The large ttbar production cross-section at the LHC suggests the use of top
quark decays to calibrate several critical parts of the detectors, such as the
trigger system, the jet energy scale and b-tagging.Comment: 6 pages, 5 figures. Talk given at `V Workshop Italiano sulla Fisica
  pp a LHC', Perugia, Italy, 30 January - 2 February 200

Acharya, B. S.

Cavallari, F.

Corcella, G.

Di Sipio, R.

Petrucciani, G.

English

arXiv

The large $t\bar{t}$ production cross-section at the LHC suggests the use of top quark decays to calibrate several critical parts of the detectors, such as the trigger system, the jet energy scale and b-tagging. The ATLAS and CMS experiments will both take advantage of these facts and a brief review of their capabilities is given.The large ttbar production cross-section at the LHC suggests the use of top quark decays to calibrate several critical parts of the detectors, such as the trigger system, the jet energy scale and b-tagging

Acharya, Bobby Samir

CERN Document Server

Available on CMS information server CMS CR-2008/014The Compact Muon Solenoid ExperimentMailing address: CMS CERN, CH-1211 GENEVA 23, SwitzerlandConference Report28 April 2008Commissioning ATLAS and CMS with top quarksBobby Samir Acharya, Francesca Cavallari, Gennaro Corcella, Riccardo Di Sipio, Giovanni Petrucciani on thebehalf of the ATLAS and CMS collaborationsAbstractThe large tt¯ production cross-section at the LHC suggests the use of top quark decays to calibrateseveral critical parts of the detectors, such as the trigger system, the jet energy scale and b-tagging.The ATLAS and CMS experiments will both take advantage of these facts and a brief review of theircapabilities is given.Presented at Quinto workshop italiano sulla fisica p-p ad LHC,30/01/2008 - 02/02/2008,Peugia,Italy1 IntroductionEvents in which top-quark pairs are produced will be extremely important at the LHC, as they will provide a uniqueenvironment to study physics within the Standard Model and beyond [1]. Final states in tt¯ events are classifiedin three categories, according to the W -decay mode in top decay t → bW : fully hadronic, semileptonic or fullyleptonic. Semileptonic tt¯ decays produce complex signatures within the detector, involving missing transverseenergy, charged leptons, light-particle jets and b-jets. Therefore, in order to study these events accurately at theLHC, the understanding of all the parts of the detectors is mandatory. In particular, the following should bemastered:• Trigger system;• Lepton and jet reconstruction;• Calculation of missing transverse energy;• b-tagging.Conversely, the top quark is an excellent instrument, thanks to the large tt¯ cross-section at the LHC, σ(tt¯) ∼830 pb, more than 100 times larger than at the Tevatron accelerator [2]. Semileptonic tt¯ events link all these itemstogether and can therefore be used to make what is commonly referred to as an in-situ calibration.2 TriggersAt the LHC collisions will happen with a frequency of up to 40 MHz, and this number has to be compared withthe capabilities of the ATLAS and CMS mass storage systems of about 200 Hz. So, the trigger system of ATLASand CMS has been designed to select one event out of 10 millions, when running at the highest design luminosity.The ATLAS and CMS trigger systems both have a hardware-based level 1 and a software-based high-level trigger[3,4]. Level 1 makes use of the muon detectors and calorimeters in order to identify particles, while higher levelsperform a more refined reconstruction. In order to choose interesting events and reduce the output rate, the twoexperiments designed their trigger systems in two different ways.ATLAS makes use of Regions of Interest (RoI), a technique that gives access to high-granularity information onlyfor the regions flagged as interesting by the Level 1 Trigger. The CMS High Level Trigger can access the fulldetector readout, but it performs only the minimal amount of reconstruction needed to determine if an event has tobe accepted or dropped. At the end of the process, both ATLAS and CMS trigger systems will write data with afrequency of about 100 Hz and a latency of few µs.In the first days of data-taking, close attention will be paid to the study of the single-lepton triggers. In fact, a largenumber of important processes involve the production of at least one isolated charged lepton, and leptonic decaysof top quarks are amongst these. Fig. 1 shows the efficiency of the level-1 single-lepton triggers in tt¯ events,calculated with respect to the offline reconstruction.Moreover, semileptonic events are triggered from jet triggers, too, giving the possibility to measure directly theefficiency of the leptonic triggers. Thus, the very large cross section of tt¯ events can be successfully exploited tocalibrate the triggers.For example, a sample of events can be collected according to the offline selection defined in Tab. 1. Then, asub-sample is extracted, containing only events that fired the single-lepton trigger. From this sub-sample, one caneasily calculate the fraction of tt¯ events that fired the jet trigger. This technique can be subsequently applied, e.g.,to several jet triggers for each lepton trigger, leading to a very good determination of combined trigger efficiencies[4].Top-quark production is also suitable to study other triggers, such as double-lepton (for full-leptonic decays), jetand missing-ET triggers. In fact, two leptons give a very clean signature for triggering, albeit limited in statisticsat the very beginning.With early data, the fully hadronic channel is extremely challenging triggerwise, due to the large QCD background.Reasonably, this channel will be studied accurately in a subsequent phase of the experiment.ATLAS and CMS will also estimate the single-lepton trigger efficiency as a function of its momentum from pro-cesses which do not involve top quarks, such as Z → ee / µµ. However, since the jet energy scale and underlying2event might be different between Z → ee and tt¯ processes, it is clearly preferable to calculate efficiencies fortt¯ events by using tt¯ events themselves.3 Jet Energy ScaleThe cone algorithm for jet reconstruction, with a cone radius R = 0.4 or 0.5, is commonly used both in ATLASand CMS, since it provides a good compromise between energy reconstruction and angular resolution [5].Due to a HCAL resolution lower than ATLAS, CMS found better results using the particle flow, a useful techniquewhen dealing with low-granularity calorimeters. Preliminary studies show an overall efficiency similar to that ofATLAS.The (mis)calibration of the Jet Energy Scale (JES) appears as an important source of systematic uncertainty onMW and Mt. The a priori knowledge of jet-energy calibration is about 10%. The goal of 1 GeV error on Mtrequires understanding the JES to 1%. The Jet Energy Scale can be evaluated using the method of pT -balanceapplied to Z/γ+jets events. Here, the well reconstructed Z/γ transverse momentum can be balanced against thejets in the events, allowing a pT -dependent jet calibration. These processes are also useful for the b-jet energyscale, when jets are tagged as b-jets.However, as stated before, it would be better to measure the JES for tt¯ events by means of tt¯ events themselves,at least for two main reasons:• tt¯ selection cuts can lead to JES different from that of Z/γ+jets;• the underlying event (UE) may be different for the two processes.To this end one could exploit the t → Wb → jjb decay chain, since it gives an identifiable W → jj sample(in-situ calibration). The jj invariant mass should of course yield the well-known W -boson mass.ATLAS will determine the JES by studying the reconstructed MW after the offline selection (as defined in Tab 2).Its impact, after varying the reconstructed jet energies by±1%, has been evaluated. Unfortunately, offline selectionintroduces a bias caused by the pT -cut which is important near threshold [5]. To handle this problem, fittingtechniques are applied to determine the jet-energy-scale factors as a function of the jet energy and pseudorapidityη.Starting from the same principles, CMS will calculate MW by combining the two jets. The light-quark jet energyis scaled by a global correction factor ∆C, chosen to fit the reconstructed W mass within the world average, asshown in Fig. 2. Studies show that the main sources of systematic uncertainty on ∆C are the pile-up and b-tagefficiency [6]. More refined techniques are under study, based on a kinematic fit of Mbjj in t → Wb → jjbdecays, so that one can also measure the b-JES.4 b-taggingIdentification of b-jets is crucial in many analyses at the LHC, such as the searches for the Higgs boson, supersym-metry and other New Physics scenarios. Thus, to calibrate b-tagging algorithms, one would like to isolate a sampleof b-jets as pure as possible.Again, the large tt¯ cross section offers the possibility of an in-situ calibration with several advantages, since almostevery tt¯ event contains two b-quarks. In fact, semileptonic tt¯ events are identifiable without b-tagging and hencegive a handle on b-tagging mechanisms [7]. With an integrated luminosity of 100/pb, several hundred events areexpected. To gain more statistics, di-jet events could be used but for b-tagging calibration. However the b-taggingefficiency, - like the JES - is sample and analysis dependent. For this reason a measurement of the efficiency fromtop events themselves is preferable.The default ATLAS b-tagger uses a likelihood algorithm weight w, constructed from the impact parameter and thesecondary-vertex taggers. Choosing a threshold on the weight translates into an efficiency to recognize correctlythe b-jets (b) and to reject the jets originated from the lighter quarks (Rl−jets= 1/l−jets). As shown in Fig. 3,w is large for b-jets and low for light-jets, proving itself as a good quantity to distinguish b-jets from light-jets.For example, setting the cut w > 6, an overall efficiency b = 63% and light-jet rejection Rl−jets= 250 can beachieved [7].3In a recent CMS study an attempt was made to evaluate the b-tagging efficiency directly from data. In this study, asimple algorithm was used, mainly based on track counting and track probability. One can associate to each trackinside the jet an impact parameter and a secondary vertex location. If these values are greater than their thresholds,the track is ‘counted’. Then, the jet is b-tagged if it contains more ‘counted’ tracks than a minimum. Tagging oneof the b-jets hardly allows to have a rather pure b-jets sample from the other top and evaluate the performance ofthe b-tagging. However, this study shows that with 1fb−1 one can reach an uncertainty on the b-tagging efficiencyof ∼5%.Other strategies, such as the soft lepton method, are also under study.Assuming that each selected event actually contains two b-jets, b can be measured from the data themselves,counting the number of tagged jets as b-jets. To take into account mistagged events and backgrounds, a morerefined likelihood function can be written, with b and Rl−jets as parameters.Overall, the main sources of systematic uncertainty are light-jet rejection, JES,W+jets background contamination,and the uncertainty on the measurement of the top mass.5 ConclusionsTop-quark events and, in particular, those with semileptonic decays, will be a poweful source of data for themeasurement of trigger efficiencies, jet energy scale and b-tagging performance. Both ATLAS and CMS aredeveloping methods and algorithms to capitalise on this opportunity.tt¯ events, especially semileptonic, allow one to trigger on both leptons and jets independently, thereby allowingthe possibility to measure trigger efficiencies.Moreover, the presence of W -bosons, light jets and b-jets in tt¯ events, allows one to measure the JES for bothlight jets and b-jets. The goal of measuring the top-quark mass with an uncertainty of 1 GeV requires a 1% erroron the JES, which can be achieved with at least 1/fb of data.Since almost every decaying top quark produces a b quark, tt¯ events supply a pure sample of b-jets, which couldbe used to calibrate the b-taggers. Much work is in progress to develop these techniques in preparation for the firstdata-taking.References[1] Beneke M. et al., Top quark physics, hep-ph/0003033.[2] Bonciani R., Catani S., Mangano M.L. and Nason P., , Nucl. Phys. B, 529 (1998) 529.[3] The CMS Collaboration, CMS High Level Trigger, CERN/LHCC 2007-021 (2007).[4] Collins N., Thomas J., Watson A. et al., Triggering top quark events in ATLAS, CSC Note T5 (2008).[5] The ATLAS Collaboration, Light jets in tt¯ events, ATLAS Note in preparation.[6] The CMS Collaboration, Measurement of jet energy scale corrections using top quark events, CMS PASTOP-07-004 (2008).[7] Bachacou H., Gorfine G., Harrington R., Hawkings R. and Sandhoff M., Flavour tagging calibration withtt¯ events in ATLAS.[8] The CMS Collaboration, CMS Physics Technical Design Report, Volume 1, section 12.2.8, CERN/LHCC2006-001 (2006). CMS TDR 8.1.Isolated e or µ with pT > 20 GeV≥ 4 jets with pT > 40 GeV2 b-jetsEmissT > 20 GeVTable 1: Offline selection cuts for semileptonic decays.4Isolated e or µ with pT > 20 GeV≥ 4 jets with pT > 40 GeV2 b-jetsEmissT > 20GeVMjjj ∼MtTable 2: Offline selection cuts for selecting a sample suitable for jet energy scale determination.Figure 1: ATLAS Level-1 efficiencies for one-lepton trigger menu e251 (left) and mu20i (right). Efficiencies are calculatedwith respect to the offline reconstruction.Figure 2: CMS will calculate JES by fitting the reconstructedMW to the world average. Corrections are applied by multiplyingM recoW by a constant term ∆C.Figure 3: The ATLAS b-tagger weight w (left) is low for light jets and high for b-jets. A generic selection is made applying acut w > 6. CMS will make use of track counting and track probability taggers (right).5

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