10 May 2006Paul Dauncey1 ALICE EMCAL Technical Proposal: First Discussion Paul Dauncey, Michel Gonin, Junji Haba

10 May 2006Paul Dauncey2 EMCAL is lead-scintillator Shashlik sampling EM calorimeter In addition to PHOS, a crystal EM calorimeter Covers  = ±0.7,  = Much larger than PHOS,  = ±0.12,  = 100 0, 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

10 May 2006Paul Dauncey3 Total is ten full plus two half supermodules Takes up effectively the whole space mechanically available Each full supermodule covers  = 0.7,  = 20 0 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

10 May 2006Paul Dauncey4 Reuse a lot of PHOS readout electronics APD and preamplifier FEE card (with shaper shortened from 1  s 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

10 May 2006Paul Dauncey5 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 quenching 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

10 May 2006Paul Dauncey6 Jet energy bias Fragmentation function in terms of x = p t hadron /E t jet p t hadron measured only for charged particles by TPC E t 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 E t jet best measured by charged particles and photons Necessitates a large solid angle EM calorimeter But will still potentially have some remaining bias

10 May 2006Paul Dauncey7 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

10 May 2006Paul Dauncey8 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

10 May 2006Paul Dauncey9 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

10 May 2006Paul Dauncey10 The 11 × “10 0 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 “10 0 module” is 0.17 in  and extends the full length in  E.g. reducing from 11 to 8 “10 0 modules” gives roughly half the acceptance resulting in roughly equivalent size in  and  Cost/acceptance trade-off hard to judge without knowing systematic limits

10 May 2006Paul Dauncey11 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