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Learn about the LHC's impressive progress in managing luminosity challenges & optimizing the CMS L1 Trigger in 2012. Explore strategies, data, and upgrades for improved performance.
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-90 kHz rate Level-1 Triggerin2012 CMS run 201624 LHC Fill 2998 Manfred Jeitler HEPHY Vienna - 3% deadtime
challenges and performance in 2012 • LHC very successfully increased luminosity • up to almost 8e33 • PeakLumi = 7573.567×1030cm-2sec-1 • PeakPileup = 34.117 • in run 201624 (fill 2998) • cope with this without losing good data!
challenges and performance in 2012 plot by Darin Acosta
challenges and performance in 2012 more and more trigger algos must be disabled ! (mapopripped) plot by Darin Acosta
muon rate strongly suppressed... ...while keeping the efficiency • mainly by better pt assignment in CSC Track Finder • also by more sophisticated merging scheme in GMT • Global Muon Trigger • rate down by 50% • over full range • 30% for |η|<2.1 shown in plot
calorimeter jet seed threshold • jet an especially HT rates are strongly pileup dependent • HT is ET summed over identified jets above (low) energy threshold • 10 GeV • overall illumination of calorimeter creates fake jets • cure: introduce threshold for jet “seed” • currently 5 GeV • must be done in GCT • Global Calorimeter Trigger, upstream of GT (Global Trigger) • outside of “Level-1 trigger menu” • very effective in reducing rates
L1 trigger close to its present limit ...... but not at the end of its tether! • with LHC’s impressive progress we have had to also improve the trigger • by just raising thresholds we would be losing physics already • substantial improvements achieved this year • some more still be possible • but to profit from the LHC upgrade CMS also must upgrade the L1 trigger
L1 upgrade stage zero:optical links between GCT and GT • fast galvanic serial links between GCT and GT had been a worry • no spares available, no data check possible • new electronic modules with optical links installed before 2012 run • “Optical PSB” (or “oGTI”) • learning experience for mostly optical connections in upgrade project • CRC check recently implemented • must be careful when touching a running system • due to misconfiguration calo monitoring data were not correctly read for some time • now fixed, everything working fine • don’t forget certain “features” such as GCT input masking!
Sorry for keeping you waiting ... • ... but we know (now) that you enjoy your L1 trigger just as much if it’s served 12 BX later! • ... and it will taste better if cooked a bit longer!
Why just-in-time delivery of L1A? • the upgraded trigger will be better than the present system • but not faster : • carry out more complex operations • collect and use more information for decision making • use of serial optical links allows using more information (higher bandwidth) and is less error prone (fewer cables that can break) but requires time for SERialization / DESerialization (SerDes)
latency + 12 BX test • finding the maximum admissible latency is not trivial because: • Tracker and Preshower might become gradually inefficient at too high latency • can only be checked realistically with data • +12 BX on top of present (production) latency is theoretical maximum • at higher latency inefficiency must occur at some level • the test in June showed that +12 BX is safe! • took a full fill with this latency, data are OK • rolled back for production afterwards • we will certainly use this budget for the L1 Trigger upgrade
L1 and BPTX • present CMS configuration relies on BPTX beam monitor being fully available all the time • have to tell collision bunches from gaps between: • for suppressing HF prefiring (see below) • for calculating the trigger dead time • for special checks (e.g., beamgas events in unpaired bunches) • this dependence is dangerous • as shown by a few (rare) hickups • other option: just use filling scheme published by LHC • also this is not always failsafe • ATLAS relied on it and lost some data
BPTX : coping with the dependence • ideal: take LHC record and cross-check with BPTX before stable beams • cannot change after ramp • implementation in progress • in the meantime: have created special configuration key for running without BPTX • not ideal for data but reduced dependence • early warning system in case of BPTX problems • DQM plots developed to warn about problems already during LHC ramp • what about long-term maintenance of BRM / BPTX ? • small group • importance tends to be understimated by funding agencies
HF Prefiring • prefiring of HF from particles hitting the photomultipliers directly • instead of creating scintillation light in the detector, as a well-behaved particle should • suppress now by vetoing bunch crossings before a collision crossing • this is why we need BPTX most badly! • “PreBPTX veto” • impossible in 25-ns running • for upcoming 25-ns test: will probably not use jet triggers • HF will replace all photomultipliers during LS1 • hope that this will fix the problem • if not: remove HF from single-object triggers
forward physics: collaboration with TOTEM • CMS and TOTEM are two separate experiments with separate DAQ • might change over LS1 • but their triggers cooperate • CMS receives several bits from TOTEM over LVDS signals • some “Technical Trigger” bits, some used in Algorithms • TOTEM receives from CMS the L1A and some BGOs • via an unused “APVE output” • including BC0 (“bunch crossing zero”) and OC0 (“orbit counter reset”) • can merge CMS and TOTEM data off line • several successful low-lumi runs taken jointly • CMS will use TOTEM’s “T2” for minimum-bias trigger in upcoming p-Pb test run
where is TOTEM? Roman Pots 9.5 < η < 13 140 m, 220 m
BCM1F for beam-gas veto? • last year’s beam gas problems prompted studies to reduce effect at Level-1 by using the “BCM1F” detector • fight beamgas-induced high trigger rate by veto signal • such events may be worse than normal events because of high Pixel occupancy • Previously Known as PKAMs • other idea: use HF asymmetry • beam gas much less of a problem in 2012 • hard to study the veto performance • but also less important! • no conclusion yet if this approach will (have to) be used
give me more ... backup slides
physics triggers L1A @ BX=a BC0 L1A @ BX=a+m L1A @ BX=a+m+n
calibration trigger BC0 calib @ BX=3490+m+n TEST ENABLE @ same BX as before calib @ BX=3490+m calib @ BX=3490