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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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)






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


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)


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


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


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


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


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


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)


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)


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


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)



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


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


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.


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.


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


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.


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


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


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


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


Space-charge distortion correction36


Occupancy • Average pileup = 5 MB events

– 2500 tracks in average – ~7500 tracks is maximum

• Maximum occupancy : 70% at IROC

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Future Upgrade and Physics Perspectives of the ALICE TPC

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