CMS will use dijets to search for physics beyond the standard model during
early LHC running. The inclusive jet cross section as a function of jet
transverse momentum, with 10 inverse picobarns of integrated luminosity, is
sensitive to contact interactions beyond the reach of the Tevatron. The dijet
mass distribution will be used to search for dijet resonances coming from new
particles, for example an excited quark. Additional sensitivity to the
existence of contact interactions or dijet resonances can be obtained by
comparing dijet rates in two distinct pseudorapidity regions.Comment: 10 pages, 4 figures, accepted for publication in J. Phys. G: Nucl.
  Part. Phy

Anwar Bhatti

Benjamin Bollen

CMS Collaboration

David Mason

Esen S Harris R

Frank Chlebana

Gumus K Akchurin N Esen S Harris R

Hatakeyama K (CDF Collaboration)

Konstantinos Kousouris

Manoj K Jha

Marco Cardaci

Marek Zielinski

Pumplin J

Robert M Harris

Selda Esen

Sjostrand T Lonnblad L Mrenna S

English

arXiv

CMS will use dijets to search for physics beyond the standard model during early LHC running. The inclusive jet cross section as a function of jet transverse momentum, with 10 inverse picobarns of integrated luminosity, is sensitive to contact interactions beyond the reach of the Tevatron. The dijet mass distribution will be used to search for dijet resonances coming from new particles, for example an excited quark. Additional sensitivity to the existence of contact interactions or dijet resonances can be obtained by comparing dijet rates in two distinct pseudorapidity regions

Bhatti, Anwar

Bollen, Benjamin

Cardaci, Marco

Esen, Selda

Jha, Manoj K.

Chlebana, Frank

Harris, Robert M.

Kousouris, Konstantinos

Mason, David

Zielinski, Marek

CERN Document Server

arXiv:0807.4961v2  [hep-ex]  6 Oct 2008C M S search plans and sensitivity to new physicsw ith dijetsA nw ar B hatti1,B enjam in B ollen2,M arco C ardaci2,FrankC hlebana3,Selda Esen4,R obert M .H arris3,M anojK .Jha5,K onstantinos K ousouris3,D avid M ason3 and M arek Zielinski61 RockefellerUniversity,New York,NY,USA2 UniversiteitAntwerpen,Antwerp,Belgium3 Ferm ilab,Batavia,IL,USA4 Brown University,Providence,RI,USA5 University ofDelhi,India and INFN,Bologna,Italy6 University ofRochester,Rochester,NY,USAE-m ail:rharris@fnal.govA bstract. CM S willuse dijets to search for physics beyond the standard m odelduringearlyLHC running.Theinclusivejetcrosssection asafunction ofjettransversem om entum ,with 10 pb  1 ofintegrated lum inosity,issensitiveto contactinteractionsbeyond thereach oftheTevatron.Thedijetm assdistribution willbeused tosearch fordijetresonancescom ing from new particles,forexam plean excited quark.Additionalsensitivity to the existenceofcontactinteractionsordijetresonancescan beobtainedby com paring dijetratesin two distinctpseudorapidity regions.PACS num bers:12.38.Q k,12.60.Rc,13.87.CeSubm itted to:J.Phys.G:Nucl.Part.Phys.CM S search plansand sensitivity to new physicswith dijets 2TheLargeHadron CollideratCERN willproducem any eventswith two energeticjetsresulting from proton-proton collisionsatps = 14 TeV.These dijeteventsresultfrom parton scattering,produced by thestrong interaction ofquarks(q)and gluons(g)insidetheprotons.Thispaperdiscussesplansto usedijetsin thesearch fortwo signalsofnew physics: contactinteractionsand resonancesdecaying into dijets. Two m odelsofquark com positenesshavebeen considered forthisgenericsearch.Therstm odelisa contactinteraction [1]am ong left-handed quarksatan energy scale+ in theprocessqq! qq,m odeled with theeectiveLagrangian L qq = (2=2)(qLqL)(qLqL)with+ chosen forthesign.Thesecond isa m odelofan excited quark (q*)[2]in theprocessqg ! q ! qg,detectable asa dijetresonance.Allprocessespresented herehave beensim ulated using PYTHIA version 6.4 [3].A detailed description ofthe Com pact M uon Solenoid (CM S) experim ent can befound elsewhere [4,5].The CM S coordinatesystem hasthe origin atthe centerofthedetector,z-axispointsalong the beam direction toward the west,with the transverseplane perpendicular to the beam . W e dene  to be the azim uthalangle, to bethe polarangle and the pseudorapidity as  =   ln(tan[=2]). The centralfeature ofthe CM S apparatus is a superconducting solenoid,of6 m internaldiam eter. W ithinthe eld volum e are the silicon pixeland strip tracker, and the barreland endcapcalorim eters (jj < 3): a crystalelectrom agnetic calorim eter (ECAL) and a brass-scintillator hadronic calorim eter (HCAL). Outside the eld volum e, in the forwardregion,there is an iron-quartz ber hadronic calorim eter (3 < jj< 5). The HCALand ECAL cellsare grouped into towers,projecting radially outward from the origin,fortriggering purposesand to facilitatethejetreconstruction.In theregion jj< 1:74these projective calorim eter towers have segm entation  =  = 0:087,and the and  width progressively increases at higher values of. The energy in the HCALand ECAL within each projectivetowerissum m ed tond thecalorim etertowerenergy.Towers with jj< 1:3 contain only cells from the barrelcalorim eters,towers in thetransition region 1:3 < jj< 1:5 contain a m ixture ofbarreland endcap cells,andtowersin theregion 1:5< jj< 3:0 contain only cellsfrom theendcap calorim eters.Jetsare reconstructed using both the iterative and m idpointcone algorithm s[5],with indistinguishable results for this analysis. Below we willdiscuss three types ofjets: reconstructed, corrected and generated. The reconstructed jet energy, E , isdened as the scalar sum of the calorim eter tower energies inside a cone of radiusq()2 + ()2 = 0:5, centered on the jet axis. The jet m om entum , ~p, is thecorresponding vector sum : ~p =PE iu^i with u^i being the unit vector pointing fromthe origin to the energy deposition E i inside the cone. The jettransverse m om entum ,pT,is the com ponent of~p in the transverse plane. The E and ~p ofa reconstructedjetarethen corrected forthenon-linearresponseofthecalorim eterto a generated jet.Generated jets com e from applying the sam e jet algorithm to the Lorentz vectors ofstablegenerated particlesbeforedetectorsim ulation.On average,thepT ofa correctedjetisequaltothepT ofthecorrespondinggenerated jet.Thecorrectionsestim ated froma GEANT [6]sim ulation ofthe CM S detector increase the average jetpT by roughlyCM S search plansand sensitivity to new physicswith dijets 350% (10% )for70 GeV (3 TeV) jets in the region jj< 1:3. The applied correctionsdepend on jet  as wellas pT. The jet m easurem ents presented here are within theregion jj< 1:3,wherethesensitivity to new physicsisexpected to bethehighest,andwhere the reconstructed jet response variations as a function of are both m oderateand sm ooth. Further details on jet reconstruction and jet energy corrections can befound elsewhere [5,7].Thedijetsystem iscom posed ofthetwojetswith thehighestpT in an event(leadingjets),and thedijetm assisgiven by m =q(E 1 + E 2)2   (~p1 + ~p2)2.Theestim ated dijetm assresolution variesfrom 9% ata dijetm assof0.7 TeV to 4.5% at5 TeV.CM S will record events that pass a rst level trigger followed by a high leveltrigger. For an instantaneous lum inosity of 1032 cm   2s  1, we consider three eventsam plescollected by requiring atleastone jetin the high leveltriggerwith correctedtransverse energy above 60,120 and 250 GeV,prescaled by factorsof2000,40 and 1,respectively. For an integrated lum inosity of100 pb  1,the three event sam ples willeectively correspond to 0.05,2.5,and 100 pb  1.Thersteventsam plewillbeused tom easurethetriggereciency ofthesecond sam ple.Thesecond and third eventsam pleswillbeused to study dijetsofm assabove330 and 670 GeV,respectively,forwhich thetriggerecienciesareexpected to behigherthan 99% [8].Backgrounds from cosm ic rays, beam halo, and detector noise are expected tooccasionally produce events with large or unbalanced energy depositions. They willbe rem oved by requiring 6E T=PE T < 0:3 andPE T < 14 TeV,where 6E T (PE T) isthe m agnitude ofthe vector (scalar) sum ofthe transverse energies m easured by allcalorim etertowersin theevent.Thiscutisestim ated to bem orethan 99% ecientforboth QCD jeteventsand the signalsofnew physicsconsidered. In the high pT regionrelevantforthissearch,jetreconstruction isfully ecient.CM S planstosearch forcontactinteractionsusingthejetpT distribution.Figure1showssim ulationsofthe inclusive jetdierentialcrosssection asa function ofpT,forjetswith jj< 1.Considering rsttheQCD processes,thereconstructed and correctedquantitiesarecom pared with theQCD prediction forgenerated jets.Aftercorrections,thereconstructed and generated distributionsagree.Theratioofthecorrected jetcrosssection to thegenerated jetcrosssection variesbetween 1.2 atpT = 100 GeV and 1.05atpT = 500 GeV,rem aining roughly constantforhigherpT.Thedeviation ofthisratiofrom 1 isattributed to thesm earing eectofthejetpT resolution on thesteeply fallingspectrum . The m easured spectrum in data could be further corrected for resolutionsm earing,and this ratio from sim ulation isan estim ate ofthe size ofthatcorrection.Them easurem entuncertaintiesarepredom inantly system atic.Theinsetin Fig.1showsthe eecton the jetrateofa 10% uncertainty in the jetenergy correction. Fig.2 alsoshows the eect ofthis uncertainty on a lowest orderQCD calculation. This levelofjetenergy uncertainty could beexpected in early running,foran integrated lum inosityaround 10 pb  1.Thisexperim entaluncertainty isroughly an orderofm agnitudelargerthan the uncertainties from parton distributions,asestim ated using CTEQ6.1 ts[9]and shown in Fig.2.Figures1 and 2 show thattheeectofnew physicsfrom acontactCM S search plansand sensitivity to new physicswith dijets 4interaction with scale + = 3 TeV is convincingly above what could be expected form easurem entuncertaintieswith only 10 pb  1. Forcom parison,a Tevatron search hasexcluded contact interactions with scales + below 2.7 TeV [10]. The results ofthelowestordercalculationsin Fig.2 arethesam easthesim ulation resultsin theinsettoFig.1.CM S plansto search fornarrow dijetresonancesusing thedijetm assdistribution.Figure 3 showsthe dierentialcrosssection versusdijetm ass,where both leading jetshavejj< 1,and them assbinshaveawidth roughly equaltothedijetm assresolution.Considering rsttheQCD processes,thecrosssection forcorrected jetsagreeswith theQCD prediction from generated jets. To determ ine the background shape either theM onte Carlo prediction ora param eterized tto the data can be used. The inset toFig.3 showsa sim ulation ofnarrow dijetresonanceswith a q* production crosssection.Forq* m assesof0:7,2:0 and 5:0 TeV thecrosssectionsforjetjj< 1 are795,9:01 and0:0182 pb,respectively. This is com pared to the statisticaluncertainties in the QCDprediction,including triggerprescaling.Thiscom parison showsthatwith an integratedlum inosity of100 pb  1 a q* dijet resonance with a m ass of2 TeV would produce aconvincing signalabove the statisticaluncertainties from the QCD background. Forcom parison,a Tevatron search has excluded q* dijetresonances with m ass,M ,below0:87 TeV [11]. The heaviestdijetresonancesthatCM S can discover(atve standarddeviations) with 100 pb  1 ofintegrated lum inosity, using this search technique andincluding the expected system atic uncertainties [12,13],are: 2.5 TeV forq*,2.2 TeVforaxigluons[14]orcolorons[15],2.0 TeV forE 6 diquarks[16],and 1.5 TeV forcoloroctettechnirhos[17].Studiesofthejet cuthaveconcluded thattheoptim alsensitivityto new physicsisachieved with jj< 1:3 fora 2 TeV spin 1 dijetresonancedecaying toqq [18].CM S plansto search forboth contactinteractionsand dijetresonancesusing thedijetratio,r= N (jj< 0:7)=N (0:7< jj< 1:3),whereN isthenum berofeventswithboth jets in the specied jjregion. The dijet ratio is sensitive to the dijet angulardistribution. For the QCD processes, the dijet ratio is the sam e for corrected jetsand generated jets, and is constant at r = 0:5 for dijet m asses up to 6 TeV [18].Figure4 showsthedijetratio from contactinteractionsand dijetresonances,com paredtotheexpected statisticaluncertainty on theQCD processes,for100pb  1 ofintegratedlum inosity,includingtriggerprescaling.Thesignalfrom acontactinteraction with scale+ = 5 TeV riseswellabove theQCD statisticalerrorsathigh dijetm ass.System aticuncertaintiesin thedijetratioareexpected tobesm all,sincethey predom inantly cancelin the ratio aspreviously reported [12,19]. Using the dijetratio,CM S can discoveracontactinteraction atscale + = 4,7 and 10 TeV with integrated lum inositiesof10,100,and 1000 pb  1,respectively [18]. The signalfrom a 2 TeV spin 1/2 q* producesa convincing peak in the dijet ratio,because ithas a signicant rate and a relativelyisotropic angular distribution com pared to the QCD t-channelprocesses. Fixing thecrosssection ofthe2 TeV dijetresonanceforjj< 1:3 at13:6 pb (from theq* m odel),thedijetratioin thepresenceofQCD background increasesby approxim ately 6% whenCM S search plansand sensitivity to new physicswith dijets 5considering a spin 2 resonancedecaying to both qqand gg (such asa Randall-Sundrumgraviton [20]),and the dijetratio decreases by approxim ately 4% when considering aspin 1 resonance decaying to qq (such as a Z0,axigluon,or coloron) [18]. Hence,thesensitivity toa2TeV dijetresonancedependsonly weakly on thespin oftheresonance.To m easurethespin,weneed both thedijetratio and an independentm easurem entofthe cross section ofthe resonance,for exam ple,from the dijet m ass dierentialcrosssection.Nevertheless,with sucientlum inosity,thissim plem easureofthedijetangulardistribution,ora m orecom pleteevaluation oftheangulardistribution,can beused toseethesesm allvariationsand inferthespin ofan observed dijetresonance.In conclusion,CM S plansto use m easurem entsofrateasa function ofjetpT anddijetm ass,aswellasaratioofdijetratesin dierent regions,tosearch fornew physicsin the data sam ple collected during early LHC running. W ith integrated lum inositysam ples in the range 10{100 pb  1,CM S willbe sensitive to contact interactions anddijetresonancesbeyond thosecurrently excluded by theTevatron.A cknow ledgm entsW e gratefully acknowledge the assistance ofour colleagues at CM S in preparing thispaper. For their constructive com m ents and guidance, we would like to thank ouranalysis review com m ittee: Richard Cavanaugh, Dan Green and Luc Pape. W eappreciate the m any usefulcom m entson prose and style provided by CarlosLourencoand theCM S publicationscom m ittee.Finally,wewould liketo thank GigiRolandiforhisencouragem ent and forleading the collaboration wide review ofthispaper,whichserved asa testofthepublication processatCM S.R eferences[1]Eichten E,LaneK and Peskin M 1983 Phys.Rev.Lett.50 811[2]BaurU,Hinchlie Iand Zeppenfeld D 1987 Int.J.M od.Phys.A 2 1285[3]Sjostrand T,Lonnblad L and M renna S 2008 preprinthep-ph/0108264[4]CM S Collaboration 2008 The CM S experim entatthe CERN LHC(accepted for publication in JINST)[5]CM S Collaboration 2006 PhysicsTDR Volum e IC ER N -LH C C -2006-001[6]AgostinelliS etal.(G EANT4 Collaboration)2003 Nucl.Instrum .M eth.A506 250[7]CM S Collaboration 2008 CM S Physics AnalysisSum m ary JM E-07-003 athttp://cm s-physics.web.cern.ch/cm s-physics/public/JM E-07-003-pas.pdf[8]Esen S and HarrisR 2006 CM S Note 2006/069[9]Pum plin J etal.(CTEQ Collaboration)2002,JHEP 07 012[10]AbbottB etal.(D0 Collaboration)1999 Phys.Rev.Lett.82 2457[11]Hatakeyam a K etal.(CDF Collaboration)2008 recentpublic resultsathttp://www-cdf.fnal.gov/physics/exotic/r2a/20080214.m jjresonance 1b/[12]CM S Collaboration 2006 J.Phys.G:Nucl.Part.Phys.34 995[13]G um usK ,Akchurin N,Esen S and HarrisR 2006 CM S Note 2006/070[14]BaggerJ,Schm idtC and K ing S 1988 Phys.Rev.D 37 1188[15]Sim m onsE 1997 Phys.Rev.D 55 1678[16]HewettJ and Rizzo T 1989 Phys.Rep.183 193 and referencesthereinCM S search plansand sensitivity to new physicswith dijets 6[17]LaneK and M renna S 2003 Phys.Rev.D 67 115011[18]CM S Collaboration 2007 CM S Physics AnalysisSum m ary SB M -07-001 athttp://cm s-physics.web.cern.ch/cm s-physics/public/SBM -07-001-pas.pdf[19]Esen S and HarrisR 2006 CM S Note 2006/071[20]RandallL and Sundrum R 1999 Phys.Rev.Lett.83 3370CM S search plansand sensitivity to new physicswith dijets 7 (GeV)TJet p0 500 1000 1500 2000 2500 3000 3500 (pb/GeV)T/dpσd-710-610-510-410-310-210-110110210310410 (GeV)TJet p200 400 600 800 1000 1200 1400Number of Jets / 50 GeV110210310410510610CMS SimulationFigure 1. The inclusive jet pT dierentialcross section expected from Q CD forjj< 1,for generated jets (points),reconstructed jets (triangles),and corrected jets(open circles).Theinsetshowsthenum berofgenerated jetsexpected in 50 G eV binsforan integrated lum inosityof10pb  1.ThestandardQ CD curve(solid)ism odied byasignalfrom contactinteractionswith scale+ = 3TeV (dotted)and 5TeV (dashed).Theshaded band representsthe eectofa 10% uncertainty on the jetenergy scale.CM S search plansand sensitivity to new physicswith dijets 8 (GeV)TJet p200 400 600 800 1000 1200 1400Fractional Difference from QCD-1-0.500.511.522.53LO CalculationsQCD & Stat. Err.Energy Err. (10%)PDF Err. (CTEQ 6.1)  = 3 TeV+Λ = 5 TeV+ΛFigure 2. The fractionaldierence from the Q CD jet rate resulting from a 10%uncertainty on the jet energy scale (dashed), uncertainties in parton distributions(dotted),and signals from contact interactions with scale + = 3 TeV (boxes) and+ = 5TeV (triangles).Statisticaluncertaintiesexpected foran integrated lum inosityof10 pb  1 (verticalbars)areshown on the Q CD prediction (points).CM S search plansand sensitivity to new physicswith dijets 9Dijet Mass (GeV)0 1000 2000 3000 4000 5000 6000 7000/dm (pb/GeV)σd-610-510-410-310-210-110110210310410CMS SimulationDijet Mass (GeV)0 1000 2000 3000 4000 5000 6000 7000(Signal-QCD)/QCD-0.100.10.20.30.40.50.60.7Figure 3. Thedijetm assdierentialcrosssection expected from Q CD forjj< 1fromgenerated jets(points),reconstructed jets(triangles),and corrected jets(open boxes).The inset shows dijet resonancesreconstructed using corrected jets,com ing from q*signals[13]ofm ass0:7,2,and 5 TeV.The fractionaldierence (histogram )betweenthe q* signaland the Q CD background iscom pared to the statisticaluncertaintiesinthe Q CD prediction (verticalbars)foran integrated lum inosity of100 pb  1.CM S search plansand sensitivity to new physicswith dijets 10Dijet Mass (GeV)500 1000 1500 2000 2500 3000 3500|<1.3)η|<0.7) / N(0.7<|ηDijet Ratio = N(|0.30.40.50.60.70.80.911.1CMS SimulationFigure 4. The dijet ratio for corrected jets expected from Q CD (horizontalline),with statisticaluncertainties(verticalbars)foran integrated lum inosity of100 pb  1,iscom pared to Q CD + contactinteraction signalswith a scale + = 5 TeV (dashed)and 10 TeV (dotted),aswellasto Q CD + dijetresonancesignals(histogram )with q*m assesof0.7 and 2 TeV.

CMS search plans and sensitivity to new physics with dijets

Crossref

CMS search plans and sensitivity to new physics with dijets

Abstract

Similar works

Full text

Available Versions

CERN Document Server

Crossref