29 june 2006paul dauncey1 alice emcal technical proposal: response to questions paul dauncey, michel...

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29 June 2006 Paul Dauncey 1 ALICE EMCAL Technical Proposal: Response to questions Paul Dauncey, Michel Gonin, Junji Haba

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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

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Page 1: 29 June 2006Paul Dauncey1 ALICE EMCAL Technical Proposal: Response to questions Paul Dauncey, Michel Gonin, Junji Haba

29 June 2006 Paul Dauncey 1

ALICE EMCAL Technical Proposal: Response to questions

Paul Dauncey, Michel Gonin, Junji Haba

Page 2: 29 June 2006Paul Dauncey1 ALICE EMCAL Technical Proposal: Response to questions Paul Dauncey, Michel Gonin, Junji Haba

29 June 2006 Paul Dauncey 2

• EMCAL is lead-scintillator Shashlik sampling EM calorimeter• Covers = ±0.7, = 1100

• 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

Page 3: 29 June 2006Paul Dauncey1 ALICE EMCAL Technical Proposal: Response to questions Paul Dauncey, Michel Gonin, Junji Haba

29 June 2006 Paul Dauncey 3

• 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

Page 4: 29 June 2006Paul Dauncey1 ALICE EMCAL Technical Proposal: Response to questions Paul Dauncey, Michel Gonin, Junji Haba

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Ideal detector, pp, 100GeV jetsAnswer: Sources of resolution

pT > 0 GeV1 GeV2 GeV

Plus underlying background

Out-of-cone fluctuations only

Resolution degrades: from 21% to 28%Difference between fast and full

simulation is smallConclusion: fast simulation sufficient

Full and fast simulation, R=0.4

Page 5: 29 June 2006Paul Dauncey1 ALICE EMCAL Technical Proposal: Response to questions Paul Dauncey, Michel Gonin, Junji Haba

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• 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

Page 6: 29 June 2006Paul Dauncey1 ALICE EMCAL Technical Proposal: Response to questions Paul Dauncey, Michel Gonin, Junji Haba

29 June 2006 Paul Dauncey 6

B/S = 5

B/S = 100

PPR Vol II Fig 6.374

1.8 GeV 0.7 GeV

PPR Vol II Fig 6.368

PPR Vol II Fig 6.386

Statistical error10,000 jets Et = 100 GeV

Systematic error estimate at low z• Systematic error arises from background subtraction• Consider example: ~ 5 region

• S/B ~ 1/100, dNch/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(105) jets needed to reach systematic

limit

Page 7: 29 June 2006Paul Dauncey1 ALICE EMCAL Technical Proposal: Response to questions Paul Dauncey, Michel Gonin, Junji Haba

29 June 2006 Paul Dauncey 7

• 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

Page 8: 29 June 2006Paul Dauncey1 ALICE EMCAL Technical Proposal: Response to questions Paul Dauncey, Michel Gonin, Junji Haba

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• 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

Page 9: 29 June 2006Paul Dauncey1 ALICE EMCAL Technical Proposal: Response to questions Paul Dauncey, Michel Gonin, Junji Haba

29 June 2006 Paul Dauncey 9

CMS

loose track cuts

• Unique in ALICE• PID 0 – 50 GeV (‘jet composition’)• Very low pt < 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

Page 10: 29 June 2006Paul Dauncey1 ALICE EMCAL Technical Proposal: Response to questions Paul Dauncey, Michel Gonin, Junji Haba

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• 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

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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

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number of 10 degree segments

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Area coverage [supermodules]

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• 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 104 to pT > 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=0.1-0.2 jet trigger cone and explore systematics => 104 sample to pT ~

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

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• 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

Page 14: 29 June 2006Paul Dauncey1 ALICE EMCAL Technical Proposal: Response to questions Paul Dauncey, Michel Gonin, Junji Haba

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• “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

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• 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