29 June 2006Paul Dauncey1 ALICE EMCAL Technical Proposal: Response to questions Paul Dauncey, Michel Gonin, Junji Haba

29 June 2006Paul Dauncey2 EMCAL is lead-scintillator Shashlik sampling EM calorimeter Covers  = ±0.7,  = Sufficient to contain jet using cone R ~ 0.4 Reminder of proposed EMCAL Relevant physics is jet quenching Provides jet trigger and improved jet energy reconstruction Will also provide EM trigger and 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

29 June 2006Paul Dauncey3 The general statements made at the last meeting were that the energy resolution with the EMCAL was completely dominated by physics fluctuations and so a full simulation was not needed. However, the plot shown (of the energy for leading particle, charged-only and charged+photons) does not include fluctations from the underlying events or any ambiguity in the jet clustering algorithm. There is also an issue with subtraction of track-associated energy to prevent double-counting. It would give us much more confidence to see results on these things from the full simulation. Question: Simulation issues

29 June 2006Paul Dauncey4 Ideal detector, pp, 100GeV jets Answer: Sources of resolution p T > 0 GeV 1 GeV 2 GeV Plus underlying background Out-of-cone fluctuations only Resolution degrades: from 21% to 28% Difference between fast and full simulation is small Conclusion: fast simulation sufficient Full and fast simulation, R=0.4

29 June 2006Paul Dauncey5 Jurgen told us verbally that the systematic limit on the fragmentation function measurements would be equivalent to approximately 10,000 events. Can you indicate where this number comes from and what assumptions are behind it (in terms of jet energy, fragmentation model, etc)? Answer: Jet quenching shifts high z particles to low z so measurements of high and low z should both show effects. They claim there is no significant systematic error in the high z region of the fragmentation function. The issue is measuring the low z region. Systematic is estimated from STAR results. Question: Systematic errors

29 June 2006Paul Dauncey6 B/S = 5 B/S = 100 PPR Vol II Fig GeV 0.7 GeV PPR Vol II Fig PPR Vol II Fig Statistical error 10,000 jets E t = 100 GeV Systematic error estimate at low z Systematic error arises from background subtraction Consider example:  ~ 5 region S/B ~ 1/100, dN ch /dy ~ 6000 Systematic error ~ ± 10% (extrapolating from STAR) To be systematics limited, need 5-10% or better statistical precision 10,000 events, Stat error ~ ± 15% Conclusion: O(10 5 ) jets needed to reach systematic limit

29 June 2006Paul Dauncey7 There are a wide variety of predictions for jet quenching. However, it would be very helpful to have some quantitative results on at least one or two indicative models, to demonstrate how well jet quenching can be measured with and without the EMCAL. One useful measure would be how much integrated luminosity would be needed to reject a given model, again with and without the EMCAL. This study would of course need to consider the systematics involved also. Answer: “Detailed exploration of jet quenching models in ALICE simulations is compute intensive. This is part of the ongoing Mock Data Challenge but ~ 6 months will be require to have quantitative results” Conclusion: They have not done the background work; fast simulation (by their own argument) would have been sufficient Question: Jet quenching sensitivity

29 June 2006Paul Dauncey8 How will the ALICE quenching measurements compare quantitatively with those from ATLAS and CMS, with or without the EMCAL? How do the trigger/reconstruction/systematics vary? Answer: “We do not feel sufficiently knowledgeable to give quantitative statements about the capabilities of Atlas or CMS. Below are some qualitative differences. The capabilities of ALICE & Atlas/CMS are very complementary and TOGETHER they should cover very well all relevant aspects for jet quenching.” Question: Comparison to ATLAS/CMS

29 June 2006Paul Dauncey9 CMS loose track cuts Unique in ALICE PID 0 – 50 GeV (‘jet composition’) Very low p t < 1-2 GeV (‘medium response’) Better in ALICE Tracking < 5 GeV (‘radiated energy’, low z fragmentation function) Detailed study of underlying event (‘background subtraction’) Comparison to ATLAS/CMS Comparable Calorimeter resolution (dominated by physics fluctuations) Better in Atlas/CMS Large acceptance calorimeters (=> statistics at high Et, jet-jet correlations) Conclusion: No quantitative study done but reasonable argument that low z region will be best done by ALICE. High z region is statistics dominated and so ATLAS and CMS likely to do better due to larger acceptance

29 June 2006Paul Dauncey10 The size (and hence cost) of the EMCAL seems to have been determined by the available space. What determines the optimal size of this detector? A reduction in size would reduce the rate but if the measurements will be systematics limited, this does not matter so much. Conversely, if the EMCAL will be statistics limited in jet quenching measurements, then should it be bigger, e.g. by taking the space currently occupied by the PHOS as well? It seems unlikely the best cost/performance point happens to coincide exactly with the available space within ALICE. Answer: EMCAL could be no larger as already at limit of support weight possible; load taken through L3 magnet doors. A smaller EMCAL would be possible. Question: Size

29 June 2006Paul Dauncey11 Trigger acceptance For high energy jets, completely statistics limited EMCAL trigger gives factor > ×10 Even with trigger, does not exceed systematic limit within one year at 200GeV Due to width of jet, trigger acceptance falls quickly with size Conclusion: rapid loss with fewer modules. Loss of statistics would have an impact for highest energy jets Area coverage [supermodules]

29 June 2006Paul Dauncey12 Before the 2010 run, you will have only a partial EMCAL. Can you detail quantitatively what you will be able to do with the data before this time? Will you need to use a smaller cone size, for example, and if so, what effect will this have? Answer: This depends on the year: 2008 First super module installed, ~10% acceptance for single particle trigger 10 4   to p T > 30 GeV/c => hadron quenching measurement already exceeding the kinematic and statistical reach of the RHIC experiments Inclusive electrons to ~15 GeV/c => Heavy quark cross section, flow and quenching 2009 Reasonable acceptance (up to 50%) but minimal for R=0.4 trigger Use small R= jet trigger cone and explore systematics => 10 4 sample to p T ~ 140 GeV/c Conclusion: Some physics can be done but not primary goals. Depends on amount of useful Pb-Pb running in these years also. Question: Partial EMCAL performance

29 June 2006Paul Dauncey13 It would be very useful to have a clear statement from ALICE on your priorities for completion of the EMCAL and the PHOS in the coming years. Both detectors will start with one module installed and we would all hope both will be completed with no problems. However, if there are resource limits, where would your priorities lie? Also, the European greoups on the EMCAL are also involved in the ITS and muon spectrometer; how will their priorities between their existing work and the EMCAL be balanced? We think it is not realistic to assume the new work will be able to be done with zero impact on the existing commitments. Question: Priorities

29 June 2006Paul Dauncey14 “PHOS: currently ~ 80% funded (~ 4/5 modules) ALICE gives absolutely highest priority to the completion of the ITS and muon projects (production/installation/commissioning) Vast majority of resources is ‘new’ US institutes Grenoble/Frascati have joined recently, explicitly to work on EMCAL Overlap in terms of groups/individuals is minor Catania/Strasbourg play minor roles (until current commitments are fulfilled) Nantes: exclusively people not currently involved in ITS/muon arm Time schedules are largely sequential ITS/muon arm production & installation finished by April 2007” Conclusion: They are confident the PHOS will be funded so they do not see this as an issue. They are confident there will be no significant impact. We are not so convinced… Answer: Priorities

29 June 2006Paul Dauncey15 We feel the background work has not been done in detail There is no detailed simulation of the EMCAL impact on jet quenching measurements They did the minimum amount of work to answer our questions The addition of the EMCAL will certainly not harm ALICE’s physics ability It is also likely to enhance it but it would be nice to be sure Technically the detector is well designed and we have no issues The inclusion of US groups in ALICE would be very welcome They would help finance computing and the common fund We think the Technical Proposal should be (unenthusiastically) recommended for approval Although with the caviat that the case is not really proven yet Another round of discussion would delay planned applications for funding in Sept this year Conclusions and recommendation