Failures of the ATLAS Tile calorimeter would affect the jet energy resolution and would fake tails of missing transverse energy. Significant effects are expected in processes involving high transverse momentum jets ($p_T > 100$ GeV). These effects, their consequences, as well as methods to minimize them, are studied using simulated data for various degradation topologies and for different physics processes

Arabidze, G

Mermod, P

Milstead, D

Stanek, R

CERN Document Server

ATLAS NOTEATL-COM-TILECAL-2008-010August 27, 2008Effects of ATLAS Tile calorimeter failures on jets and missing transverseenergy measurementsP. Mermod1, G. Arabidze2, D. Milstead1 and R. Stanek31Stockholm University, Sweden2University of Athens, Greece3Argonne National Laboratory, USAAbstractFailures of the ATLAS Tile calorimeter would affect the jet energy resolution and wouldfake tails of missing transverse energy. Significant effects are expected in processes involv-ing high transverse momentum jets (pT > 100 GeV). These effects, their consequences, aswell as methods to minimize them, are studied using simulated data for various degradationtopologies and for different physics processes.1 IntroductionStudies of physics processes at the LHC presently rely on simulated data. Simulations of the detectorinclude intrinsic performance effects due to, e.g., cracks or miscalibration [1]. However, simulated datausually do not take into account accidental hot or dead channels (or even whole dead regions) in thedetectors. Such detector failures might substantially affect the measurement precision or the discoverypotential in a physics analysis. In particular, an accurate measurement of the missing transverse energy(/ET ), which relies on the calorimeters, is crucial in many physics searches [2].In this paper, the impact of a failing ATLAS Tile (hadronic) Calorimeter (TileCal) is investigated insome detail. In a recent cosmic ray data study [3], it has been shown that noisy cells in the calorimeterscan induce large amounts of fake /ET . In the present study, we assume that hot or noisy cells are identifiedand masked, and consider only the effects of dead cells in TileCal. Trigger issues are not considered here.This paper is organized as follows : descriptions of the MC processes and of the TileCal degradationtopologies used for the study are given in sections 2 and 3 ; a discussion of the relative fraction of jetenergy deposited in TileCal is presented in section 4 ; results on the effects of TileCal degradation onjets and /ET are presented and discussed in sections 5 and 6 ; and a summary and conclusions are givenin section 7.2 MC SamplesThe MC data used in this paper passed through a full ATLAS simulation1). Three different physicschannels were considered ; they are listed below with the corresponding MC models in parenthesis :• t ¯t (MC@NLO, considering all decays except all-hardonic)• J5 (Pythia, QCD processes with 280 < pˆT < 560 GeV)• J7 (Pythia, QCD processes with 1120 < pˆT < 2240 GeV)The t ¯t process is an important source of background in many analyses, and is also a very interestingstandard model process in itself. The J5 and J7 processes are important when studying very high-pTphysics. They are used here as benchmarks to quantify the effects of a failing TileCal on jets and /ETmeasurements.3 ScenariosSix illustrative TileCal degradation topologies have been considered (for a complete description of Tile-Cal, see, e.g., [4]) :• No degradation.• Whole TileCal off.• 9 disabled drawers2) at random in the long barrels 3). This scenario is illustrated in Fig. 1(a).1)The data were reconstructed with Athena release 12.0.31. The jet definition was Cone4TowerParticleJets, with a coneradius of 0.4, Pt of seeds of 1 GeV, and a split/merge fraction of 0.5 ; the towers were made on the EM scale and calibratedwith a global H1 algorithm. The /ET definition was MET Final, with calibrated cell energies in topoclusters (H1 algorithm) andtaking into account muons (Moore algorithm) and cryostat corrections (from Kt jets).2)There are 64 drawers in total for each TileCal barrel.3)The TileCal long barrels LBA and LBC cover the pseudorapidity range 0 < |η | < 0.9.2(a) 9 modules at random in the long barrels(b) 9 contiguous modules in LBAFigure 1: Drawing of the TileCal modules in the long barrels LBA and LBC. The modules shown inblack correspond to the nine disabled drawers in the scenarios considered for the present study.3• 144 disabled DMUs4) at random in the long barrels (only J7).• 9 contiguous disabled drawers in the long barrel (only J7). This scenario is illustrated in Fig. 1(b).• 9 disabled drawers at random in the extended barrels5) (only J7).The extreme case where the whole TileCal is disabled gives us an idea of the role played by thehadronic calorimeter in different physics cases. Nine whole drawers turned off at random positions inthe long barrels (LB) is a quite pessimistic scenario, but not unrealistic. We might also have a certainamount of bad channels (e.g., noisy PM tubes) distributed over the whole barrel ; to illustrate this, herewe consider 144 DMUs - the equivalent of 9 drawers in number of channels - at random in the LB. Weconsider also the failure of 9 contiguous drawers 6) : in this case, we expect larger amounts of /ET , as theundetected part of the hadronic showers tends to act in the same direction. Finally, with the last scenario,we study the extended barrels (EB). We will discuss further the implications of these different situationsin section 6.4 The non-electromagnetic fractionjet energy (GeV)0 500 1000 1500 2000 2500 3000fraction of jet pt in TileCal00.10.20.30.40.50.6long barrelsextended barrelsFigure 2: Profile histogram showing the mean fraction of jet pT in TileCal as a function of the jet energy.The η regions corresponding to the LB and EB are plotted as closed and open triangles, respectively.Both the J5 and J7 processes were used to produce this figure.The profile histograms plotted in Fig. 2 show the mean fraction of transverse energy deposited inTileCal as a function of the jet energy. The fraction of energy in TileCal is seldom greater than 40%since most of the shower is captured by the LAr electromagnetic calorimeter [5].As expected, this fraction increases with the jet energy as the shower penetrates deeper into the thecalorimeters. However, there are non-linearities. In the LB (|η | < 0.9, black triangles), the fraction4)The DMUs are part of the TileCal drawer digitizers. There are 16 DMUs in each drawer, each of them serving 3 channels.5)The TileCal extended barrels EBA and EBC cover the pseudorapidity range 0.9 < |η | < 1.7.6)For instance, the failure of a 200 V power supply with a spare channels would switch off 8 contiguous drawers ; there areother failures which can take down individual drawers.4flattens around 1000 GeV and even starts to decrease a bit for very high jet energies. For the EB (0.9 <|η | < 1.7, open triangles), we observe two plateaux. This behavior predicted by the MC might notreproduce real data accurately, as the way very energetic hadronic showers develop in the calorimetersis currently based on models which have been tested experimentally only with pion test beams [5]. Butjets behave very differently from single pions, as 300 GeV pions would release much more energy inTileCal than 300 GeV jets, for instance [4]. The amount of electromagnetic calorimeter material anddead material is larger in the EB region [5], and consequently, for the EB, we see, on average, about 5 to10% less energy deposited in TileCal.5 Effects on jet resolutionPt_jet (GeV)0 500 1000 1500 2000 2500110210310410J7No degradation9 DrawersWhole TileCal(Pt_jet-Pt_true)/Pt_true-1 -0.8-0.6-0.4-0.2 -0 0.2 0.4 0.6 0.8 10200040006000800010000Figure 3: Jet pT distribution (left panel) and jet pT resolution (right panel) for J7. The solid linecorresponds to no degradation, the black dots to 9 randomly failed drawers, and the open squares to thewhole TileCal off. The plots are normalized to 10000 events.As discussed in the previous section, TileCal is expected to account for at most for 40% of the jetpT measurement for high-pT jets. The J7 process was used to produce Fig. 3, where the maximaleffect is seen by comparing the open squares (TileCal disabled) with the solid line (fully operationnalTileCal). The right panel of the figure shows the relative difference to the true jet pT . The peak isshifted by about 0.02 when disabling 9 Drawers in the LB (black dots), and the width of the distributiongets larger by about 0.01. A shift of the peak by roughly 2% is expected from this simple argument :considering the long barrels only (which leads to a slight overestimation of the overall effect), 9 drawerscorrespond to 9/128=7% of the detector, and 40% of the energy is expected to go undetected for jets inthe corresponding regions ; this gives on average 0.07x0.4=2.8% less detected energy. In the scenariowhere the 9 drawers are in the EB (not shown in the figure), the shift is only about half a percent.56 Effects on missing transverse energyA mismeasurement of the jet energy when the jet falls into a dead calorimeter region can cause largeamounts of fake /ET . In this section, we will look at /ET tails for different degradation topologies, investi-gate how the fake /ET depends on the jet η and pT , and discuss possible remedies.MET (GeV)0 200 400 600 800 1000Events110210310J7No degradation144 DMUs9 Drawers at random9 Drawers in a rowMET (GeV)0 200 400 600 800 1000Excess (relative to no degradation)-202468101214Figure 4: /ET distribution (left) and relative difference in the number of events with and without degra-dation as a function of /ET (right) for J7. The solid line corresponds to no degradation, the dots to 144DMUs, the squares to 9 drawers and the triangles 9 contiguous drawers (always in LB). The plots arenormalized to 10000 events.The left panel of Fig. 4 shows /ET distributions for the J7 sample and various scenarios. The threedegradations correspond to the same amount of dead channels 7) (always in LB), but with decreasinghomogeneousness (from DMUs spread over the whole barrels to several contiguous drawers). We seethat the /ET tails get more important as the bad channels get more concentrated in the same region ofTileCal. In the right panel, we see by how much the number of events increases : up to a factor 10 forlarge /ET when 9 drawers are disabled, and by about 100% above 200 GeV in the case of 144 dead DMUs.Whether this is a problem or not in a real physics analysis depends, e.g., on the expected backgroundcontribution from hard QCD jets.For the J5 sample (not shown in the figure), the effect is not as pronounced : we get about 100%more events for /ET > 100 GeV for 9 randomly failed drawers.For the t ¯t sample (not shown in the figure), disabling 9 drawers does not cause significant changesin the /ET distribution. There are two reasons for the t ¯t process to have little sensitivity to disabling partsof TileCal : the jet energy is, on average, less than 100 GeV, which means that little of it is measured byTileCal (see Fig. 2) ; and there are anyway relatively large amounts of natural /ET coming from physics,which dilutes the effects of instrumental /ET considered here. However, this last argument does not applyfor the all-hadronic t ¯t process, which is not considered in the present study.7)Although the degradation effects on the reconstruction performance are different : if only one of the two channels cor-responding to the same scintillator is switched off, the energy is corrected by doubling the output of the other channel ; this6fake MET (GeV)0 100 200 300 400 500 600 700 800 900 1000Events110210310hard jet in bad regionno hard jet in bad regionno hard jet close to bad region(a) J7, 9 drawers in the long barrelsfake MET (GeV)0 100 200 300 400 500 600 700 800 900 1000Events110210310 hard jet in bad regionno hard jet in bad regionno hard jet close to bad region(b) J7, 9 drawers in the extended barrelsfake MET (GeV)0 50 100 150 200 250Events110210310 hard jet in bad regionno hard jet in bad regionno hard jet close to bad region(c) J5, 9 drawers in the long barrelsfake MET (GeV)0 100 200 300 400 500 600 700 800 900 1000Events110210310(d) J7, 144 DMUs in the long barrelsFigure 5: Fake /ET distributions. Solid line : at least one hard jet in the dead region. Dashed line : nohard jet in the dead region. Dotted line : no hard jet in the dead region extended by 0.1 in eta and phi(corresponds to excluding also adjacent drawers).7Fake /ET 8) distributions are shown in Fig. 5. For the solid lines, a selection was made requiring atleast one hard jet (pT > 500 GeV for J7 and pT > 150 GeV for J5) to fall inside a dead region. ForJ7 with 9 dead drawers in the LB (Fig. 5(a)), these events exhibit an average fake /ET of 270 GeV. If weconsider the same dead drawers in the EB (still J7, Fig. 5(b)), the average value is 130 GeV. For J5 (9drawers in LB, Fig. 5(c)), the average fake /ET with jets falling in the bad region is 50 GeV.The fake /ET can be greatly reduced by rejecting events containing hard jets in the bad regions (dashedlines), and even further by also rejecting events with hard jets in drawers adjacent to the dead ones (dottedlines), since the same jet might spread over several TileCal modules. But this is at the cost of efficiency :with 9 dead drawers in the LB, we lose 11% of the events with the first, tight rejection, and 32% with thesecond, looser one9). Also, we see that even when the degraded regions are very well defined and vetoed,there is still some fake /ET . This is probably due to low-energy jets or jets which are reconstructed withlow energy (or not reconstructed at all) due to the lack of Tile calorimeter information. Remember thatwe exclude only events with hard jets in the bad regions. We can imagine that if the LAr calorimeter wasalso failing in the same region, there would be very little information telling us if there were a jet there.In such a case, one would have to be very careful with events with a large /ET in the direction of the deadzone in the calorimeters. It would also introduce a bias for events containing true (physical) /ET .In the case where we have many dead channels spread over the whole TileCal LB (Fig. 5(d)), thefake /ET due to TileCal degradation extends up to several hundreds GeV for J7, with about 10% of theevents with a fake /ET greater than 50 GeV and about 2% above 200 GeV10). This can probably not betrivially reduced.7 Summary and conclusionVarious TileCal degradation scenarios were investigated, and the effect on jet energy measurement andits repercussions on the /ET measurement were studied.A jet falling into a completely dead TileCal region would be seen with 20-40% less transverse mo-mentum on average, the effect being most important for high-energy and central jets. The changes interms of jet resolution are not significant, but the resulting fake /ET is a serious issue.If there are whole malfunctioning TileCal drawers, the fake /ET can be almost completely reduced byrejecting events with hard jets falling near to the corresponding TileCal modules, but this is at the costof detector acceptance. In case there are many small dead spots to deal with, it is necessary to take intoaccount the irreducible /ET tails in the analysis. The typical state of TileCal will be known while takingdata. The overall behavior of fake /ET may not depend on the details of the actual degradation topology,thus hopefully allowing to run the Monte-Carlo reconstruction only once with a typical topology.8 AcknowledgementsMany thanks to C. Clement, L. Fiorini and J. Proudfoot for their interest and good ideas.happens more often for 144 DMUs than fot 9 drawers.8)In this work, fake /ET means the difference in /ET relative to no TileCal degradation (for the same events being reconstructedin different scenarios). It does not include the fake /ET already present without adding any degradation.9)These numbers come from the study, they are not clear from the figure only.10)Idem.8References[1] ATLAS Collaboration, ATLAS Detector and Physics Performance Technical Design Report,CERN/LHCC99-14 1 (1999) 261.[2] Naoko Kanaya, SUSY Physics with Early Data : Understanding the ATLAS Detector and the Back-grounds, CP903, SUSY06, 14th International Conference 1 (2007) 201.[3] R.J. Teuscher et al., Fake Missing Transverse Energy from ATLAS Calorimeter Cosmic Ray Data,ATL-CAL-2008-002 (2008).[4] ATLAS Collaboration, Tile Calorimeter Technical Design Report, CERN/LHCC96-42 (1996).[5] ATLAS Collaboration, Calorimeter Performance Technical Design Report, CERN/LHCC96-40(1997).9

Effects of ATLAS Tile calorimeter failures on jets and missing transverse energy measurements

Abstract

Similar works

Full text

Available Versions

CERN Document Server