This article will discuss the status and the performances of the second-level muon trigger of the ATLAS experiment at the LHC; to perform these studies, data from the first collisions at √s = 7TeV have been used

Artoni, G.

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DOI 10.1393/ncc/i2011-10996-4Communications: SIF Congress 2010IL NUOVO CIMENTO Vol. 34 C, N. 5 Settembre-Ottobre 2011Performance study on the second-level muon triggerof the ATLAS experiment at the LHCG. Artoni“Sapienza” Universita` di Roma - Rome, ItalyINFN, Sezione di Roma 1 - Rome, Italy(ricevuto il 30 Dicembre 2010; revisionato il 21 Febbraio 2011; approvato il 21 Febbraio 2011;pubblicato online il 5 Ottobre 2011)Summary. — This article will discuss the status and the performances of thesecond-level muon trigger of the ATLAS experiment at the LHC; to perform thesestudies, data from the first collisions at√s = 7TeV have been used.PACS 29.20.-c – Accelerators.PACS 29.85.Ca – Data acquisition and sorting.PACS 29.90.+r – Other topics in elementary-particle and nuclear physics experi-mental methods and instrumentation.1. – Brief introduction to the ATLAS experimentATLAS is one of the four major experiments working at the LHC and, in particular,it is the biggest among them: 44m in length and 25m in height. It is a general purposeexperiment and this means that it has to look for not just the Higgs boson missingfrom the Standard Model, but also for new particles in a range of energy that has neverbeen explored so far. In order to accomplish this task, the detector has been designedin a way that many signatures are possibly reconstructed, thus allowing for variouspossible discoveries. The sub-detectors involved in the muon reconstruction chain arethe following:– Inner Detector - Around the beam pipe and with a radius of about 1m we havethe tracking system, divided into three different sub-detectors which use varioustechnologies. They are the Pixel Detector, the SemiConductor Tracker and theTransition Radiation Tracker. These detectors are able, thanks to the presenceof a 2T solenoidal magnetic field, to reconstruct all charged-particles tracks with|η| < 2.4(1).(1) In ATLAS it is defined a Cartesian coordinate system, having the z-axis along the beamc© Societa` Italiana di Fisica 1920 G. ARTONI– Calorimeters - Immediately outside the InnerDetector there are the calorimeters ofthe experiment, the Electromagnetic Calorimeter and the Hadronic Calorimeterrespectively. They have been designed to detect many physical signatures, such asphotons, electrons and jets. Muons will also be seen by the calorimeters, but beingminimum ionizing particles they will lose only about 2GeV(2) of their energy andthey will not be stopped in the detector.– Muon Spectrometer - This is the farthest sub-detector in ATLAS, as muons havethe peculiarity of being the only particle that passes through the whole detec-tor. The Muon Spectrometer has been designed to reconstruct all these tracks us-ing a toroidal magnetic field and different technologies have been used for differentregions of the detector. In the region where |η| < 1.05, which is called the barrel re-gion, we have Resistive Plate Chambers (RPC ) to provide a fast identification of themuon and Monitored Drift Tubes (MDT ) for the full reconstruction; the same hap-pens in the endcap region where we have Thin Gap Chambers (TGC ) for the triggersignals and MDT s plus Cathode Strip Chambers (CSC ) for the reconstruction.The second-level muon trigger will use all these sub-detectors to perform a selection;further details on the experiment can be found here [1].2. – The ATLAS muon triggerThe ATLAS muon system is the biggest sub-detector of the whole experiment and itis capable of reconstructing muons in standalone mode or using the combination withthe Inner Detector measurement. As all the trigger systems in ATLAS it is divided intothree different levels, that go under the simple names of Level 1 (L1), Level 2 (L2) andEvent Filter (EF). Here we will briefly explain how these three different levels work:L1 - The first level has been designed to reduce the trigger rate from about 1GHz to200 kHz. To accomplish this it uses only RPCs in the barrel and only TGCs inthe endcap, since the total processing time available for this level is about 2.5μs;this means that at the L1 the trigger is completely hardware. The task of the firstlevel is to identify all those events in which there is at least one muon and thento pass the information about the region in which this muon was found (Region ofInterest, RoI) to the L2.L2 - The second level receives the RoI from the first level and uses software-based al-gorithms to reduce the trigger rate from 200 kHz to about 1 kHz. In order toaccomplish this it performs a simplified reconstruction of the muon track, usingalso precision data from the MDTs, just in the RoI flagged by the L1; this is neces-sary if we do not want this level to exceed the total processing time available. Thefinal reconstructed muon is then passed to the last trigger level, the EF.direction, the x-axis pointing towards the center of the accelerating ring and the y-axis pointingin the upper direction. These coordinates do not provide a good system for physics analysis at ahadronic collider and thus we tend to use the z coordinate, the φ coordinate which is the anglein the x-y plane and the θ coordinate, which is the angle in the y-z plane. Moreover, θ does notrepresent a good choice because it is not a Lorentz invariant with respect to boosts along thez-direction; this leads to the usage of another coordinate, η = − ln(tan(θ/2)).(2) In this article we will use the convention c = 1, so all momenta will be given in units ofGeV.PERFORMANCE STUDY ON THE SECOND-LEVEL MUON TRIGGER OF THE ATLAS ETC. 21EF - The EF has to reduce the rate to about 100Hz, which is what we can then writeon disk and save for further analysis; since there are about 2 s for processing, it isfinally possible to reconstruct the event in the whole detector, starting from theRoI identified and refined by the L1 and the L2.3. – The ATLAS second-level muon triggerThe second-level muon trigger is a chain of different algorithms, each one with aparticular aim in the reconstruction. The first algorithm in the chain is μFast, thenthere is μComb and finally μIso; the total processing time available for these algorithmsis about 10ms and this forces us to simplify as much as possible all the operationsperformed by these algorithms.3.1. μFast . – μFast, as the name suggests, is the algorithm dedicated to a fast recon-struction of the muon track within the muon spectrometer. It uses all the informationcoming from the L1 and selects hits from the MDTs performing a pattern recognition;after this it uses the MDTs drift times to perform a fit of the track and then a mea-surement of the pT (3) is given by a Look-up-table. Before passing the RoI to the nextalgorithm in the chain, μFast applies a cut on the pT of the muon, thus rejecting muonsof transverse momentum lower than the configured threshold.3.2. μComb. – μComb has been developed to perform the combined reconstructionof the muon, using the track in the Muon Spectrometer and the tracks from the InnerDetector ; it starts from μFast track and then extrapolates this back to the primaryvertex, using a Look-up-table. After this it looks for the track in the Inner Detector thatbest matches in ΔR the back-extrapolated track. If a suitable track is found, μCombcombines the parameters of the two tracks to give the new pT , η and φ of the combinedtrack. At last, a cut on this combined pT is applied.3.3. μIso. – The last algorithm of the chain is μIso, that has been designed to rejectmuons coming from jets or from semi-leptonic decays of kaons and pions. In order toaccomplish this, two different ways of functioning for the algorithm have been prepared,one relying on the Inner Detector, and the other one on the Electromagnetic and HadronicCalorimeters.Track-based Algorithm - This configuration of the algorithm uses the Inner Detectortracks within the RoI to establish if the track is isolated or not. In particular itcalculates this quantity:(1) Iso = ΣΔR<0.2pT − pμT (4),where pT is measured in GeV. Of course isolated muons should have Iso  0 anda cut can be applied on this variable to preserve only muons with Iso under aparticular threshold.(3) pT =pp2x + p2y is the projection of the muon momentum on the x-y plane. It is very usefulbecause we can apply conservation laws only in the x-y plane, given the fact that we cannotknow the parton momentum along the z-direction.(4) ΔR =pΔη2 + Δφ2 is used to measure the distance between two tracks in the detector.22 G. ARTONICalorimeter-based Algorithm - Another way of determining if the muon is isolated is tolook at the calorimeter deposits around the muon track. The algorithm defines twodifferent cones, one called Inner Cone and containing only those cells in which themuon has passed, and another one called Outer Cone, around the first one. Forthe Outer Cone is then evaluated this quantity:(2) Iso = ΣOuterConecells ET − ΣInnerConecells ET (5)Looking at this variable it should be straightforward that isolated muons haveIso  0; we can then apply a cut to reject all muons having Iso greater than theconfigured threshold.4. – Second-level muon trigger performancesIn this section we will go through the performances of the various algorithms, mea-sured on the first 94 nb−1 data collected by the ATLAS experiment in 2010.4.1. Muon pT resolution. – As already pointed out in sect. 3, μFast and μComb havebeen designed to reconstruct the track parameters of the muon, in the Muon Spectrometeronly and combined with the Inner Detector, respectively, and then a hypothesis cut isapplied on the muon transverse momentum. Thus a good resolution on this quantity isnecessary for the trigger to work properly. To evaluate the pT resolution we decided touse this quantity:(3) rpT =1/pofflineT − 1/palgoT1/pofflineT= 1− pofflineTpalgoT,where palgoT can be alternatively pμFastT or pμCombT and pofflineT is the pT of the offlinetrack, which is the best reconstruction available in the ATLAS experiment. It is easy tonotice that, if everything is working as it should, rpT will be centered at zero, meaningthat the energy scales of the offline algorithm and of the L2 algorithm are the same, andthe width of the distribution should give us the resolution of the algorithm. It is possibleto see this for μFast in fig. 1 and for μComb in fig. 2, for different ranges in η and in pT .For what concerns μFast, we notice that there is some unbalance between offline andμFast pT , but this is well modeled by the Monte Carlo(6), showing that the performanceof the μFast algorithm are exactly as expected. In the barrel region, for pT > 6GeV,we can see that the width of the distribution from data is wider than that from MonteCarlo, indicating that the resolution of the algorithm is worse than expected. This smalleffect is due to the fact that the L2 muon trigger has never been tested on collision dataat√s = 7TeV before and thus some discrepancies in terms of resolution are expected.Moving on to μComb, we can see that there is a very good agreement between data andMonte Carlo, indicating that the algorithm is working exactly as expected. In the last(5) As for pT , ET =pE2x + E2y .(6) The Monte Carlo sample used in this article is composed by minimum bias events, i.e. eventsfor which minimal requirements have been imposed; for example one of these requirements wasthat a collision took place in the ATLAS detector and another one was that there was at leastone track with pT > 500MeV.PERFORMANCE STUDY ON THE SECOND-LEVEL MUON TRIGGER OF THE ATLAS ETC. 23pTr-0.3 -0.2 -0.1 0 0.1 0.2 0.3Entry/Total Entry00.020.040.060.080.10.120.140.16Entry/Total EntryATLAS Preliminary=7TeV, Data 2010s|<1.05η|<6GeVT4GeV<pDataMC(a)pTr-0.3 -0.2 -0.1 0 0.1 0.2 0.3Entry/Total Entry00.020.040.060.080.10.120.140.160.180.2Entry/Total EntryATLAS Preliminary=7TeV, Data 2010s|<1.05η|<8GeVT6GeV<pDataMC(b)pTr-0.3 -0.2 -0.1 0 0.1 0.2 0.3Entry/Total Entry00.050.10.150.20.25Entry/Total EntryATLAS Preliminary=7TeV, Data 2010s|<1.05η|<20GeVT8GeV<pDataMC(c)pTr-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8Entry/Total Entry00.010.020.030.040.050.060.070.080.09Entry/Total EntryATLAS Preliminary=7TeV, Data 2010s|>1.05η|DataMC<6GeVT4GeV<p(d)pTr-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8Entry/Total Entry00.020.040.060.080.1Entry/Total EntryATLAS Preliminary=7TeV, Data 2010s|>1.05η|<8GeVT6GeV<pDataMC(e)pTr-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8Entry/Total Entry00.010.020.030.040.050.060.070.08Entry/Total EntryATLAS Preliminary=7TeV, Data 2010s|>1.05η|<20GeVT8GeV<pDataMC(f)Fig. 1. – μFast pT resolution, indicated by the variable rpT (see eq. (3)), for different regionsand ranges in pT : for the barrel region in (a), (b) and (c) while for the endcap region in (d),(e) and (f). Also (a) and (d) have been made for 4GeV < pT < 6GeV, (b) and (e) for6GeV < pT < 8GeV and (c) and (f) for 8GeV < pT < 20GeV.plot on fig. 2 it is possible to see that the resolution from data is slightly worse than thatfrom Monte Carlo and that the width of rpT grows with the muon transverse momentum,as it should be.4.2. Efficiency . – The purpose for which μFast and μComb have been designed is toreject muons below preconfigured thresholds in pT and preserve all muons above these24 G. ARTONIT/pT pΔ-0.1 -0.05 0 0.05 0.1Events0100200300400500600700800 DataMCATLAS Preliminary=7TeV, Data 2010s<4.0T2.0<p(a)T/pT pΔ-0.1 -0.05 0 0.05 0.1Events0100200300400500600700800900DataMCATLAS Preliminary=7TeV, Data 2010s<6.0T4.0<p(b)T/pT pΔ-0.1 -0.05 0 0.05 0.1Events020406080100120140160180200DataMCATLAS Preliminary=7TeV, Data 2010s<8.0T6.0<p(c)T/pT pΔ-0.1 -0.05 0 0.05 0.1Events020406080100 DataMCATLAS Preliminary=7TeV, Data 2010s<20T8.0<p(d) [GeV]TOffline CB p2 4 6 8 10 12 14 16 18 20 resolutionTp00.0020.0040.0060.0080.010.0120.0140.0160.0180.020.0220.024DataMCATLAS Preliminary=7TeV, Data 2010s(e)Fig. 2. – μComb pT resolution, indicated by the variable rpT (see eq. (3)), for different rangesin pT : (a) has been made for 2GeV < pT < 4GeV, (b) for 4GeV < pT < 6GeV, (c) for6GeV < pT < 8GeV and (d) for 8GeV < pT < 20GeV. The last plot, (e), shows the widths ofthe four previous distributions as a function of the offline pT .thresholds, in order to reduce the trigger rate without losing interesting high-pT events.The efficiency of the pT cut is then crucial in order to understand if the algorithm isworking in the proper way and can then be used to reject events online, allowing theexperiment to bear greater luminosities. The efficiency is evaluated as follows:(4)  =Number of events passing the pT cutTotal events coming from the previous levelPERFORMANCE STUDY ON THE SECOND-LEVEL MUON TRIGGER OF THE ATLAS ETC. 25 [GeV]TOffline SA p0 5 10 15 20 25 30 35 40L2 Efficiency00.20.40.60.81ATLAS Preliminary=7 TeV, Data 2010s | < 1.05η | DATAMC(a) [GeV]TOffline SA p0 5 10 15 20 25 30 35 40L2 Efficiency00.20.40.60.81ATLAS Preliminary=7 TeV, Data 2010s | > 1.05η | DATAMC(b)Fig. 3. – μFast efficiency curves as a function of pT , for barrel in (a) and endcap in (b) with thethreshold set at 4GeV, with respect to the previous level, L1.and it is easy to notice that we should have   0 below the threshold and   1 abovethreshold. We can see in fig. 3 the efficiency for μFast and in fig. 4 for μComb; we showhere the turn-on curve as a function of the offline pT in which there is a very goodagreement between data and Monte Carlo, showing that the L2 is perfectly capable ofrejecting events and is then reliable. We should note that the strange behavior in fig. 4(b)is due to the lack of events in that pT region, thus producing effects that disappear withthe increase of statistics.4.3. μIso commissioning . – So far we have showed only the performance of the firsttwo algorithm in the chain of the second-level muon trigger. Here we show the firstresults on μIso, whose commissioning will be done in a second phase since its rejectionis needed only at high luminosities (starting from 1032 cm−2 s−1). In fig. 5(a) we can seea very good agreement between data and Monte Carlo for the track-based isolation. Itis possible to observe that the peak at zero represents those muons that are isolated andthus do not contain any other track in a cone of ΔR = 0.2 around them. This criteriumto establish if the muon is isolated or not will then be used in the experiment as soon asthe trigger rate requires it, since it appears to be perfectly working. For what concerns [GeV]Tp0 5 10 15 20 25 30 35 40Efficiency00.20.40.60.81DataMCATLAS Preliminary=7TeV, Data 2010s(a) [GeV]Tp0 5 10 15 20 25 30 35 40Efficiency00.20.40.60.81DataMCATLAS Preliminary=7TeV, Data 2010s(b)Fig. 4. – μComb efficiency curves as a function of pT for two different thresholds, 4GeV in (a)and 6GeV in (b) with respect to the previous algorithm, μFast.26 G. ARTONI[GeV]Tμ-pT pΣ0 5 10 15 20 25 30 35 40Events/GeV110210DataMCATLAS Preliminary=7TeV, Data 2010s(a) (Tile) [GeV]outer coneTE0 2 4 6 8 10 12 14Events10210DataMCATLAS Preliminary=7TeV, Data 2010s(b)Fig. 5. – μIso reconstructed variables, Iso = ΣΔR<0.2pT−pμT in (a) for the track-based algorithmand ΣOuterConecells ET in (b) for the calorimetric-based algorithm.the calorimetric-based isolation, we can see in fig. 5(b) a plot showing the energy ofall the cells in the Outer Cone of the Hadronic Calorimeter. In this case it appears tobe a good agreement with Monte Carlo at low energies and some discrepancies at highenergies, probably due to the lack of statistics.4.4. Trigger rates. – The last part of this article deals with the trigger rate issue,already explained in sect. 2. In fig. 6(a) we can see different chains of triggers using dif-ferent thresholds and their rate as a function of the instantaneous luminosity measuredby ATLAS. We can notice that the rate exponentially grows with the instantaneous lu-minosity and this has allowed experts to predict the trigger rate for higher luminosities.For what concerns fig. 6(b), we show the rate reduction from the L1 (which is the ref-]-1s-2cm29Luminosity [10-110 1L2 Muon Trigger Rate [Hz]-310-210-1101ATLAS Preliminary= 7 TeV, Data 2010sL2_mu4L2_mu6L2_mu10L2_2mu4(a)Trigger LevelL1 L2 EFTrigger Rate relative to L1-210-1101mu4mu6mu102mu4ATLAS Preliminary= 7 TeV, Data 2010s(b)Fig. 6. – Trigger rate behavior for the first 94 nb−1 of data collected by ATLAS. In (a) it ispossible to see the different trigger rates for various trigger chains: the number at the end ofeach chain indicates the pT cut that the chain performs. In (b), instead, we can see the ratereduction induced by the L2 in the muon trigger.PERFORMANCE STUDY ON THE SECOND-LEVEL MUON TRIGGER OF THE ATLAS ETC. 27erence) to the EF. We need to point out that the major part of the reduction has beendone by the L2 trigger, as it is expected, and this shows once again the importance ofthis sub-system within the whole muon trigger system.5. – ConclusionsIn this article we have shown the first results obtained using collision data at√s = 7TeV for the second-level muon trigger of the ATLAS experiment. We haveexplained how the whole trigger chain works and shown the measured performances ofall the algorithms, in terms of resolutions (subsect. 4.1), efficiencies (subsect. 4.2) andreconstructed variables (subsect. 4.3). We have pointed out the good agreement betweendata and Monte Carlo and thus the very good behavior of all the algorithms, indicatingthat they are ready to be used online for rejecting events.REFERENCES[1] ATLAS Collaboration, The ATLAS Experiment at the CERN Large Hadron Collider,JINST, 3 S08003 (2008).

Performance study on the second-level muon trigger of the ATLAS experiment at the LHC

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Performance study on the second-level muon trigger of the ATLAS experiment at the LHC

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