Many physics channels of interest at the LHC involve the measurement of jets and missing transverse momentum. The various physics effects playing a role in the chain that goes from an initial parton produced in the hard scattering process to a reconstructed and calibrated jet in the calorimeter are reviewed. The most relevant issues for a good performance in missing transverse momentum measurement are described. The expected performances of the ATLAS and CMS detectors are presented for some of the characteristics physics channels at the LHC. Jet and missing transverse energy triggers are discussed

Bosman, M

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JET AND MISSING TRANSVERSE ENERGYAT THE LHCMartine BosmanInstitut de Fsica d'Altes Energies, IFAE-UAB BarcelonaFor the ATLAS and CMS CollaborationsAbstractMany physics channels of interest at the LHC involve the measure-ment of jets and missing transverse momentum. The various physics anddetector eects playing a role in the chain that goes from an initial par-ton produced in the hard scattering process to a reconstructed and cali-brated jet in the calorimeters are reviewed. The most relevant issues fora good performance in missing transverse momentum measurement aredescribed. The expected performances of the ATLAS and CMS detectorsare presented for some of the characteristics physics channels at the LHC.Jet and EmissTtriggers are discussed.1 IntroductionVarious roles will be played by jets in the LHC Physics. The measurement of jetmultiplicity and ETdistribution will be done in the context of QCD, SUSY orother models. Resonances will be identied by their decay to jets like W ! jj,t! bW , Z ! bb, Z0! jj. Central low pTjet veto will be applied to eliminatemulti-jet background like tt. Forward jets will be tagged to select boson fusionmechanisms. Large EmissTwill be an important signature for new physics. ThepmissTvector will be used in invariant mass reconstruction of decays involvingneutrinos like A=H !  decays.2 ATLAS and CMS calorimetryThe requirements set on the calorimetry to achieve the physics goals at theLHC are listed hereafter. The pseudorapidity () coverage should extend toj  j' 5 for good EmissTresolution and forward jet tagging capability. TheTalk presented at the IX Int. Conf. on Calorimetry in Particle Physics - Annecy, Oct.9-14, 20001ATL-CONF-2002-00121/01/2002detector should be hermetic: the necessary gaps for services should not formuninstrumented "cracks" pointing to the interaction region in order to avoid tailsfrom badly reconstructed jets. The calorimeter thickness should reach at least 9 interaction lengths () to avoid longitudinal leakage of high energy pions andreduce punch-through in the muon detector. The granularity is adapted to thelateral hadronic shower size: dd = 0:0870:087 in the case of CMS and dd = 0:10:1 for ATLAS. The calorimeters are composed of the electromagnetic(EM) calorimeter followed by an hadronic (HAD) compartment with optimizedlongitudinal segmentation (see g.1). In case of ATLAS[1], the EM calorimeterATLAS CMSFigure 1: Schematic representation of the longitudinal segmentation of the AT-LAS and CMS barrel sections of the calorimeter.consists of 24 radiation lengths (X0) or 1.2  of Pb/LAr calorimeter with threelongitudinal sections for (; jet) separation. The EM calorimeter is precededby a preshower to correct for the energy loss in the solenoid coil and cryostatwall situated just in front of the calorimeter. The outer cryostat wall (0:4)separates the EM from the Fe/Scintillating Tile hadron calorimeter. In theend-cap region ( 1:7 <j  j< 3) Cu/LAr technology is used. While theforward region, covering  3 <j  j< 5, consists of a Cu/LAr EM sectionfollowed by a W/LAr section. In CMS[2], the PbW04 crystal EM calorimeter( 1:1) is combined with a Cu/Scintillating Tile calorimeter, both in the barreland end-cap region. The latter is segmented in a thin section (0:3) followedby the bulk of the calorimeter (5.6 ). It is surrounded by the solenoid coil(1:4). Two instrumented layers in the muon system provide an additional 2:5of active calorimeter. The forward region is equipped with a Fe calorimeterinstrumented with quartz bers.Both CMS and ATLAS calorimeters are non-compensated. The degree ofnon-compensation of the CMS hadronic section is e=h ' 1:4 while for ATLAS2the degree is e=h ' 1:35. The degree of non-compensation of the EM sectionis substantially higher in both cases. This results in a non-linearity of the pionresponse of about 15% between 20 and 300 GeV for CMS and about 12% forATLAS. The resolution and linearity depend on the algorithm used for energyreconstruction. CMS used a simple sum Etot= EEM+ ( H1+ H2+ H3).A large signal in H1indicates that a signicant amount of hadronic energy wasdeposited in the EM, the coeÆcient  corrects for the fact that this energyis underestimated because of non-compensation. The resolution is E=E =122%=p(E)  5%. ATLAS used in the test beam an algorithm that includesa rst order correction for non-compensation (quadratic term in EEM) and aninterpolation term between the last EM section (EEM3) and the rst hadronicsection (EHAD1) to estimate the energy loss in the cryostat: Etot= EEM+  E2EM+   EHAD+ Æ pEEM3EHAD1. The resolution is E=E =50%=pE  3:4% 1:0=E applying a 2 noise cut and reconstructing the pionenergy in a cone of dR = 0:3.Ekin(GeV)E rec/Ekin0.60.70.80.9110 10 2 10 3Figure 2: Left: jet energy measured with the calorimeters calibrated at the EMscale in cone sizes of dR=1.5 (black circles), dR=0.7 (open triangles), dR=0.4(black triangles); Right: jet energy resolution at  = 0:3 for various cone sizesafter applying a jet energy calibration algorithm (ATLAS).3 Jet ReconstructionThere are various factors that play a role in the chain between the initial partonand the reconstructed jet. Among the experimental factors are the dierentresponse to neutral and charged component (non-linearity), the lateral showersize, dead material and cracks, longitudinal leakage, and magnetic eld. Thefactors related to physics are fragmentation, initial state radiation, nal stateradiation, underlying event and minimum bias events.300.050.10.150.20 0.05 0.1 0.15 0.2IntrinsicCalorimeterTotal1/ Ö  Eparton  (GeV-1/2)s/EFigure 3: Left: jet energy resolution (calorimeter contribution) without pile-up(open squares) and with high-luminosity pile-up events (stars) for a cone size ofdR=0.4. Right: the intrinsic physics contribution to the resolution (open cir-cles) and the calorimeter related resolution (open crosses); combined in quadra-ture (black dots) they represent the resolution with which the parton energy ismeasured.The ratio of reconstructed jet energy to the particle level energy inside thecone, when the calorimeters are calibrated at the EM scale, is typically of theorder of 0.8 (see g.2 left). It increases with jet energy, as the pions from thefragmentation get more energetic, and decreases for smaller cone size due tolateral shower containment eects. The jet energy reconstruction algorithmhas to correct for these eects. The calorimeter contribution to the jet energyresolution, measured in ATLAS using the particle level energy as a reference,is shown in g.2 at  = 0:3. The jet calibration algorithm included energydependent weights for the dierent calorimeter compartments and correctionterms for energy loss in dead materials. The resolution is E=E = 48%=pE 1:7% for a very large cone (dR=1.5). The sampling term increases to 52% (62%)for a cone size of dR=0.7 (dR=0.4) due to the uctuations of out of cone lossesfor low energy jets. For high energy jets the particles are very collimated and donot suer from these losses. But for larger , the pseudorapidity lines becomeprogressively denser, hence out-of-cone losses start to aect also high energyjets, resulting in an increase of the constant term for small cone sizes.The electronic noise is another detector related eect that inuences theresolution. In ATLAS, the level of noise is 200 MeV in a tower dd = 0:10:1and 0.7 GeV (1.4 GeV) in a cone of dR=0.4 (0.7). In CMS, the level is 150 MeVin a tower of dd = 0:0870:087. Once the jet energy calibration is applied,4the electronic noise contributes a 1:7 GeV=E term to the jet energy resolutionfor a dR=0.4 cone. At high luminosity, minimum bias events generate 0.5 GeVof transverse energy in a tower d  d = 0:1  0:1 and 3.5 GeV (14 GeV)in a cone of radial aperture dR=0.4 (0.7) (el. noise included). Minimum biasdeteriorates notably the resolution of low ETjets (see g 3 left). The resolutionbecomes E=E = 62%=pE 1:5% 4:7=E. Large cones like dR=0.7 cannot beused at high luminosity.In the chain that goes from the parton to a reconstructed jet in a "cone",physics related eects also contribute to the resolution: uctuation in the frag-mentations, FSR, underlying events, magnetic eld sweeping particles out ofthe cone (pTcuto of 0.5 GeV ATLAS and 0.9 GeV in CMS). This "intrinsic"contribution is shown in g.3 together with the calorimeter contribution for axed cone algorithm (dR=0.4) applied to Z0+ jet events.4 Jet AlgorithmsThere are two basic approaches but many possible variations. In "cone-like"algorithms a cone is drawn around a seed. There are variant on how the conedirection is iterated, how jet overlap is handled, etc. "Clustering" algorithms(QCD inspired) pair "particles" (approximated by calorimeter towers) startingfrom the "closest" particles. They stop at a xed jet multiplicity or a maximum"size", etc. The various algorithms suer from dierent energy bias as a functionof EToriginating from physics eect like pile-up (luminosity dependent) orexperimental eects like detector non-linearity and shower size eects. Theseeects depend on jet particle composition and reconstructed size. The choice ofjet algorithm will depend on the physics channel and luminosity conditions. Forexample, dierent algorithms will be used for QCD jet multiplicity study at lowluminosity or for high pTW ! jj reconstruction at high luminosity. The jetenergy calibration will be a complex issue because of the combination of physicsand detector eects that depend on the jet algorithm and the luminosity. In-situphysics processes like Z0+ jet and W ! jj will be used, combined with testbeam information.5 Reconstruction of resonancesReconstruction of resonances decaying to jets will be used in many physicsanalysis. W ! jj decays are of interest in a wide range of pTof the W . Anexample of jj invariant mass reconstructed with ATLAS at low luminosity isgiven in the g.4 (left) forW with pTin the range 120-150 GeV. The resolution5mjj (GeV)Events / 2 GeV02040608010025 50 75 100 12501020304050607080900 50 100 150 200 250 300  8.395    /     9Constant   76.25Mean   105.5Sigma   14.80minv (b,b ) [GeV/c2]events / 5 GeV/c2resolution1 = 14.0%Figure 4: Left:  150 GeV pTW ! jj decays reconstructed in ATLAS atlow luminosity. Right: H ! bb decay at high luminosity reconstructed in CMSbefore calibration (dotted line) and after calibration (shaded area).is  8 GeV ; it increases to  13 GeV at high luminosity. The tail at low massis due to a bias in the jet direction when the jets overlap, a feature that getsmore pronounced with the boost due of a high pTdecaying object. An exampleof a 110 GeV Higgs decaying to bb as reconstructed at high luminosity with theCMS detector is shown in g.4 (right). The resolution is  14 GeV at highluminosity.6 Jet triggerThe jet cross-section is very steeply falling with pT: d=dpT 1=p3T. Thesharpness of the eÆciency curve is important to avoid that the trigger rate isdominated by lower pTjets. At the Level-1 trigger, both ATLAS and CMS usea sliding window algorithm searching for a local maximum in the (dd) pro-jected transverse energy map. The optimum window size depends on the jet ETthreshold and the luminosity conditions (programmable in the case of ATLAS).CMS applies a two-parameter  dependent energy correction algorithm that im-proves the resolution and consequently the sharpness of the eÆciency curve asshown in g.5 (left). At Level-2 or Level-3, standard oine jet algorithms canbe applied with a better jet energy calibration. A reduction of rate of about afactor two is seen in ATLAS with respect to the Level-1 rate. If the allocatedbandwidth for jet triggers is set for example to 25 Hz, then the correspondingETthresholds to be applied are 360 GeV, 150 GeV, 100 GeV for single-, three-,four-jet triggers respectively at low luminosity.6L1   jets   efficiency  curves    with /  without   jet  energy  corrections   0 10100200300ET gen   (GeV)Efficiency0100200300ET gen   (GeV)0 150150250350ET gen   (GeV)0 150150250350ET gen   (GeV)40 GeV80 GeV120 GeV160 GeV10 10210310410550100150200250ET [GeV]LVL-2 rate  [Hz]Njet ≥ 1Njet ≥ 2Njet ≥ 3Njet ≥ 4Figure5:Left:CMSLevel-1triggereÆciencycurvesforvariousthresholdswithandwithoutjetenergycorrections;Right:ATLASLevel-2triggerratesforvariousjetmultiplicitiesatlowluminosity.Figure6:200GeVA=H!!jjdecaysreconstructedatlowluminosity(left)andhighluminosity(right)withtheCMSdetector.7jetsjetsarenarrow,isolatedjetsassociatedto1(3)track(s).Onthebaseofthesecharacteristics,theyareseparatedfromQCDjets(thesecriteriacanalsobeappliedattriggerlevel).Forexample,ATLAScanreachintheoinerecon-structionafactorof100QCDjetrejectionforaeÆciencyof50%.TheinvariantmassofresonancesdecayingtojetscanbereconstructedfromthevisibledecayproductsandthepmissTvectorasameasureoftheneutrinocomponent.OneinterestingexampleistheA=H!decay.Fig.6showsA=H!!jjdecaysreconstructedwiththeCMSdetectoratlowandhighluminosity.70510150 100 200 300 400 500ΣET (GeV)σ(p xymiss) (GeV)full simulation in |η|<3full simulation in |η|<510-410-211021041050 50 100 150 200ETmiss trigger rate (kHz)(GeV)Figure 7: Left: resolution of the two components of the EmissTvector as a func-tion of the total transverse energy in the calorimeters at low luminosity. Right:inclusive EmissTtrigger rates from QCD jet events as a function of the EmissTthreshold applied at Level-1.8 EmissTreconstructionThe pmissTvector is used for invariant mass reconstruction like the A=H ! decay, henceforth a good resolution is important. Large EmissTwill be an im-portant signature for new physics like for example in SUSY models. Tails fromfake EmissTdue to instrumental eects like "cracks", generating tails in the re-constructed jet energy, have to be minimized. The pmissTvector is reconstructedfrom cell (or tower) energies. The important factors for the resolution as studiedby ATLAS with A=H !  decays are the following. A calorimeter coverageup to j  j 5 is needed. The resolution (pmissX;Y), measured at particle level,increases from 2.3 GeV to 8.3 GeV if the coverage decreases from j  j 5 toj  j 3. The particle level resolution (pmissX;Y) increases to 7 GeV when thedetector response is fully simulated. The contribution to the calorimeter reso-lution depends on j  j. The contribution from the dierent regions are: barrel(5 GeV), end cap (4 GeV), forward (3 GeV) decreasing because the averagetransverse energy in the various regions decreases. The electronic noise (a 1:5 cut is applied) increases the resolution from 8.3 GeV to 9 GeV. The pmissX;Yres-olution can be parameterized as (pmissX;Y) = 0:46pET(see g.7). At highluminosity, the pile-up events contribute an additional 15 GeV to the resolution.At the Level-1 trigger the EmissTis calculated from tower ETwith a granularityof d  d = 0:348 0:348 in CMS and 0:2 0:2 in ATLAS. In both cases theleast signicant bit is 1 GeV. At Level-3, EmissTcan be recalculated with nergranularity and better calibration constants. The Level-1 rate is dominated byQCD Dijet events and pile-up. An ATLAS estimate of the trigger rate at lowluminosity is shown in g.7. Below 60 GeV, minimum bias events increase the8Full simulation no pile-up ET>15 GeVconstant fake tag rate 10%0.30.40.50.60.70.80.912 2.5 3 3.5 4 4.5 5Tagging EfficiencyPseudorapidity0204060801001202 2.5 3 3.5 4 4.5 5Pseudorapidityp T of quark ATLFAST no pile-up Full simulation with pile-up Higgs with DICEHiggs with ATLFASTttbar with DICEttbar with ATLFAST00.10.20.30.40.50.60.70.80.910 15 20 25 30 35 40Threshold (GeV)Jet Veto efficiencyHiggs with DICE, High Luminosity Pile-upFigure 8: Left: Average pTof forward quarks and forward jet tagging eÆciencyin ATLAS at low and high luminosity. Right: Jet veto eÆciency for heavy Higgsan tt events for fast simulation and full simulation (DICE) in ATLAS.rate by a factor  5; above 100 GeV the pile-up has no eect anymore. At highluminosity, the rates increase by a factor  103for a 100 GeV threshold and 10 for a 200 GeV threshold.9 Forward jet tagging and low pTjet vetoForward jet tagging will be used to select boson fusion processes (see g.8 left).A tagging eÆciency of  90% can be achieved starting to decrease at j  j 4because of the decreasing average pTof the quark. At high luminosity, aneÆciency of  80% can be achieved for a fake tag rate of  10%. Central jetveto is used to reject high jet multiplicity background, usually tt. Fig.8 showsthat in ATLAS an eÆciency of  60% is achieved for a heavyH !WW ! ljjsignal and  10% for the tt background.10 ConclusionThere is a lot of interesting physics involving jets and EmissTat the LHC. TheATLAS and CMS detectors have shown that their calorimeters are prepared totrigger and reconstruct these quantities in the challenging environment of a highluminosity hadron collider.11 AcknowledgementThe author would like to thank S.Arcelli, S.Abdullin, ... from CMS, C.Santoniand many more colleagues from ATLAS for material and useful discussions.9References[1] The ATLAS calorimeter Performance Technical Design Report,CERN/LHCC 96-40.[2] The CMS Hadron Calorimeter Project Technical Design Report,CERN/LHCC 97-31.[3] The ATLAS Detector and Physics Performance Technical Design Report,CERN/LHCC 99-14.[4] Studies of the response of the prototype CMS Hadron calorimeter, includingmagnetic eld eects to pion, electron and muon beams. V.V. Abramov etal.,hep-ex/0007045,Jul. 200010

Jet and missing transverse energy at the LHC

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