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ALICE EMCAL Technical Proposal: First Discussion

Paul Dauncey, Michel Gonin, Junji Haba

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• EMCAL is lead-scintillator Shashlik sampling EM calorimeter• In addition to PHOS, a crystal EM calorimeter

• Covers = ±0.7, = 1100

• Much larger than PHOS, = ±0.12, = 1000, but larger granularity• Sufficient to contain jet using cone R ~ 0.4

• Relevant physics is jet quenching• Provides jet trigger and improved jet energy reconstruction• Will also extend statistics and low energy range of 0 spectrum

• Groups from US, France and Italy• Total cost ~ CHF 8.6M• US would fund ~80% of this; US not currently members of ALICE• Rest funded by France and Italy; one new group from each country but

others three groups already members of ALICE

• Installation: 10% for 2008, 50% for 2009, 100% for 2010

Overview

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• Total is ten full plus two half supermodules• Takes up effectively the whole space mechanically available

• Each full supermodule covers = 0.7, = 200

• Sampling frequency 1.44mm/1.76mm determines resolution• Simulation gives EM resolution = 6.9%/E 1.4%• Exceeds physics requirement of 12%/E 2% but comes “free”

Proposed detector

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• Reuse a lot of PHOS readout electronics• APD and preamplifier• FEE card (with shaper shortened from 1s to

100ns for late neutron rejection)• Trigger also from FEE cards but needs extra

electronics (not specified in detail in TP)

Readout electronics

• Electronics cost is CHF 3.2M• Out of the total of CHF 8.6M• Trigger electronics is small fraction of total• Design granularity is twice Moliere radius• Reducing to one MR would need ×4

channels and push cost up enormously

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• Jet spectrum will be softened by QCD bremsstrahlung as hadrons pass through nuclear material

• Observe through softer fragmentation function of jets with given energy in Pb-Pb compared to p-p.

Jet quenchingSensitivity to medium properties

2.0

A. Morsch, ALICE

Measurements at pT~1 GeV are crucial

0.7 GeVEJet=100 GeV:

• Usual measure of quenching is energy loss of leading parton• Some models predict average loss is independent of jet energy• Desirable to measure this over a wide range of jet energies; up to 200 GeV

• Effect is very model dependent• Size of energy loss varies; could be ~30-40 GeV, could be much less• Not clear if signal would ever be visible, particularly in high energy jets• Even setting limit on parton energy loss would still restrict models

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Jet energy bias• Fragmentation function in terms of x = pt hadron/Et jet

• pt hadron measured only for charged particles by TPC

• Et jet must be unbiased

• Observed energy dominated by energy fluctuations in/out of jet cone, R = 0.4• Reasonably insensitive to

detector resolution• Tails can cause bias; upwards

fluctuations more critical than downwards

• Et jet best measured by charged particles and photons• Necessitates a large solid angle EM calorimeter• But will still potentially have some remaining bias

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• Raw jet rate in EMCAL acceptance above 100 GeV is around 300k/year and above 200 GeV is around 10k/year

• Without EMCAL trigger, would get ~30k/year and ~1k/year

• Need to bin by jet E bins and impact parameters (and other reality factors)

• Trigger is required to get high energy (i.e. up to 200 GeV) jet rate to useful level

Rates and trigger• Trigger enhances jet rate by ~10 for Pb-Pb, ~50 for p-p and p-A

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• This would bring US into ALICE• They would pay ~80% of EMCAL• They would also contribute between 6% and 10% of computing• They would also contribute to the Common Fund, removing the deficit

• The other ~20% is France and Italy• Some groups already in ALICE; involved in ITS and Spectrometer• ALICE are confident this will have no impact on existing responsibilities

• Only one other detector in ALICE currently not fully funded • This is the other EM calorimeter, the PHOS• Very important for 0 and direct measurements• Already staged with completion only by 2010 (assuming funding found)

Issues: 1 – Funding and effort

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• Both the quenching effect and the jet energy bias are unknown• Very model dependent• Size of systematic errors on fragmentation function not known• We have not seen an estimate even using non-quenched PYTHIA• Cannot tell where systematic limit is and where more statistics will be useful

• Need real jet data before these effects can be determined• Could be done for jet E < 100 GeV with TPC alone with 2008 Pb-Pb data• But would probably delay installation of EMCAL to after 2012• Not yet clear if higher energy (~200 GeV) jets will be interesting

• Installation of EMCAL in time for first few years of data is a risk• Not to physics; the EMCAL will definitely improve the physics performance

of ALICE• The risk is financial; the systematic limit may be too large so effectively no

improvement for jet quenching beyond TPC-only is found

Issues: 2 – Bias/resolution limit

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• The 11 × “100 module” design fills available space• How would performance degrade with fewer modules?

• Basic measure of required size due to R = 0.4 jet cone definition• Roughly; centre of jet must be more than 0.4 from edge

• EMCAL design is × = 1.4×1.9

Issues: 3 – Size of EMCAL

• Gives ~0.6×1.1 acceptance for central jet direction

• Each “100 module” is 0.17 in and extends the full length in

• E.g. reducing from 11 to 8 “100 modules” gives roughly half the acceptance resulting in roughly equivalent size in and

• Cost/acceptance trade-off hard to judge without knowing systematic limits

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• Technically, the design seems robust• No challenging new detector technologies• Reusing existing electronics designs

• The issues are mainly to do with physics outcome• Will the extra statistics due to the trigger be useful for quenching?• Will including the EM energy in the jet reconstruction reduce the bias

sufficiently?

• In addition, should consider• There are PHOS modules which have no funding• Some EMCAL European groups are working in other systems

• What happens next?• We collect questions and forward them to ALICE• Rediscuss and make a recommendation at the next LHCC

Conclusions