We present a study of the CMS trigger system in heavy-ion collisions.
Concentrating on two physics channels, dimuons from decays of quarkonia and
single jets, we evaluate a possible trigger strategy for Pb+Pb running that
relies on event selection solely in the High-Level Trigger (HLT). The study is
based on measurements of the timing performance of the offline algorithms and
event-size distributions using full simulations. Using a trigger simulation
chain, we compare the physics reach for the jet and dimuon channels using
online selection in the HLT to minimum bias running. The results demonstrate
the crucial role the HLT will play for CMS heavy-ion physics.Comment: 4 pages, 4 fugures, contribution to QM'06 conferenc

Betts R

CMS Collaboration

G Roland

the CMS Collaboration

English

arXiv

We present a study of the CMS trigger system in heavy-ion collisions. Concentrating on two physics channels, dimuons from decays of quarkonia and single jets, we evaluate a possible trigger strategy for Pb+Pb running that relies on event selection solely in the High-Level Trigger (HLT). The study is based on measurements of the timing performance of the offline algorithms and event-size distributions using full simulations. Using a trigger simulation chain, we compare the physics reach for the jet and dimuon channels using online selection in the HLT to minimum bias running. The results demonstrate the crucial role the HLT will play for CMS heavy-ion physics.We present a study of the CMS trigger system in heavy-ion collisions. Concentrating on two physics channels, dimuons from decays of quarkonia and single jets, we evaluate a possible trigger strategy for Pb+Pb running that relies on event selection solely in the High-Level Trigger (HLT). The study is based on measurements of the timing performance of the offline algorithms and event-size distributions using full simulations. Using a trigger simulation chain, we compare the physics reach for the jet and dimuon channels using online selection in the HLT to minimum bias running. The results demonstrate the crucial role the HLT will play for CMS heavy-ion physics.We present a study of the CMS trigger system in heavy-ion collisions. Concentrating on two physics channels, dimuons from decays of quarkonia and single jets, we evaluate a possible trigger strategy for Pb+Pb running that relies on event selection solely in the High-Level Trigger (HLT). The study is based on measurements of the timing performance of the offline algorithms and event-size distributions using full simulations. Using a trigger simulation chain, we compare the physics reach for the jet and dimuon channels using online selection in the HLT to minimum bias running. The results demonstrate the crucial role the HLT will play for CMS heavy-ion physics.We present a study of the CMS trigger system in heavy-ion collisions. Concentrating on two physics channels, dimuons from decays of quarkonia and single jets, we evaluate a possible trigger strategy for Pb+Pb running that relies on event selection solely in the High-Level Trigger (HLT). The study is based on measurements of the timing performance of the offline algorithms and event-size distributions using full simulations. Using a trigger simulation chain, we compare the physics reach for the jet and dimuon channels using online selection in the HLT to minimum bias running. The results demonstrate the crucial role the HLT will play for CMS heavy-ion physics.We present a study of the CMS trigger system in heavy-ion collisions. Concentrating on two physics channels, dimuons from decays of quarkonia and single jets, we evaluate a possible trigger strategy for Pb+Pb running that relies on event selection solely in the High-Level Trigger (HLT). The study is based on measurements of the timing performance of the offline algorithms and event-size distributions using full simulations. Using a trigger simulation chain, we compare the physics reach for the jet and dimuon channels using online selection in the HLT to minimum bias running. The results demonstrate the crucial role the HLT will play for CMS heavy-ion physics.We present a study of the CMS trigger system in heavy-ion collisions. Concentrating on two physics channels, dimuons from decays of quarkonia and single jets, we evaluate a possible trigger strategy for Pb+Pb running that relies on event selection solely in the High-Level Trigger (HLT). The study is based on measurements of the timing performance of the offline algorithms and event-size distributions using full simulations. Using a trigger simulation chain, we compare the physics reach for the jet and dimuon channels using online selection in the HLT to minimum bias running. The results demonstrate the crucial role the HLT will play for CMS heavy-ion physics

Roland, G.

CERN Document Server

arXiv:nucl-ex/0702041v1   21 Feb 2007Triggering on hard probes in heavy ion collisionswith CMSG Roland for the CMS CollaborationMassachusetts Institute of Technology, Cambridge, MA 02139, USAE-mail: rolandg@mit.eduAbstract. We present a study of the CMS trigger system in heavy-ion collisions.Concentrating on two physics channels, dimuons from decays of quarkonia and singlejets, we evaluate a possible trigger strategy for Pb+Pb running that relies on eventselection solely in the High-Level Trigger (HLT). The study is based on measurementsof the timing performance of the oﬄine algorithms and event-size distributions usingfull simulations. Using a trigger simulation chain, we compare the physics reach for thejet and dimuon channels using online selection in the HLT to minimum bias running.The results demonstrate the crucial role the HLT will play for CMS heavy-ion physics.1. Trigger strategy in CMS Pb+Pb runningThe key component in exploiting the CMS capabilities in heavy-ion collisions is thetrigger system, which is crucial in accessing rare probes such as high ET jets and photons,Z0 bosons, D and B mesons, and high-mass dileptons from the decay of quarkonia. Theunique CMS trigger architecture only employs two trigger levels: The Level-1 trigger isimplemented using custom electronics and inspects events at the full bunch crossing rate.All further online selection is performed in the High-Level Trigger (HLT) using a largecluster of commodity workstations (the “filter farm”) running “oﬄine” reconstructionalgorithms on fully assembled event information. The trigger system was designed todeal with the unprecedented luminosities in LHC p+p running, yielding an expectedevent rate of 40 MHz, with 25 superimposed p+p collisions for each event. Out of the40 MHz p+p event rate, a 100 kHz data stream will be selected by the Level-1 triggerfor further processing in the HLT. The HLT will reduce the 100 kHz input stream to150 Hz of events written to permanent storage.The Pb+Pb design luminosity L = 1027 cm−2s−1 at the beginning of a store issmaller than the p+p design luminosity by 7 orders of magnitude. The correspondinginitial Pb+Pb collision rate is ≈8 kHz, and the average collision rate over the duration ofa store will be ≈3 kHz. Therefore, even the maximal event rate for Pb+Pb collisions ismuch smaller than the 100 kHz input rate for the HLT in p+p collisions after the Level-1selection. This allows a trigger strategy in Pb+Pb running that can be summarized asfollows: Every Pb+Pb collision identified by the Level-1 trigger will be sent to the HLTTriggering on hard probes in heavy ion collisions with CMS 2 [GeV/c]Tmin [GeV] or part. pTminjet E0 50 100 150 200 250 300 350) [1/s]Tmin>pTN(p-810-710-610-510-410-310-210-110110210310410DAQ bandwidth/event size-/h+hjetpromptγψJ/Υ+jet*γ+jet0Z = 7800 mbtotPbPbσ-1L = 0.40 (mb s) = 72 mbNNσ = 400minbiascollN [GeV/c]Tmin [GeV] or part. pTminjet E1 10 210) [1/s]Tmin>pTN(p-810-710-610-510-410-310-210-110110210310410jetψJ/ΥHLTMin.bias = 7800 mbtotPbPbσ-1L = 0.40 (mb s) = 72 mbNNσ = 400minbiascollN v0.09Figure 1. Left: Expected production rates in minimum bias Pb+Pb collisions at√sNN= 5.5 TeV, assuming design luminosity. Right: Expected rates to tape for jets,J/ψ and Υ channels for minimum bias (dashed lines) and HLT triggered data sets(solid lines).filter farm. At the HLT, the full event information will be available for each event. Allrejection of Pb+Pb collisions will be based on the outcome of HLT trigger algorithmsthat are identical to the corresponding oﬄine algorithms or optimized versions of theoﬄine algorithms. Therefore, algorithms like the oﬄine jet finder will be run on eachPb+Pb event in the CMS interaction region, optimizing the CMS physics reach. Thisstrategy relies on the fact that the HLT in its final configuration will provide sufficientinput bandwidth to accept all collision events and sufficient computing power to runfull oﬄine algorithms on all events.The event size and computing time constraints were evaluated using full GEANT4 based simulations of Pb+Pb events. For events with a charged hadron multiplicity ofdN/dη ≈ 3000 for central events, the event size was found to be approximately linearin the charged hadron multiplicity, ranging from 330 kByte/event for a b = 12 fmsample to 8.5 MByte/event for a b = 0 fm sample. Averaging over impact parameterand adjusting for additional noise, backgrounds and diagnostic information, we obtainan event size of 2.5 MByte per minimum bias event for running at design luminosity.Including all uncertainties, we expect that the bandwidth of 225 MByte/sec will allowa rate of Pb+Pb events to mass storage between 10 and 100 Hz. A large part of thisuncertainty will only be resolved once the first LHC data are taken, underscoring theneed for a flexible high-level trigger scheme.Triggering on hard probes in heavy ion collisions with CMS 3The HLT online computing farm is expected to consist of about 1500 servers. Theprojected CPU budget per event, in units of todays 1.8 GHz Opteron CPUs on whichour timing measurements were performed, will be ≈ 1.5 s at the beginning of each store(8 kHz collision rate), and ≈ 4 s averaged over the duration of the store (3 kHz collisionrate).We measured the timing of three key algorithms in the CMS ORCA framework:the jet finding algorithm, the stand-alone muon finder using muon chamber informationand the full muon finder including the silicon tracker information. Averaging over theGlauber impact parameter distribution, the execution time of the modified iterativecone jet finding algorithm, including background subtraction, is 〈t〉 = 250 ms. Morethan 50% of the execution time was spent in unpacking the calorimeter data. Thestand-alone muon algorithm has an average execution time of 〈t〉 = 80± 20 ms, usingmuon candidates from the Level-1 trigger. Both algorithms therefore fit comfortablyinto the CPU budget per event discussed above.The full muon algorithm extends the tracks found in the muon system to the silicontracker and provides a significant improvement in momentum resolution and backgroundrejection. This is particularly important for low pT dimuons, which are expected to takeup the largest fraction of the output bandwidth to tape. This algorithm is called forless than 2% of all events. The current L3 execution time corresponds to about 10±3 sper minimum bias event. Work on porting the the present oﬄine algorithms to a newframework, CMSSW, and optimizing or significantly modifying the present algorithmsfor use in the HLT event selection is ongoing. [GeV/c]Tp0 2 4 6 8 10 12 14 16 18 20 22(Y)σ)/,(Yσ00.10.20.30.40.50.60.70.8parton gas modelsHLT, one run year, dN/dy=2500mini-jet models [GeV/c]Tp0 50 100 150 200 250AAR00.20.40.60.811.21.4 0-10% central, jet triggeredFigure 2. Left: Υ’ over Υ ratio vs pT . Statistics correspond to 106 s of data taking(one nominal year of LHC heavy ion running). Shown in comparison to the statisticaluncertainty are calculations in different theoretical scenarios. Right: The nuclearmodification factor RAA as a function of pT for charged particles, for minimum biasdata (left panel) and for data triggered on high-ET jets (right panel), for 106 s of datataking.Triggering on hard probes in heavy ion collisions with CMS 42. HLT simulation resultsUsing results from event size and timing studies, a simulation chain was set upto translate the production cross-sections into rates to tape. The simulations usedparametrizations of the acceptance, efficiency and background rates of the oﬄine jetfinding and muon finding algorithms which are expected to form the basis of the HLTalgorithms. Details of the model calculations and reconstruction algorithms can befound in [3].In Figure 1 (right) we show the rates of signal events to tape for minimum biasrunning (no event selection in HLT) in comparison to those for event selection using theHLT. The rates were calculated for design average luminosity, using a trigger table thatdevoted about 30% of the output bandwidth to dimuon chanels and about 35% to jetchannels.Using the HLT, a gain in statistics of more than an order of magnitude is achievedfor jets at large ET and for dimuons. Correspondingly, the usable range in ET (pT ) forthe jet and dimuon measurements is extended by more than than factor of 2 and 3,respectively. Note that, for this comparison, the HLT rate for each process was onlycounted in the corresponding trigger stream.Two key examples of the physics benefit of the HLT for quarkonium and jet relatedmeasurements are shown below. The left plot of Fig. 2 shows the ratio of Υ’ to Υ yieldsas a function of transverse momentum. The projected statistical resolution is comparedto four model calculations. This measurement, which relies on the added statisticsprovided by the HLT selection, allows a clear distinction of the different scenarios, andmay therefore serve as a sensitive probe of the initial QCD medium.In the right plot of Fig. 2 we show the nuclear modification factor RAA for eventsselected by an HLT trigger on high ET jets (right). Compared to a minimum bias dataset, the triggered sample extends the useful range in pT by more than a factor of 2.5,to more than 200 GeV/c. Predictions for RAA in Pb+Pb collisions at the LHC havebeen made for several models of the parton energy loss in the QCD medium. Thepredictions differ most markedly in the high pT region, which can only be measuredwith high precision in the jet-triggered event sample. In summary, the flexibility ofthe HLT system will allow us to allocate bandwidth to trigger channels differentiallyas a function of rapidity, y, and pT of the trigger object and as a function of collisioncentrality, using full oﬄine algorithms. This sophisticated triggering system will becritical in maximizing the overall physics reach of CMS in heavy ion running.[1] CMS Collaboration, “The TriDAS Project Technical Design Report, Volume 2: Data Acquisitionand High-Level Trigger,”, CERN/LHCC2002-26 (2002). CMS-TDR-006-add-2.[2] CMS Collaboration, “CMS physics : Technical Design Report v.2 : Physics performance”,CERN/LHCC2002-021 (2002). CMS-TDR-008-2.[3] see Betts R, for the CMS Collaboration, these proceedings, and CMS Collaboration, “Heavy IonPhysics Technical Design Report” , under preperation.

Triggering on Hard Probes in Heavy Ion Collisions with CMS

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Triggering on hard probes in heavy-ion collisions with CMS

Triggering on hard probes in heavy ion collisions with CMS

Triggering on hard probes in heavy ion collisions with CMS

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