Dilepton-jet final states are used to study physical phenomena not predicted
by the standard model. The ATLAS discovery potential for leptoquarks and
Majorana Neutrinos is presented using a full simulation of the ATLAS detector
at the Large Hadron Collider. The study is motivated by the role of the
leptoquark in the Grand Unification of fundamental forces and the see-saw
mechanism that could explain the masses of the observed neutrinos. The analysis
algorithms are presented, background sources are discussed and estimates of
sensitivity and the discovery potential for these processes are reported.Comment: 6 pages, 16 figures, To be published in the proceedings of DPF-2009,
  Detroit, MI, July 2009, eConf C09072

Bansal, Vikas

English

arXiv

Dilepton-jet final states are used to study physical phenomena not predicted by the standard model. The ATLAS discovery potential for leptoquarks and Majorana Neutrinos is presented using a full simulation of the ATLAS detector at the Large Hadron Collider. The study is motivated by the role of the leptoquark in the Grand Unification of fundamental forces and the see-saw mechanism that could explain the masses of the observed neutrinos. The analysis algorithms are presented, background sources are discussed and estimates of sensitivity and the discovery potential for these processes are reported

CERN Document Server

arXiv:0910.2215v1  [hep-ex]  12 Oct 2009Proceedings ofthe DPF-2009 Conference,Detroit,M I,July 27-31,2009 1ATLAS Sensitivity to Leptoquarks, W R and Heavy Majorana Neutrinosin Final States with High-pT Dileptons and Jets with Early LHC Data at14 TeV proton-proton collisionsVikas Bansal (ATLAS Collaboration)University of Pittsburgh, 3941 O’Hara St., Pittsburgh, PA 15260, USAD ilepton-jet nal states are used to study physical phenom ena not predicted by the standard m odel. TheATLA S discovery potential for leptoquarks and M ajorana N eutrinos is presented using a fullsim ulation ofthe ATLA S detector at the Large H adron Collider. The study is m otivated by the role ofthe leptoquark inthe G rand U nication offundam entalforces and the see-saw m echanism that could explain the m asses oftheobserved neutrinos. The analysis algorithm s are presented,background sources are discussed and estim ates ofsensitivity and the discovery potentialforthese processes are reported.1. IntroductionTheLargeHadronCollider(LHC)willsoonopen upa new energy scale thatwilldirectly probe forphysi-calphenom enaoutsidethefram eworkoftheStandardM odel(SM ).M any SM extensionsinspired by G randUnication introduce new,very heavy particles suchasleptoquarks. Extending the SM to a largergaugegroup thatincludes,e.g.Left-RightSym m etry (LRS)[1], could also explain neutrino m asses via the see-saw m echanism .TheLRS-based Left-RightSym m et-ric M odel(LRSM )[10]used asa guide forpresentedstudies,extends the electroweak gauge group oftheSM from SU(2)L  U(1)Y to SU(2)L  SU(2)R U(1)B   L and thereby introducesZ0and right-handedW bosons.IftheLRS breaking in natureissuch thatallneutrinosbecom e M ajoranas,the LRSM predictsthesee-saw m echanism [2]thatelegantly explainsthem assesofthe threelightneutrinos.2. Search for scalar leptoquarksLeptoquarks(LQ )arehypotheticalbosonscarryingboth quark and lepton quantum num bers,aswellasfractionalelectric charge [3,4]. Leptoquarks could,in principle,decay into any com bination ofany avorlepton and any avorquark. Experim entallim itsonlepton num berviolation,avor-changing neutralcur-rents,and proton decay favorthreegenerationsoflep-toquarks.In thisscenario,each leptoquark couplestoalepton and a quark from thesam eSM generation[5].Leptoquarks can either be produced in pairs by thestrong interaction orin association with a lepton viatheleptoquark-quark-lepton coupling.Figure1showsFeynm an diagram s for the pair production oflepto-quarksatthe LHC.Thiscontribution describesthe search strategy forleptoquarksdecayingtoeitheran electron and aquarkoram uon and aquark leadingtonalstateswith twoleptonsand atleasttwo jets.The branching fractionFigure 1: Feynm an diagram s for the pair production ofscalarleptoquarksvia gluon-gluon fusion (a-d)and quarkanti-quark annihilation (e-f).ofa leptoquark to a charged lepton and a quark isdenoted as1.M C-sim ulatedsignaleventshavebeen studied[6]us-ing M onte Carlo (M C) sam ples for rst generation(1st gen.) and second generation (2nd gen.) scalarleptoquarks sim ulated at four m asses of 300 G eV,400 G eV,600 G eV,and 800 G eV with theM C gener-atorPythia [7]at14 TeV pp center-of-m assenergy.The nextto leading order(NLO )crosssection [8]forthe abovesim ulated signaldecreaseswith leptoquarkm ass from a few pb to a few fb with m ass point of400 G eV at(2.24 0.38)pb.1 = 1 would m ean that leptoquarks do not decay intoquarksand neutrinos.2 Proceedingsofthe DPF-2009 Conference,Detroit,M I,July 27-31,2009Table I Partialcross-section (pb)thatrem ainsaftereach selection criterion forthe rstgeneration leptoquark channel.Baselineselection correspondstotherstselection oftwoelectron candidatesand twojetsfrom thereconstructed objects.TheZ/DY crosssection isfortheregion M (ee) 60 G eV.VB pairscorrespondsto thediboson processesofW W ,W Z,and,ZZ.Physics Before Baseline ST M ee M1lj -M2lj m asswindowsam ple selection selection  490 G eV  120 G eV [320-480 ]-[320-480 ][G eV]LQ (400 G eV) 2.24 1.12 1.07 1.00 0.534Z/DY  60 G eV 1808. 49.77 0.722 0.0664 0.0036tt 450. 3.23 0.298 0.215 0.0144VB pairs 60.94 0.583 0.0154 0.0036 0.00048M ultijet 108 20.51 0.229 0.184 0.0Table II Partial cross-section (pb) that rem ains after each selection criterion for the second generation leptoquarkchannel.Baselineselection correspondsto therstselection oftwo m uon candidatesand two jetsfrom thereconstructedobjects. The Z/DY crosssection isforthe region M () 60 G eV.VB pairscorrespondsto the diboson processes ofW W ,W Z,and ZZ.Physics Before Baseline pT 60 G eV ST M () M lj m asswindowsam ple selection selection pjetT 25 G eV  600 G eV  110 G eV [300 -500 ][G eV]LQ (400 G eV) 2.24 1.70 1.53 1.27 1.23 0.974Z/DY 60 G eV 1808. 79.99 2.975 0.338 0.0611 0.021tt 450. 4.17 0.698 0.0791 0.0758 0.0271VB pairs 60.94 0.824 0.0628 0.00846 0.00308 0.00205M ultijet 1080.0 0.0 0.0 0.0 0.02.1. Reconstruction and objectsselectionSignalreconstruction requiresselection oftwo highquality leptons and at least two jets. Each signaljet and lepton candidate is required to have trans-verse m om entum (pT )> 20 G eV.Thishelpsto sup-press low pT background predicted by the SM .Lep-tons are required to have pseudorapidity jj below2.5,which istheinnerdetector’sacceptance,whereasjets are restricted to jj < 4:5 to suppress back-grounds from underlying event and m inim um biasevents that dom inate in the forward region of thedetector. In addition, leptons are required to passidentication criteria,which,in case ofelectrons,arebased on electrom agnetic-shower shape variables inthe calorim eterand,in the case ofm uons,are basedon nding a com m on track in them uon spectrom eterand the inner detector together with a m uon isola-tion2 requirem entin the calorim eter.Electron candi-datesarealsorequired tohaveam atchingtrackin the2E isoT=pT 0:3, where pT is m uon candidate’s transversem om entum and E isoTis energy detected in the calorim eters ina cone ofR =p( 2 +  2)= 0.2 around m uon candidate’sreconstructed trajectory.inner detector. Furtherm ore,it is required that sig-naljetcandidatesarespatially separated from energyclustersin theelectrom agneticcalorim eterthatsatisfyelectron identication criteria. Finally,a pairoflep-toquark candidatesare reconstructed from lepton-jetcom binations. G iven the factthatthese fourobjectscan be com bined to give two pairs,the pairthathasm inim um m assdierencebetween thetwo leptoquarkcandidatesisassum ed to be the signal.2.2. Background StudiesThe m ain backgrounds to the signal com e fromtt, and Z=D Y + jets production processes. M ultijetproduction where two jets are m isidentied as elec-trons, represents another background to the dielec-tron(1stgen.) channel. In addition,m inor contribu-tions arise from diboson production. O ther poten-tialbackground sources, such as single-top produc-tion,werealso studied and found to be insignicant.Thebackgroundsaresuppressed and thesignalsig-nicance isim proved by taking advantageofthe factthatthenalstateparticlesin signal-likeeventshaverelatively large pT . A scalar sum oftransverse m o-m enta ofsignaljets and lepton candidates,denotedby ST ,helps in reducing the backgrounds while re-taining m ostofthesignal.Theothervariableused toincreasethesignalsignicanceistheinvariantm assofProceedings ofthe DPF-2009 Conference,Detroit,M I,July 27-31,2009 3the two leptons,M ll. The distributions ofthese twovariablesfortherstgeneration channelareshown inFig.2. [GeV]TS0 200 400 600 800 1000 1200 1400 1600-1Events / 50 GeV / 100 pb024681012141618202224Signal - 400 GeVDrell-YanttWW,ZZ,WZATLASFirst generation [GeV] eeM100 200 300 400 500 600 700 800-1Events / 20 GeV / 100 pb012345678ATLASFirst generationSignal - 400 GeVDrell-YanttWW,ZZ,WZFigure 2: ST (top)and M ee ofthe selected electron pairafterST requirem ent(bottom )in 1stgen.leptoquark M Cevents(m L Q = 400 G eV).Both distributionsare norm al-ized to 100 pb  1ofintegrated pp lum inosity.After applying optim ized selection on these twovariables,ST and M ll,relativecontributionsfrom thebackground processes from tt, Z=D Y , diboson andm ultijet are 22% , 7% , 0.4% and 18% , respectively.Partialcross-sectionforthesignaland thebackgroundprocesses passing the selection criteria are shown intables I and II for the rst and second generationchannels,respectively. Figure 3 shows the invariantm asses3 of the reconstructed leptoquark candidatesbefore and after background suppression criteria areapplied to the M C data.2.3. Sensitivity and Discovery PotentialATLAS’ssensitivity to leptoquark signalfora 400G eV m ass hypothesis and with an integrated pp lu-m inosity of100 pb  1 is sum m arized in Fig.4. The3These distributions contain two entries per event corre-sponding to the two reconstructed leptoquark candidates. [GeV]ljM0 100 200 300 400 500 600 700 800-12 * events / 10 GeV / 100 pb-110110210310 ATLASSignal - 400 GeVDrell-YanttWW,ZZ,WZ [GeV]ljM0 100 200 300 400 500 600 700 800-12 * events / 10 GeV / 100 pb0510152025 ATLASSignal - 400 GeVDrell-YanttWW,ZZ,WZFigure 3: Reconstructed electron-jet invariant m ass for1st gen.leptoquark (m L Q = 400 G eV) in signaland back-ground M C eventsafterbaseline selection (top)and afteradditionalselection criteria based on ST and m ll (bottom )have been applied. Both distributions are norm alized to100 pb  1 ofintegrated pp lum inosity.cross-sectionsincludesystem aticuncertaintiesof50% .Leptoquark-like events in the ATLAS detector aretriggered by single leptonswith an eciency of97% .ATLAS is sensitive to leptoquark m asses of about565 G eV and 575 G eV for 1st and 2nd generations,respectively,atthegiven lum inosity of100 pb  1 pro-vided thepredicted cross-sectionsforthepairproduc-tion ofleptoquarksarecorrect.3. Search for W R bosons and heavyMajorana neutrinosW R bosonsaretheright-handed counterpartoftheSM W bosons.These right-handed interm ediate vec-tor bosons are predicted in LRSM s and can be pro-duced atthe LHC in the sam e processesasthe SM ’sW and Z. They decay into heavy M ajorana neutri-nos. The Feynm an diagram for W R production andsubsequent decay to M ajorana neutrino is shown inFig.5.This section describes the analysis ofW R produc-tion and itsdecaysW R ! eN e and W R ! N ,fol-4 Proceedingsofthe DPF-2009 Conference,Detroit,M I,July 27-31,2009Table III LRSM dielectron analysis. Partial cross-section (pb) that rem ains after each selection criterion for thedielectron channel.Physics Before Baseline M (ejj) M (eejj) M (ee) STsam ple selection selection  100 G eV  1000 G eV  300 G eV  700 G eVLRSM 18 3 0.248 0.0882 0.0882 0.0861 0.0828 0.0786LRSM 15 5 0.470 0.220 0.220 0.215 0.196 0.184Z/DY  60 G eV 1808. 49.77 43.36 0.801 0.0132 0.0064tt 450. 3.23 3.13 0.215 0.0422 0.0165VB pairs 60.94 0.583 0.522 0.0160 0.0016 0.0002M ultijet 10820.51 19.67 0.0490 0.0444 0.0444Table IV LRSM dim uon analysis. Partialcross-section (pb)thatrem ainsaftereach selection criterion forthe dim uonchannel.Physics Before Baseline M (jj) M (jj) M () S Tsam ple selection selection  100 G eV  1000 G eV  300 G eV  700 G eVLRSM 18 3 0.248 0.145 0.145 0.141 0.136 0.128LRSM 15 5 0.470 0.328 0.328 0.319 0.295 0.274Z/DY  60 G eV 1808. 79.99 69.13 1.46 0.0231 0.0127tt 450. 4.17 4.11 0.275 0.0527 0.0161VB pairs 60.94 0.824 0.775 0.0242 0.0044 0.0014M ultijet 1080.0 0.0 0.0 0.0 0.0lowed by thedecaysN e ! eq0qand N  ! q0q,whicharedetected in nalstateswith (atleast)two leptonsand two jets.The two leptonscan be ofeithersam e-sign oropposite-sign chargedue to the M ajorana na-tureofneutrinos.Thisanalysisin both thedielectronand thedim uon channelshasbeen perform ed withoutseparating dileptonsinto sam e-sign and opposite-signsam ples.Studies [6]ofthe discovery potentialfor W R andM ajorana neutrinosN e and N  have been perform edusingM C sam pleswhereM (N l)= 300G eV;M (W R )=1800 G eV (referred to as LRSM 18 3) and M (N l)= 500 G eV; M (W R ) = 1500 G eV (referred to asLRSM 15 5),sim ulated with PYTHIA according to aparticularim plem entation [9]ofLRSM [10].Thepro-duction cross-sections(pp(14 TeV)! W R X ) tim esthe branching fractions(W R ! lN l ! lljj)are 24.8pb and 47pb forLRSM 18 3 and LRSM 15 5,respec-tively.3.1. Reconstruction and objectsselectionSignaleventcandidatesarereconstructed usingtwoelectron orm uon candidatesand twojetsthatpassthestandard selection criteria asdiscussed in section 2.1.Thetwo signaljetcandidatesarecom bined with eachofthe signalleptons and the com bination that givesthe sm allerinvariantm assisassum ed to be the newheavyneutrinocandidate.Theotherrem ainingleptonisassum ed to com edirectly from thedecay oftheW Rboson.Ifsignalelectronsand signaljetsoverlapin Rwithin 0.4 then,to avoid double counting,only thetwo signaljets are used to reconstruct the invariantm assesofthe heavy neutrino candidate and W R .3.2. Background StudiesThem ain backgroundsto theLRSM analysesstud-ied herearethesam easm entioned in section 2.2.Thesam ebackground suppression criteria asin thelepto-quark analysesarealso eectivehere,nam ely ST andm ll. The distributions ofthese two variablesfor thedim uon channelare shown in Fig.6. Partialcross-section for the signaland the background processespassing the selection criteria are shown in tables IIIandIV forthedielectronanddim uon channels,respec-tively.Figure7showstheinvariantm assoftherecon-structed W R candidatesbefore and afterbackgroundsuppression criteria areapplied to the M C data.3.3. Sensitivity and Discovery PotentialSignalsignicance for W R analyses in the dielec-tron and dim uon channelsasa function ofintegratedpp lum inosity at14 TeV issum m arized in Fig.8.Theresultsincludesystem aticuncertaintyof45% and 40%fordielectron and dim uon channel,respectively. TheProceedings ofthe DPF-2009 Conference,Detroit,M I,July 27-31,2009 5]-1Integrated luminosity [pb0 50 100 150 2002 β0.10.20.30.40.50.60.70.80.91ATLAS1st gen.2nd gen.1st gen. (no syst. unc.)2nd gen. (no syst. unc.) discovery contoursσ5 Leptoquark mass [GeV]300 350 400 450 500 5502 β0.10.20.30.40.50.60.70.80.91ATLAS1st gen.2nd gen.Figure 4: 5 discovery potential for 1st and 2ndgen.400 G eV scalarleptoquark versus2,with and with-out background system atic uncertainty (top). M inim um2ofscalarleptoquarkversusleptoquarkm assat100pb  1ofintegrated pp lum inosity at5 (background system aticuncertainty isincluded)(bottom ).q l-q-'WR NlWR*l  or l-qq-'two jetsFigure 5: Feynm an diagram for W R production and itsdecay to the M ajorana neutrino N ‘ atthe LHC.eventsin thisanalysisarealso triggered by singlelep-tonswith an eciency of97% .4. ConclusionsDilepton-jetbased nalstateshave been discussedin both electron and m uon channels. Discovery po-tential for leptoquarks and LRSM with early LHCdata havebeen investigated with the predicted cross-sectionsforthesem odels.Assum inga  = 1,both 1st0 500 1000 1500 2000 2500ST ( GeV ), dimuon channel10-11101102103Events / 60 GeV per 100 pb-1ATLAS100 600 1100 1600M(µµ) ( GeV )10-11101102Events / 40 GeV per 100 pb-1ATLASFigure 6: LRSM analysis. The distributionsofST (top)and M (‘‘)(bottom )forsignalsand backgroundsnorm al-ized to 100pb  1 ofintegrated pp lum inosity afterbaselineselection in the dim uon analysis.and 2nd generationsleptoquarkscould be discoveredwith m asses up to 550 G eV with 100 pb  1 ofdata.Two LRSM m asspointsLRSM 18 3 and LRSM 15 5for the W R bosons and heavy M ajorana neutrinoshavebeen studied.The discovery ofthese new parti-cleswith such m asseswould require integrated lum i-nositiesof150 pb  1and 40 pb  1,respectively.6 Proceedingsofthe DPF-2009 Conference,Detroit,M I,July 27-31,2009500 1000 1500 2000 2500M(µµjj) ( GeV )10-11101102Events / 50 GeV per 100 pb-1ATLAS500 1000 1500 2000 2500M(µµjj) ( GeV )10-11101102Events / 50 GeV per 100 pb-1ATLASFigure 7: LRSM analysis. The distributions ofthe re-constructed invariant m asses for W R ! N  candidatesin background and signal(LRSM 18 3 and LRSM 15 5)events before (top) and after (bottom ) background sup-pression is perform ed in dim uon channelanalysis. Bothdistributionsare norm alized to 100 pb  1ofintegrated pplum inosity.References[1]Pati,J.C.and M ohapatra,R.N.,Phys.Rev.D11,566 (1975).[2]M ohapatra,R.N.and Senjanovic,G .,Phys.Rev.D 23,165 (1981).[3]Buchm uller, W .and W yler, D., Phys.Lett.B177,377 (1986).[4]G eorgi,H.and G lashow,S.L.,Phys.Rev.Lett.32,438 (1974).[5]Leurer,M .,Phys.Rev.D 49,333 (1994).[6]Aad, G . et al. [The ATLAS Collaboration],arXiv:0901.0512 [hep-ex].[7]T.Sjostrand,etal.,JHEP 05 (2006)026."[8]K ram er, M . et al., Phys. Rev. D 71, 057503(2005).[9]Ferrari,A.etal.,Phys.Rev.D 62,013001(2000).[10]Huitu,K .etal.,Nuc.Phys.B 487,27 (1997).0 100 200 300Integrated pp luminosity ( pb-1 )0.02.55.07.510.0Significance (dielectron channel)ATLAS0 100 200 300Integrated pp luminosity ( pb-1 )0.02.55.07.510.0Significance (dimuon channel)ATLASFigure 8: LRSM analysis. Expected signalsignicancesversusintegrated pp lum inosity forN e,N  and W R m asshypotheses,according to signalM C sam ples LRSM 18 3and LRSM 15 5.O pen sym bolsshow sensitivitieswithoutsystem atic uncertainties. Sensitivities shown with closedsym bols include an overall relative uncertainty of 45%(40% ),estim ated for background contributions in the di-electron (dim uon)analysis.

ATLAS Sensitivity to Leptoquarks, W_R and Heavy Majorana Neutrinos in Final States with High-pt Dileptons and Jets with Early LHC Data at 14 TeV proton-proton collisions

ATLAS Sensitivity to Leptoquarks, W_R and Heavy Majorana Neutrinos in
  Final States with High-pt Dileptons and Jets with Early LHC Data at 14 TeV
  proton-proton collisions

ATLAS Sensitivity to Leptoquarks, W_R and Heavy Majorana Neutrinos in Final States with High-pt Dileptons and Jets with Early LHC Data at 14 TeV proton-proton collisions

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