Performance of the LHC, ATLAS and CMS in 2011 DanielFournier1,a LAL,UnivParis-Sud,CNRS/IN2P3,Orsay,France Abstract. The path taken by the LHC team to reach 3.6 1033 cm 2 s 1 instantaneous luminosity, and to − − deliver5.6fb 1perexperimentissummarized.Themainperformancesofthetwoexperimentsarehighlighted, 2 − inparticularthewaytheymanagedtocopewiththealreadyhighlevelof“pile-up”.SelectedStandardModel 1 andtopphysicsresultsaregiven,andthestatusofthelimitsontheHiggsbosonsearchbyeachexperimentis 0 2 summarized.Abriefoverviewofthesearchforsupersymmetryandexoticphenomenaismadeattheend. n a J 1 LHC running in 2011 - intensity ramp-up with up to 1380 bunches per beam (maximumpossiblewith50nsspacing) 3 2 1.1 Machine performance -emittancereduction -b ∗reductionfrom3.5mdownto1m ] With5.6fb 1ofgoodproton-protondatadeliveredtoAT- AsofearlySeptembera peakluminosityofabout3.6 x LASandCM−Satacentreofmassenergyof7TeV,theper- 1033atthebeginningofafillwasreached,andreproduced e - formanceofthe LHCis consideredbyeveryoneinvolved untiltheendoftheperiod(endOctober),allowingtorecord p asoutstanding.Themachinealsodeliveredabout1.2fb 1 abouthalfofthetotalintegratedluminosityoftheyeardur- − e toLHCband5pb 1toAlice,anintegratedluminositycor- ingthelasttwomonths(Fig.1). h − respondingto the runningconditionschosenby these ex- [ periments. 1 Themainparametersofthemachine,attheendofthe v running period where the instantaneous luminosity was 1 highest,arelistedintable1,togetherwiththerunningpa- 8 rameters at the end of 2010, and in comparison with the 6 nominal parameters at 14 TeV in the centre of mass [1]. 4 . The beam crossing angle at the collision points 1 and 5 1 whereATLAS(respCMS)areinstalledwasof120micro- 0 radians, sufficient to limit long range beam-beam effects 2 forabunchspacingof50ns. 1 : v Table1.ParametersofLHCexploitation,attheendof2010,at Fig.1.Luminosityintegratedbyexperimentsin2011. i X theendof2011,anddesignparametersat14TeVinthecentreof mass. r a Theseexcellentperformanceswereobtaineddespitea Parameter 2010 2011 Nominal number of effects which limited the machine availability N(1011p/b) 1.2 1.5 1.15 and the actual length of most of the fills. Some of these k(n ) 368 1380 2808 effects[2]arebrieflydiscussedbelow: bunches Bunchspacing(ns) 150 50 25 e (m mrad) 2.4-4 1.9-2.3 3.75 -RadiationinducedfailuresofElectronics(SEU).Thecryo- b (m) 3.5 1 0.55 genics and the machine protectionsystems suffered from ∗ L(cm 2s 1) 21032 3.61033 1034 electronicsfailuresduetoradiationeffects,whoseratewas − − showntobenearlyproportionaltotheinstantaneouslumi- Energy(MJ)stored 28 110 360 nosity.Anumberofactionsweretakenduringtheyearto mitigatetheincidenceofsucheffects(relocationorshield- The path from the 2010 parameters to those obtained ingofcriticalelectronics,improvementofredundancy,...). attheendofthisyear,whichrepresentanimprovementby -Beamdumpstriggeredbyhighlosses(UFO).Suddenin- afactor20,wasmadeinsuccessivesteps: crease of beam losses duringstable conditionshave been -restartofthemachinewithtrainsofbunchesseparatedby tentatively associated to “falling objects”. By far not all 150ns these losses trigger a beam dump.While duringinjection -beamscrubbingatinjectionenergyandhighintensityto mostUFOshappeninthevicinityofkickermagnets,losses “clean-up”thesectionspronetoelectron-cloudeffects duringstablebeamaremoreorlessuniformlydistributed -runningwithshorttrainsofbunchesseparatedby50ns aroundthemachine.ItwasobservedthattherateofUFOs tendtodecreaseduringlongperiodsofreproduciblecon- a e-mail:fournier@lal.in2p3.fr ditions. EPJWebofConferences -Vacuumpressureincreases.Verysignificantpressurein- -integratedluminosityoflongphysicsperiods,isthenob- creases (factors 100 or more) appeared when trains of tainedsummingupcountsfromluminositymonitors,suit- bunchesseparatedby50nswereinjectedinthemachine. ablycorrectedforvariouseffects([3],[4]),andnormalized Theywereprincipallylocatedaroundthecollisionpoints, totheVDMscanperiod. attransitionplacesbetweencoldandwarmsectionsofthe AnexampleofVanderMeerscan,takenfromATLAS machine. Dedicated periods at high intensity and injec- isshowninFig.3. tion energy (beam scrubbing) allowed to “clean up” the criticalplaces,withpressurereductionsoftypicallyanor- der of magnitude after 15 hours. Wrapping simple coils around these places furthermore reduced the effect, con- firmingthatthesourcewasdominantly“electron-clouds”. -Heatingofbeamelementsatvariousplaces(beamscreen, kickers, collimators, and -recently discovered- damaged RF-fingers),associatedtotheelectromagneticfieldaccom- panyingthebunches,wasobservedandforcedtolimitthe bunchchargeinseveraloccasions. -RFbeamloadingandbeaminstabilitiesleadingtoemit- tanceblow-upswerealsoobservedatseveraloccasions. Fig.3.ExampleofcountingrateduringaVanderMeerscan. As a global consequence of these various effects, the duration of beam fills was in average 6 hours, with large variations,whiletheoptimumwouldhavebeenaround12 Thesizeoftheluminousregionisrelatedtothecount- hours. Another measurement of the impact of the above ingrateRby: limitations,isthroughthemachineavailability,whichwas about50%duringthe“physicsperiods”.Inturnabouthalf (cid:229) = 1 RRx(d )dd (2) ofthistimecorrespondedto“stablebeamcondition”which x √2p Rx(0) coveredthus23%of“physics”time(Fig.2). The accuracy of the method is limited by the bunch chargemeasurementaccuracy,andbynonlinearitiesinthe luminosity monitors. The precision obtained for the first partofthe2011data(untilb wasloweredto1m)is3.7% ∗ forATLASand4.5%forCMS. The reliability of running the machine with beams separated in one direction serves another important pur- pose.Itallowstorunthemachinewith“luminositylevel- ling”inLHCb,around2to31032 cm 2 s 1 asrequested − − bythisexperiment.Fig.4showsanexample,foragivenfill ofhowwellthe instantaneousluminosityiskeptconstant inthisexperiment,whileitsmoothlydecreasesinATLAS andCMSbecauseofprotonlossesandemittanceincrease. Fig.2.Piechartofmachineavailability. 1.2 Luminosity measurement and adjustments Knowingtheluminositywithprecisionisanimportantas- setformany,ifnotallphysicsmeasurements.In2010the Van der Meer method was applied to measure the luminosity with an accuracyof 3.4%in ATLAS[3] and4.0% inCMS[4].In2011thiswasrepeatedtwice.Themethod appliedattheLHCgoesasfollows: Fig.4.ExampleofafillwithluminositylevellinginLHCb. -themachineisrunwithasmallnumberofbunches - the beam current associated to each bunch is measured witha“beamcurrenttransformer”(givingn andn ) 1 2 FinallythelowluminosityinAliceisprovidedbycol- -thesizeoftheluminousregionismeasuredbystudying liding in point 2 a few dedicated bunches added to the thecountingrateasafunctionofbeamseparation,sequen- trains. tially in the horizontal and vertical directions, giving (cid:229) x As a consequence of the complex bunch structure in and(cid:229) .Thismeasurementismadeusingluminositymoni- y eachring,afew“unpairedbunches”crossthenominalcol- tors,whicharealsorunduringhighluminositydatataking. lisionpointsatatimewhenthereisnobunchcomingfrom Lisobtainedas(f istherotationfrequencyofonebunch r the other direction, thus without producing any collision. aroundthering): The data corresponding to these BCIDs (bunch crossing identifiers)arehoweverrecordedandusedtoevaluatethe L=k.f .n .n /(2p (cid:229) (cid:229) ) (1) “non-collision”background,associatedforthelargestpart r 1 2 x y HadronColliderPhysicsSymposium2011 to beam losses in the arcs. Secondaries produced when these lost protonsinteractin the magnetyokes,or muons from the subsequentdecay of the secondaries, eventually reachtheexperimentsandactivatesomelowleveltriggers. Analysing these data shows [5] that, as anticipated, they mostlyconsistofenergydepositslocatedinthe(horizontal) machine bending plane. Their rate is very low, con- firmingtheveryhighefficiencyofthecollimationprocess inthemachine.Thisbackgroundneverthelessneedstobe rejectedforsomeanalyseslookingforrareprocesseswith rather weak signatures (jets and missing transverse mo- mentumEmiss forexample). T Fig. 5. Luminosity weighted mean number of interactions per crossing. 1.3 Pile-up -SeveraldaysinSept2011werededicatedtodatataking As will be discussed in the next sections, the occurrence with high b settings (90m) in view of measuring elastic of severalindependent,inelastic, proton-protoncollisions ∗ scatteringwithdedicatedforwarddetectors(ATLAS/ALFA duringonebunchcrossingconstitutesanoise,usuallycalled andTOTEM).Thesedetectorspositionedat240moneach pile-up noise, which degrades to some extent the perfor- sideofthecollisionpoints1and5,arelocatedin“Roman manceofthereconstructionofsomeofthe“objects”used pots” and allow to triggerand recordelastically scattered forphysicsanalysis.Itisthusimportanttobeabletochar- protons with transverse momenta up to about 1.5 GeV/c acterizethepile-upconditionsonaneventbyeventbasis. [6].Thebeamconditionswerecleanenoughthatthedetec- Oneessentialvariableinthisrespectisthenumberofpri- torscouldbebroughtdownto6s fromthebeamsfordata maryverticesreconstructedpercrossing,calledN inthe VX taking. More than 1 million elastic events were recorded following.Sincetheresponseofseveraldetectors(inpar- byeachexperiment.Higherb areplannedin2012(upto ticular the calorimeters) extends over more than the time ∗ 1km)inviewofreachingtheCoulombinterferenceregion. interval between two successive crossings, it is also im- portanttohaveaviewofthepile-upconditionsinthesur- rounding bunches (so called “out of time” pile-up as op- posedto“in-time”pile-updescribedbyN ).Thevariable 2 ATLAS and CMS status VX usedfordescribingthepile-upconditionsoverallism ,the numberof interactionsper crossing. m is calculated from The structure of each of the two experiments is known thesuitablynormalizedinstantaneouscountofluminosity worldwidealready.ATLAS[7]featuresanaircoretoroid, monitors(L)andtheinelasticcross-sectionas: highgranularity“accordion”lead-liquidargonelectromag- netic calorimetry, complemented by Copper/liquid argon m =L.s inel/k.fr (3) andtungstenliquidargonhadroniccalorimetersinthefor- Summinguptheentriesoveradatatakingperiod,one warddirection,andiron-scintillatingtilesinthecentralre- obtains the histogram of the “luminosity weighted mean gion.Thecentraltracking,insidea2Tmagneticfield,uses numberofinteractionspercrossing”asshowninFig.5for Si-pixelsandSi-stripsintheinnerpart,andstrawtubesat ATLAS. The dispersion reflects the variation of charges largerradius. between bunches, the decrease of instantaneous lumino- InCMS[8]thelargesolenoidof4Tmagneticfieldcon- sityduringeachfill,andthedifferenceofinitialconditions tains the inner detector with pixel and Si-strips, embed- from fill to fill. In particular the change between b ∗ val- dedin a PbWO4 crystalcalorimeter,followedby a brass- uesisstriking,withgoingfrom6.3forb =1.5m,to scintillatingtilehadroniccalorimeter.Theforwardcalorime- ∗ 11.6forb =1.0m.InaverageN andareroughly ters are recessedat 8mfromthe collisionpointand use a ∗ VX proportional,with 0.6, the smaller num- steel-quartzfiber(Cerenkov)samplingtechnique.Bothex- VX berofreconstructedvertices∼resultingfromthelimitedde- perimentshaveasophisticated3-leveltriggersystem,mak- tector acceptance, and the minimal conditions set for a ingalargeuseoflepton(electrons,muons,taus),highen- validreconstructedvertex(typicallyatleast3tracksofat ergyjetandEmisssignatures. T least0.5GeV/ctransversemomentum). Up to now, the detectors have notsuffered significant radiationdamage.Oneobserveshoweverthattheleakage current in Si-pixel sensors is increasing, in particular in 1.4 Special runs CMSwhere,fortechnicalreasons,thepixeldetectorisfor thetimebeingoperatedatwarmtemperature.Thetypein- On top of regular “physics runs” in nominal conditions, versionisexpectedtohappensometimein2012. somerunsweretakeninpeculiarconditionstoaddressspe- During the past year about 4 1014 events were pro- cificquestions: cessed by the trigger system of each experiment, sollici- -Dataweretakenwithshorttrainsofbunchesseparatedby tatedatthe20MHzbunchcollisionfrequency.Attheother 25nsinordertoassessthetrigger,datataking,andrecon- end,about300events/s,inaverage,werewrittentoperma- structionperformanceinthefuture“nominal”conditions. nentstoragebyeachexperiment.Itisinstructivetolookat - Fat bunches, of close to twice the nominalcharge were tables2and3whichshowforeachexperiment,at31033 also collided in order to assess the pile-up effects with peak luminosity, how the bandwidth is split between the reachingvaluesofupto40. main trigger/DAQchannels,keepingin mindthatthefull EPJWebofConferences triggermenuhasabout350linesinATLAS,andevenmore Table3.CMStriggerthresholdandratesattheLVL1andafter inCMS. A goodfractionofthemareusedtomonitorthe finalselection. triggeritself byaccepting,ateachlevel, a prescaledfrac- tionoftherejectedevents.Amongthemainsignaturesare thepresenceofoneidentifiedlepton.Onecanobservethat theleptonthresholdsaresomewhatlowerinATLAS,thus taking in comparison a larger fraction of the bandwidth. Given the steep slope of the lepton (and jets) transverse momenta,it is extremelyimportantto have sharp turn-on thresholds to reduce the rate of unwanted signals. As an example,Fig.6showstheturn-oncurveforthecombined muonandhadronictautrigger,asafunctionoftheoffline tau-jettransversemomentum. 3 Physics objects and selection of SM results The new feature of 2011 data was the high level of pile- up,with reachingup to about20 atthe endofthe period.Whilethepile-up“noise”isnotexpectedtoaffect veryhighenergyjets, norleptonreconstruction,noreven b-tagging, it is on the other hand expected to affect seri- ouslyobjectsoflargesizeandlow/mediumtransverseen- ergy,andthereforeinparticular: Fig. 6. Turn-on curve for the combined muon (12 GeV) and hadronictautrigger,inCMS. – lowenergyjets – Emiss T – isolationofleptonsandphotons. Atthe storagelevel, thedata volumeis largerforAT- LAS( 2Mbyte/event)thanforCMS( 0.5Mbyte/event) ∼ ∼ wherezerosuppressioninthecalorimeterisappliedatthe 3.1 Jets and QCD dataacquisitionlevel. In the high transverse momentum (p ) range, the critical T quantitiesforjetsaretheenergyscaleandthelinearity.In Table2.ATLAStriggerthresholdandratesattheLVL1andafter ATLASjetsare reconstructedwith the anti-kTalgorithm, finalselection. withasizeparameterR=0.6(R=0.4isalsousedforcom- plex final states). Jets are built as vectorial sums of clus- tersofcalorimetercells,correctedforhadronictoelectro- magneticresponse,anddeadmateriallosses.Insitumeth- odsare usedto checkthe p scale up to 1 TeV or above: T photon-jetbalance,multi-jetbalance,track-jets,.. In 2010 the energy scale systematic uncertainty was 2.5%inawidekinematicrange.Thelargedatasetof2011 mayallowtoimprovethisuncertainty.TheATLASinclu- sivep spectrum,with1.9fb 1integratedluminosity,also T − including2010dataonthelowerp part,isshowninFig. T 7.ThedataiswellreproducedbyaPYTHIAsimulationin whichthePDFarecorrectedforNLOeffects[9].Jetswith p upto1.9TeVhavebeenobserved! T InCMSjetsarereconstructedfrom“Particleflow”clus- ters, using the anti-kT algorithm with R=0.5. A certain level of cluster merging (“jet grooming”) is made before The combined efficiency of data acquisition and data goingtophysicsdistributions.Asanillustration,theinvari- qualityselection in eachexperimentis ata high level,al- antmassspectrumofthetwojetsofhigherp isshownin T lowingtofindcloseto90%ofthedeliveredluminosityin Fig.8,comparedtoPYHTIAsimulation(withCTEQ6L1) thephysicsplots. scaledupbyafactor1.33.Theagreementisverygood,and The fraction of channels alive is also on a high stan- allowstoruleoutexcitedquarksmithmassessmallerthan dard,beingforexample99%ormoreforeachofthetwo 2.49 TeV (2.68 expected limit), Axigluons with masses electromagnetic calorimeters, and about 97% for each of lowerthan2.47TeV(2.66expected),andW’withmasses thetwopixelsystems. smallerthan1.51TeV(1.40expected). HadronColliderPhysicsSymposium2011 width approximately doubles when N goes from 1 to VX 12,meaningthattheunderlyingeventofZ0 productionis morebusythanforarandomevent. Fig.7.Inclusivejetp distributioninATLAS. T Fig.9.MuonisolationinATLAS. - Emiss. Obtained as the negativevector sum of the trans- T versemomentumofall“objects”inanevent,theEmissisa T prioriquitesensitivetopile-up.Thisdependenceislimited ifoneretainsonlyjetsaboveaminimump .Asanexam- T ple of performance, for a rather complex final state, Fig. 10showsEmiss inCMS(firstpartof2011data)forevents T with 2 leptons of opposite charge and identical flavor, in theZ0 massrange,plus2jets.Oneobservesthatthepeak of Z+ jets events remains rather narrow, and that the tail athighEmiss (greaterthanabout70GeV)isdominatedby T physicalprocesses,essentiallytt¯pairs. Fig.8.InvariantmassofleadingjetsinCMS. 3.2 Impact of pile-up on sensitive quantities - Jet energyresolution.The p balancebetween photons, T insensitivetopile-upgiventheirsmallsize,andjetsallows to control the jet energy scale and resolution. In ATLAS it was found,in the first part of the data (=6), that thejetenergyresolutionisworsenedby10%inthelowest p range (30 GeV). In CMS the effects of “in time pile- T up” and of “out of time pile-up” were separated showing Fig.10.EmissdistributionforZ+2jetseventsinCMS. T that(forN =8)thelattercontributesabout5GeVrmsto VX thecomponentperpendiculartothephotondirectionwhile the former contributes about 2 times more. These figures 3.3 W&Z Physics correspondtothepresentstatus,withoutanycorrectionto mitigatetheobservedworsening. Thisphysicsisentirelydonewiththeleptonicdecaymodes. -Leptonandphotonisolation.Inordertoreducetheback- Thefull2011datasetrepresentsineachexperimentabout ground of fake muons coming from heavy quark decays 3millionsofZdecaysinelectronpairsormuonpairs,and insidejets,anisolationcutisoftenapplied.Itconsistsei- 10 times more W decays in electron-neutrino or muon- therofatrackisolationcut(sumoftransversemomentaof neutrino. The decays to t come on top of this, with sig- tracksfallinginsidea coneofsizeD R aroundthemuon), nificantlysmaller statistics dueto thereducedtriggerand or a transverse energy cut (sum of all calorimeter trans- reconstruction efficiencies. Given the low level of back- verseenergies)oracombinationofboth.Fig.9showsthe groundunderthepeak,Zdecaysareusedtoestablishwith spectrumofthecalorimetertransverseenergyinaconeof the“tag-and-probe”methodthetriggerandreconstruction D R=0.4 around muon tracks from Z0 decays in ATLAS, efficienciesindataandinMonte-Carlosimulations.They for two pile-up conditions corresponding to N =4 and arealsousedtosettheelectronandthemuonenergyscales, VX N =8.Withhigherpile-uponeobservesabroadeningof andwherenecessaryto improvethe energy(electrons)or VX the distribution, and a shift of the mean value. The lat- themomentum(muons)reconstruction. ter can be subtracted from an estimate of the “ambient” Earlyphysicsresultswereobtainedwiththe2010data, pile-up level, but the broadening of course will stay. The illustrated by Fig. 11 which shows the W and Z fiducial EPJWebofConferences cross-sectionsinATLAS[10],comparedtocalculationsat is an importantasset for manyphysicsobjectives.Indeed theNNLOusingdifferentPDFsets.Alsoparticularlysen- a worsening of missing momentumresolutionwill some- sitiveto thePDFsisthe chargeasymmetryofleptonicW whatdegradethesituation.Thet performancesinATLAS decays. The variation of this asymmetry with the lepton aresimilartoCMS. pseudo rapidity(see Fig. 12) reflects combinedeffectsof PDFs,WpolarizationandV-AdecayoftheW.Thesignin- versionaround h =3.0,whichfallsintotheacceptanceof | | theLHCbexperimentisnicelyreproducedbysimulations. Largerdata sets, as now available, will certainly improve theknowledgeofPDFs. Fig.13.Zdecaysreconstructedinthelepton-hadrondecaymode (CMS). 3.4 B-tagging and top physics EfficientB-taggingisakeytotopphysics,tosomeHiggs Fig.11.WandZfiducialcross-sectionsinATLAS,comparedto channels,... Already commissioned with 2010 data, “ad- NNLOsimulations. vancedtaggingmethods”werevalidatedwiththe2011data set. AmongthemATLASusesacombinationofthetrack impact parameter in 3D (IP3D) and of a fit of secondary vertices(SV1).At60%efficiency,thiscombinedapproach has a rejection 4 times larger than the early “SV0” algorithm[12].Fig.14illustratesthisperformancebyshowing thefractionofjetssatisfyingtheb-taggingcutat60%efficiency,comparedtoMonte-Carlosimulation.Theagree- mentissatisfactory,andshowsthat,around100GeV,50% oftheeventspassingthecutaregenuineb-jetswhile60% oftheremainingonesareactuallycharmedjets. Fig.12.LeptonicchargeasymmetryinWdecays(ATLAS,CMS, LHCb)comparedtoNLOsimulations. Zproductionisthebestpossibleplacetoassesstheef- ficiencyandtheaccuracyoft decaysreconstruction.The triggerefficiencyisthefirstproblemtobeovercome,given the large fraction of transverse momentum taken by the neutrinosinthefinalstate.Toreachlowenoughthresholds, doubleconditionsarerequired,likeillustratedforexample inFig.6forthem -hadfinalstate.Thehadronic“t -jets”are Fig.14.Fractionofb-jetstaggedby“IP3D+SV1”inATLAS. separated fromelectron showersand from jets by a com- bination of criteria on chargedtracks (1 or 3), on shower Theproductionoftt¯pairswas alreadymeasuredwith shapes in the electromagnetic and hadronic calorimeters, 2010 data, both in the single lepton and in the dilepton and on isolation. Finally to isolate Z decays, a minimum modes,withandwithoutb-tagging.Withthefirst0.7fb 1 Emiss isrequired.Fig. 13showsthespectrumobtainedby − T of 2011 data, ATLAS measured the cross section using CMS [11] in the muon-hadron channel, still with 2010 bothmodes[13]. data.Thecleanlinessofthesignal,togetherwithaZcross- sectioninthett mode(s .BR=1.0 0.05(stat) 0.08(syst) 0.04(lumi) nb) which matches±the other le±ptonic de- s tt¯ =179.0 3.9(stat) 9(syst) 6.6(lumi) pb ±caymodesdemonstratethatt decaysaremastered,which ATLAS ± ± ± (4) HadronColliderPhysicsSymposium2011 TopphysicsisalsoanimportantpartoftheCMSpro- channel [17] are shown below as an example, CMS hav- gram.Theexperimentmeasuredthecross-section[14] ing similar performances. Events are triggered by either anelectronoramuonofhightransversemomentum.The analysis requires 4 leptons of p >15 GeV. At least two T s tt¯ =166 2.2(stat) 11(syst) 8(lumi) pb (5) pairs of opposite charge need to fall in the Z mass win- CMS ± ± ± dow(66 100 GeV) 75%prompt gg and 25% g -jets, with much smaller co∼ntributions of jet-jetan∼dDrell-Yane+e pairs. − Thankstothesamplingsindepth,electromagneticcalo- rimeterdataaloneallowtomeasurethepolarangleofeach photon, and the space angle q between the two photons. Theaccuracyofthismeasurementisillustratedbythedif- ference in the longitudinalposition of the primary vertex found by intersecting the beam line successively by each of the two photon’sdirection. This differencehas an rms of30mm,thuscorrespondingtoaprimaryvertexaccuracy Fig.18.95%CLupperlimitsfor“combined”Higgssearchesin of 15mm, well below the longitudinal spread of primary ATLAS. vertices(about56mm),andaccurateenoughtogiveacon- tributiontothemgg resolutionnegligiblecomparedtothe 282 to 296 GeV). See Fig. 18 for the expected (131-447 effectofthephotonenergiesresolution.Whenoneorboth GeVintheabsenceofaSMHiggssignal)andtheobserved ofthephotonsareconvertedintheinnerdetectorvolume, limits. the coordinate of this conversion point is used, with the - CMS excludes at 95% CL that the SM Higgs boson be shower barycentre,to give an even more accurate photon between145and400GeV,withtheexceptionoftwonar- direction. row bands, different from the ATLAS ones, (216 to 226 GeV and 288 to 310 GeV). See Fig. 19 for the expected limits(130-440GeV)andtheobservedones. Fig.19.95%CLupperlimitsfor“combined”Higgssearchesin Fig.20.ATLASgg massspectrumandassociatedlimit. CMS. The main message from these two results is that the Theenergyresponseofthecalorimeteriscalibratedus- best motivated low mass region (the EW fits give ingZ0 decaysine+e pairs.Monte-Carlosimulationsare − m <161 GeV at 95% CL) is still open to exploration, used to take into account the small differences in H whileawide“medium/high”massrangeisexcluded. response between electrons, converted photons, and un- Whilethehighmassrange(above 450GeV)should converted photons. With the available statistics, the cali- notbeprematurelydiscarded,itisclea∼rthatthelargestef- bration was made by bins of dh =0.1, without any subdi- fortintheshorttermwillbedevotedtothelowmassregion vision in azimuth.By comparingthe width of the Z0 line (m < 145 GeV), where the main channels are shape toMonte-Carlosimulations,thisprocedurealso al- H H gg , H tt , H ZZ 4l, H WW llnn lows to estimate the “constant term” of the energy ∗ ∗ and→VH, H →bb¯,V →ll,ln or→nn . → → resolution,which,forthedatasetconsideredforthisopti- Giventh→eextreme→importanceofthesechannels,andin mization(2010dataworth36pb 1)was1.1 0.5%inthe − particularofthefirstthreewhichcangiverisetoanarrow barrel(h < 1.37)and1.8 0.6%in the en±d-caps(1.52 masspeak,ashortreviewoftheexpectedperformanceof < h <| 2|.37).The estimate±d averagegg mass resolution | | theexperimentsisgivenbelow. isabout1.7GeVformgg =130GeV. HadronColliderPhysicsSymposium2011 The gg spectrum obtained with 1.08 fb 1 of data is counttakenoftherelativeamountofdataanalyzedbyeach − showninfig.20.The95%CLlimit,normalizedtotheSM experimentatthetimeoftheConference. Higgscross-section,timesthe branchingratioto thetwo- photonfinal state, is givenin a small insert, showingthat wabiothuTtthh4eistmimaamiensoetufhnfeotrSotMfondHaAtiTagLgthsAecSreopxsepsre-fsroeirmcmteiaonnntc.wesarseslaetnesdittiovethtios 2)2 GeV/c678000000 BR(H()pb)σ×→γγ95%CL0000...00234...455523CsM =S 7 p TreelVim Li n=a 1ry.66 fb-1 ±±OOM bbe12σσssd eeiEEarrxnxvvpp eeEeddexcc CBptteeeLaddcys te CCeLsLdLiimas sCnitL Lsi mLiimtit D212 appfatrrakooemm γpptt γγ 1 fake γ codhlautaatniotnhnea,lnpisuasrtoteidciuminlaptrrhloyevcienutrhtrheeencteoZnnd0s-tlcainanpetssth.eraWmpietohofp1tt0him0eitezinmaeteirogsnym,rioetrsies- vents / (450000 00..001.5501110 115 120 125 130 135 140 m1H4 (5GeV/c12)5015××σσSSMM Drell-Yan E CMS preliminary hopedthatthenominal0.7%constanttermwillbereached. 300 s = 7 TeV L = 1.66 fb-1 Theeffectofhigherpile-up(inparticularonisolation-see 200 section3.2)alsoneedstobeassessed. Theuseofadditionalvariables(transversemomentum 100 ofthegg system,presenceofadditionaljets,decayangles 0 80 90 100 110 120 130 140 150 160 170 180 inthegg system)arealsobeinginvestigated. m (GeV/c2) γγ Fig.21.CMSgg massspectrumandassociatedlimit. 4.2 Two-photon final state in CMS The absence of longitudinal segmentation in the PbWO4 4.3 Four-lepton final state in ATLAS crystal calorimeter of CMS imposes to combine the in- teraction vertex position with the shower positions in the The ZZ 4l final state combines low background and calorimetertodeterminethespaceanglebetweenthetwo ∗ → precision mass reconstruction, the main drawback being photons.Thevertexisselectedonthebasisofthesumof thep2 ofthetracksassociatedtoeachreconstructedvertex, thesmallbranchingratio.NonresonantZZproductionas T consideredinsection2.5isanirreduciblebackground,while combinedwiththep balancebetweenthetracksandthe two-photon system. FTor =6.5 correspondingto the Zbb¯andtt¯arethemainotherbackgrounds,reducedbyiso- lation and impact parameter requirements. The key point analyzeddataset,itwasestimatedbyMonte-Carlosimula- whenaddressingthelowHiggsbosonmassrangeisthere- tionthatin83%ofthecases,theselectedvertexiswithin constructionandidentificationefficienciesofleptons,and 10mmof the true vertex,a distance small enoughto give in particular electrons of transverse momenta down to 5 a negligible contributionto the invariantmass resolution. to 7 GeV. In an extended sample as compared to section Photonidentificationisbasedonshowersizeandisolation. 3.5 (up to 2.28 fb 1), 27 events were selected (6ee, 9em Asopposedto ATLAS,the sum oftracksp in the isola- − tioncone(D R=0.3)isusedonitsown,andcTombinedwith and12mm )withoneZmassrequirement(76to106GeV) while28 4wereexpected.Oneeventonlyhadamassbe- thetransverseenergyintheelectromagneticcalorimeteras ± low140GeV,asshowninFig.22. discriminatingvariable.Thecutvaluesareadjustedtogive thebestS/Bratioforaparticularsignalphotonefficiency. Intheend,thesamplepurityissimilartoATLAS. Inordertoeliminatecrystaltransparencyvariationsas a function of the luminosity integrated in the preceding fewhours/days,correctionfactorsaredeterminedfromthe crystalresponsetolaserpulsesdistributedoverthecalorime- terduringpartofthecyclewithoutcollisions(“abortgap” inparticular)[21].Thecorrectionsrangewasuptoabout 10% for the analysed data set. As in ATLAS, the Z0 line shape is used to set the energy scale, improve the cali- bration,anddeterminetheconstantterm.Theresolutionis then transported by Monte-Carlo to the two-photon final state. It ranges from less than 1.5 GeV for barrel-barrel eventstoabout3GeVforbarrel-endcapevents. The resulting gg spectrum is shown in Fig. 21. The 95%CLlimit,normalizedtotheSMHiggscross-section, timesthebranchingratiotothegg finalstateisgivenina smallinsert,showingthatwith1.70fb 1ofdatatheexper- Fig.22.ATLAS4lmassspectrum. − imentwassensitivetoabout3timestheSM Higgscross- section.Intermsofdetectorperformance,CMSismaking abigefforttoreachthenominalconstanttermwhichhad The main performanceeffortforthis channel,bothin beensetto0.5%. ATLASand CMS, is towardsimprovingthe efficiencyof Comparingthetwoexperimentsonecanseethat,com- soft leptons. On the analysis point of view the very low biningefficiency,backgroundrejection,accuracyinenergy mass range(below 140 GeV) may benefitfrom consider- andangularmeasurements,theirsensitivitiesareatpresent ingeventswherethetwo Z areoffmassshell,an attempt quitesimilar,asillustratedbytheexpectedCLslimits,ac- alreadymadebyCMS(seetalksatthisConference). EPJWebofConferences 4.4 Higgs tt in CMS 5.1 B mm s → → Thisdecaymodeisaveryusefulcomplementtogg inthe IntheStandardModelthischannel,whichhassimilarities withthehistoricallyfamousK mm decay,ispredicted lowmassrange,andakeychannelfortheMSSMHiggsfor L apulabrlgicer[a2n2g]eaonfaMnaAlyansidstwanitbhv1a.l6uefsb.R1ecoefndtlayt,aC, MusSinmgathdee mtoehtrayveisarbearalinzcehdinagsirnattihoeoMf(S3S.2M→±,0w.2it)h1l0ar−g9e.Iefnsouupgehrstaynmb- − e-m ,e-hadandm -hadfinalstates,triggeredbyeitheralep- values, the branching ratio can significantly be increased tonoraleptonplusat -jet(seesection3.3).Thet -jetsare bythecontributionofadditionalparticlesintheloops(see Fig.24)whichgoesliketanb 6. reconstructed using particle flow and identified as briefly describedin3.3.OneofthemainbackgroundsisW+jets, where the W decay leptonically, and the jet is misidenti- fied as a t -jet. This backgroundis rejected by cutting on theEmiss projectedontothebisectorofthetwot “visible” T decay products. The analysis is made using the invariant massofthetwot visibledecayproducts. ThesensitivitytotheSMHiggsisenhancedbytreating separatelyeventswith2additionaljetsinthefinalstate,in aconfigurationcompatiblewithHiggsproductionbyVec- torBosonFusion(Dh >3.5andMjj>350GeV).No jj | | significantexcessoverbackgroundisseen,allowingtoput a95%CLlimitat 10timestheSMHiggscross-section ∼ forM =130GeV,whiletheexpectedvaluerangedfrom H around7at110GeVuptoabout9at140GeV. The sensitivity to the MSSM is enhanced by treating separatelyeventswithatleastoneb-taggedjet.Noexcess Fig.24.SUSYdiagramscontributingtoBs mm . → overbackgroundisseen,allowingtoruleout,at95%CL,a largefractionoftheMSSMspace,asshowninFig.23.Itis interestingtonotethatforM <130GeVthewholetanb In CMS the analysisrequirestwo muonsof pT larger A than 4 GeV, isolated and with a highly significant flight rangeisalreadyruledout.ThesuperiorityofLHCoverthe path (L/s > 15 (barrel) or >20 (End-Caps)). Events are Tevatroninthischannelisalsoexemplifiedbycomparing countedinawindowof 75MeVaroundthenominalB the CMS limit to the D0 limit obtainedwith a more than s ± mass. The event count was found compatible with back- fourtimeslargerdataset. ground only, leading to an upper limit on the branching ratioof1.910 8atthe95%CL,foradatasetof1.14fb 1 − − [23].AsimilaranalysisinLHCb[24]ledtoalimitof1.5 10 8. With the additionof the data sets notyetanalyzed, − andimprovedanalysis,thestandardmodellimitisbecom- ingatargetsoonwithinreach. 5.2 Search for s-quarks and gluinos Searchforsupersymmetricparticleswasoneoftheprior- ity topics when data at the LHC became available. In the R-parityconservingscenarios,standardsearchesrequirea large Emiss together with several jets. Different channels T canbeaddresseddependingonthepresenceofoneormore Fig.23.ExcludedMSSMparameterspaceinCMS,usingthet leptonsin the final state, of the same sign, or of opposite t channel. − sign. A summary of the limits obtained by CMS [25] with Themain effortfor the tt final state, both in ATLAS up to 1.1fb−1 ofdata, is shownin Fig. 25. One observes (no results shown here) and CMS will be to maintain the thatthemorestringentlimitsaregivenbythe“Jets+EmTiss” performancewithvaluesofreaching20andabove. channel, for which the limit is slightly above 1 TeV for bothsquarksandgluinos,ifoneassumestheyhavesimilar masses. The limit by ATLAS (1.07 TeV) is similar. The gainofsensitivitywithrespecttotheTevatronisstriking. 5 Search for new Physics Future searches will address more exclusive final states, like s-tops,... until the energy of the LHC is significantly Thesearchforexcitedquarkstates,axigluons,andW’was increased. already mentioned in section 3.1. Only two examples of searchesforSUSYeffects,eitherdirectorindirectarebriefly describedbelow (beyondthe MSSM Higgsconsideredin 5.3 Search for heavy vector bosons (Z’) 4.4)toillustratethepresentsituation,andthestatusofthe searchforZ’inleptonpairsisgiven.Inallcases,formore Dilepton final states (ee or mm ) provide easy trigger and recentupdates,seededicatedtalksattheConference. cleansignaturesforthesearchofheavyrecurrencesofthe

Performance of the LHC, ATLAS and CMS in 2011

Similar Performance of the LHC, ATLAS and CMS in 2011