ALICE EMCAL Technical Proposal: First Discussion

1. ALICE EMCAL Technical Proposal: First Discussion Paul Dauncey, Michel Gonin, Junji Haba Paul Dauncey

2. Overview • EMCAL is lead-scintillator Shashlik sampling EM calorimeter • In addition to PHOS, a crystal EM calorimeter • Covers Dh = ±0.7, Df = 1100 • Much larger than PHOS, Dh = ±0.12, Df = 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 p0 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 Paul Dauncey

3. Proposed detector • Total is ten full plus twohalf supermodules • Takes up effectively the whole space mechanically available • Each full supermodule covers Dh = 0.7, Df = 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” Paul Dauncey

4. Readout electronics • Reuse a lot of PHOS readout electronics • APD and preamplifier • FEE card (with shaper shortened from 1ms to 100ns for late neutron rejection) • Trigger also from FEE cards but needs extra electronics (not specified in detail in TP) • 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 Paul Dauncey

5. Jet quenching • 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. • 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 Paul Dauncey

6. 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 Paul Dauncey

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

8. Issues: 1 – Funding and effort • 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 m 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 forp0 and direct g measurements • Already staged with completion only by 2010 (assuming funding found) Paul Dauncey

9. Issues: 2 – Bias/resolution limit • 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 Paul Dauncey

10. Issues: 3 – Size of EMCAL • 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 h×f = 1.4×1.9 • Gives ~0.6×1.1 acceptance for central jet direction • Each “100 module” is 0.17 in f and extends the full length in h • E.g. reducing from 11 to 8 “100 modules” gives roughly half the acceptance resulting in roughly equivalent size in hand f • Cost/acceptance trade-off hard to judge without knowing systematic limits Paul Dauncey

11. Conclusions • 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 Paul Dauncey