of ion backflow (IBF)• High rate capability• No ion tail

– Requirement• IBF < 1% at Gain =2000• dE/dx resolution < 12% for 55Fe• Stable operation under LHC condition

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Standard GEMPitch=140mmHole f=70mm

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Space Charge Distortions • Ions from 8000 events pile up in the drift volume in

50kHz Pb-Pb collisions (tion=160ms)• 1% of IBF at Gain = 2000 (e=20)

– At small r and z, dr=20cm and drf = 8cm• For the largest part of drift volume, dr<10 cm

– Corrections to a few 10-3 are required for final resolution (s(rf) ~ 200um)

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GEM TPC R&D Program• Extensive studies started in 2012.

– Technology choice• Baseline: GEM stacks of standard (S) and large-pitch (LP)• COBRA-GEM• 2 GEM + MicroMegas(MMG)

– Ion backflow – Gain stability – Discharge probability – Large-size prototype

• Single mask technology– Electronics R&D– Garfield simulations– Physics and Performance simulations

• Collaboration with RD51 at CERN

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280um

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4 GEM setup with S and LP foils • IBF and Resolution studies for baseline solution

– Different foil configurations, VGEM, transfer field ET

• IBF optimized setting = high ET1 & ET2, and low ET3, VGEM1~VGEM2~VGEM3<<VGEM4

– 0.6-0.8% IBF at s(5.9keV)~12%

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4 GEMS-LP-LP-S

140um pitch 280um pitch

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Garfield Simulations• Garfield++/Magboltz simulations

– Field calculation by ANSYS– IBF quantitatively well described by simulations

based on Garfield++.

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GEM1(S)

GEM2(LP)

GEM3(LP)

GEM4(S)

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dE/dx studies with 3 GEM Prototype12

G=1000 6000

• Prototype IROC was built in 2012.• With 3 single-mask GEMs• Beam test at PS (e/p/p) in 2012

• Good e/p separation• sdE/dx/<dE/dx> ~ 10.5%

• Comparable to the current TPC resolution (~9.5% with IROC)

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Alternative: 2 GEM + MicroMegas• IBF and Resolution studies

– VMesh, VGEM, transfer field ET

– It is possible to reach < 0.2% IBF at s(5.9keV)~12%.

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Large-scale solution and operational stability still to be verified

Ne-CO2 (90-10)Gain~1850-2150

UMMG

UGEM

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Electronics• New ASIC “SAMPA”

– Integration of the functionality of the present preamp/shaper and ALTRO ADC+DSP

• Both polarity, Continuous/Triggered RO• SAR ADC (10M or 20MSPS)

– First MWP submission in April

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Upgrade of ALICE Electronics & Trigger System(Technical Design Report)

http://cds.cern.ch/record/1603472

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Reconstruction Scheme

• Two stage reconstruction scheme:– Cluster finding and cluster-to-track association in the TPC

• Data compression by x20 : 1 TB/s 50 GB/s• Scaled average space-charge distortion map

– Full tracking with ITS-TRD matching • High resolution space-charge map (time interval~5ms) for full distortion

calibration

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Expected Performance • Space charge fluctuations (~3%) are taken into account.(Nevt,dNch/dh,etc)

• ITS-TPC track matching and pT resolution are practically recovered after 2nd reconstruction stage.

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Summary and Outlook• The ALICE program after LS2 requires an upgrade of

the TPC.• MWPC-based readout chambers will be replaced by

detectors employing micro-pattern detectors including GEMs to allow TPC operation in continuous mode.

• Extensive R&D of the GEM TPC upgrade– 4 GEMs, 2GEM+MMG

• IBF<1%, Resolution for 55Fe<12%– Performance of the present TPC will be maintained in 50kHz Pb-Pb collisions.– Stability, discharge probability under study– Beam test of IROCs at PS and SPS in 2014

• Construction (GEM, FEE) from 2015

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Backup slides

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IBF with conventional GEMs• Measurement at CERN(RD51)/TUM/FRA/Tokyo.

– 3 or 4 standard GEM settings– standard and/or large pitch foils– X-ray from top or side, – current readout from each electrode

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Other options• COBRA-GEM

– SciEnergy, 400um pitch• 2 GEM + MicroMegas

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Calculator• Parameterization of collection, extraction, gain,

resolution, and IBF vs. VGEM, Ed, Et, Eind, S/LP

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Collection vs. Ed/UGEM1 Extraction vs. ET/UGEM2

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Calculator• Parameterization of collection, extraction, gain,

resolution, and IBF vs. VGEM, Ed, Et, Eind, S/LP

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RMS/Gain vs. Total Multiplication*sqrt(collection)

# of ions in drift/Effective Gain vs. Ed/UGEM1

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Space-charge fluctuation• Source of space-charge fluctuations

– The number of pile up events, Multiplicity– Charge of the tracks, Granularity

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At 8000 ion pile up events,space-charge fluctuation is 2-3%.Dominant source:

• Nevt fluctuation • Multiplicity fluctuation

Need take into account thesefluctuations for distortion corrections.

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Space-charge map• Study of space-charge distortions based on real

Pb-Pb data

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50kHz Pb-Pb collisions.8000 pileup events in ion drift time=160msec

Overlapped 130k events are used to estimate time-averaged space-charge distortion.

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Space-charge fluctuation• Time shifted space-charge map

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Simulation inputs:Use fluctuating space-charge map for track distortion and

Correction Use time-shifted map

~5msec is the time-scale to update the space-charge map during the online-calibration procedure

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Distortion correction in 2nd stage• Simulate statistics of typical calibration interval

(~5msec. 250Hz)– Pre-reconstruct by scaled average SC map

• Then, use ITS-TRD track interpolation– Map residual local distortions and 2-D correction

analysis to get (dr, drf)

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Spatial Patterns of dr and drf are well reproduced.

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IROC Prototype• Large-size GEM foils by CERN using single mask

technology. 3 standard GEM foils in prototype

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TPC Operation without GG

• MWPC without GG– Best estimate: ion back

flow (IBF) rate of ~5% at gain = 6000

– Simulation shows a large distortion in electric field impossible

• Tolerable limit– IBF rate of 1% at gain

2000; ~20 back flow ions per electron

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Front-end Electronics

Comparison of FEE parameters for RUN 1 and 3

Data rates and bandwidth requirements

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Current TPC Performance• 98% tracking efficiency in pp. 1-3% lower for

central Pb-Pb• Momentum resolution ~ 1% at 1GeV, 5% at 50GeV• dE/dx resolution= 5.5% in pp and 7% in Pb-Pb

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Gating Grid Operation31

• GG close 100us after collisions• GG closed for 180us (ion arrival time

to the GG)• IBF<10-4 but event rate < 3.5kHz• GG open results in 5-8% IBF

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Gain Stability• 3 stacked GEMs with 90Sr for Ne/CO2 (90/10)

– Single-wire chamber as a reference for correction of the gain fluctuation due to P/T

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Gain Variation within 0.5%at gain=1800

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Prototype Beamtest at PS in 2012• IROC Prototype (3 standard GEMs) beamtest at

CERN-PS T10– e, p, p: 1-3 GeV for negative, 1&6 GeV for positive– PCA16 + ALTRO Readout from LCTPC collaboration– dE/dx resolution for standard and IBF setting

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Garfield Simulations• Garfield++ simulations

– Field calculation by ANSYS– Mis-alignment of GEMs– Measurements are understood.

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IBF and Energy Resolution• Systematic studies for 4 GEM

– different foil configurations, VGEM, transfer field ET

• IBF optimized setting = high ET1 & ET2, and low ET3, VGEM1<VGEM2<VGEM3<VGEM4

– 0.6-0.8% IBF and s(5.9keV)=11-12%

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4 GEMS-LP-LP-S

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Space-charge distortion correction36

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Occupancy • Average pileup = 5 MB events

– 2500 tracks in average – ~7500 tracks is maximum

• Maximum occupancy : 70% at IROC

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