Final states including leptons are most promising to detect early signs of
new physics processes when the Large Hadron Collider will start proton-proton
collisions at the centre of mass energy of 14\TeV. The reach for Supersymmetry
and Extra Dimension models for integrated luminosities ranging from 1 to 10/fb
is reported. Preliminary results indicate that already with 1/fb of data new
phenomena can be detected.Comment: 4 pages, 7 figures, to appear in the proceedings of 42nd Rencontres
  de Moriond - QCD and Hadronic Interactions, La Thuile, Aosta Valley, Italy,
  17-24 March 200

Kazana, M.

English

arXiv

Final states including leptons are most promising to detect early signs of new physics processes when the Large Hadron Collider will start proton-proton collisions at the centre of mass energy of 14TeV. The reach for Supersymmetry and Extra Dimension models for integrated luminosities ranging from 1 to 10/fb is reported. Preliminary results indicate that already with 1/fb of data new phenomena can be detected.Final states including leptons are most promising to detect early signs of new physics processes when the Large Hadron Collider will start proton-proton collisions at the centre of mass energy of 14\TeV. The reach for Supersymmetry and Extra Dimension models for integrated luminosities ranging from 1 to 10/fb is reported. Preliminary results indicate that already with 1/fb of data new phenomena can be detected

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arXiv:0705.3733v2  [hep-ex]  15 Jun 2007SEA R C H ES FO R N EW P H Y SIC SW IT H LEP T O N S IN T H E FIN A L STAT E AT T H E LH CM .K AZANA(on behalfofthe ATLAS and CM S Collaborations)Laboratorio de Instrum entac~ao e Fsica Experim entalde PartculasAv.Elias Garcia 14,1,1000-149 Lisboa,PortugalFinalstatesincludingleptonsarem ostprom ising todetectearly signsofnew physicsprocesseswhen the Large Hadron Collider will start proton-proton collisions at the centre of m assenergy of14TeV .The reach forSupersym m etry and Extra D im ension m odelsforintegratedlum inosities ranging from 1 to 10fb 1is reported. Prelim inary results indicate that alreadywith 1fb 1ofdata new phenom ena can be detected.By the end of 2008 the ATLAS1 and CM S2 experim ents at the LHC expect to collectbetween 0.5 and 1fb  1 of data each, which should m ake possible the rst searches for newphenom ena over the Standard M odel(SM ) background. Allsuch searches would require theprecision m easurem entofthe SM processeswith detailed understanding ofthedetectorperfor-m ance,reconstruction algorithm s and triggering. Leptons,electrons and m uons,have betterreconstruction eciency and energy resolution than taus,jets and m issing transverse energy(ET= ).They also providea clean triggering and a high background reduction.Aboveall,leptonsm ay indicate signaturesofnew physics,such asdecaysofm assive strongly interacting particlesto leptonsaccom panied by jetsand ET= and decaysofnew m assiveresonancesto di-lepton pairs.In thisreport,theprelim inary discovery lim itsforSupersym m etry(SUSY)and ExtraDim ension(ED)m odelsestim ated overa widerange ofparam eterspace are presented.Inclusive SU SY reach w ith leptonsSupersym m etry3isdescribed bym odelswhich providearealisticSUSY-breakingschem e.O neofthegeneralapproachesisgiven bytheM inim alSupergravity4 (m SUG RA)m odelwith only5freeparam eters:acom m on scalar(m 0)and ferm ion (m 1=2)m asses,atrilinearcoupling(A 0)and Higgssector param eters (tan;sgn)atthe G rand Unication (G UT)scale. In m SUG RA,assum ingR-parity3,new supersym m etric particles are produced in pairs and the lightest one (LSP) isstable and neutral. At the LHC,the SUSY production is dom inated by strongly interactingsquarks and gluinos (M SU SY  m eq;eg),which have long decay cascades with the jet em ission.The cascade ends with the LSP,which is not detected. Therefore,a generic supersym m etrysignatureisa m ulti-jetnalstatewith largeE T= .Them ain backgroundsareQ C D and tt;W ;Zwith Q C D -jetassociated production processes,which should beestim ated by usingan exactLOevaluation ofpartonicm atrix elem entsm atched with parton showersatthehard processscale5.In SUSY cascades,leptonsareproduced in decaysofcharginosorneutralinos(e.g.e02 ! e01l+ l  ,e+1 ! ell+ )and thenalstateconsistsofn 4leptons(+ jets+ E T= ).Pairsofleptonscan havetheATLAS prelim inaryA. B.Figure1:Them SUG RA discoveryreach in m 0-m 1=2 planefor xed A0= 0,tan= 10,> 0for1fb 1with system aticuncertaintiesdenoted by dash linesforthe (A)and with uncertaintiesalready included forCM S (B).sam eoroppositesign (SS orO S).Considering signatureswith atleastonelepton,substantiallyreduces the Q C D background. The inclusive SUSY searches em ploy the following strategy.First, experim entalsignatures are studied for a lim ited num ber of test points of m SUG RAparam eterspace(in m 0-m 1=2 planeforxed A 0,tan,sgn)usingthefulldetectorsim ulation andreconstruction (S& R)software.Next,theresultsareextended to otherpointsoftheparam eterspace using fastS& R.In orderto obtain the bestsignalto background (S=B )ratio the SUSYselection cuts are optim ised for each point. The expected discovery reach is evaluated forparam etersetshaving atleastvestandard deviation (5)signalsignicance6.TheATLAS collaboration studied them SUG RA m odelwith n 2 leptonsin thenalstate.Thecutshavebeen optim ised with fastS& R form 0= 100-2000G eV,m 1=2= 100-1500G eV,tan= 5,10,30,50,A 0= 0 and > 0 m odelparam etersusing the SUSY-sensitive observables: ET= ,p1stjetT,p4thjetT,SphericityT . The background consists ofthe following SM processes5: tt+ N(0-3)jets,W (! l)+N(2-5)jets,Z(! ll;)+ N(2-5)jets,N(2-6) Q C D jets. The large cut on ET= (e.g.> 400G eV) eec-tively rem oves the SM background in the wide m 1=2 region due to m ass splitting between theheaviest and the lightest SUSY particles. The m ajor theoreticaluncertainties ofbackgroundcross-sectionsarisefrom thelow parton pT cutand thesm allrenorm alization scale.Experim en-taluncertainties oflum inosity (5% ),ET= scale (5% ) and jet energy scale (5% ) are considered.The resulting discovery reach,dened by atleast10 signaleventsand S=pB > 5 for1fb  1,isshown in Fig.1A.By including uncertaintiesthediscovery potentialcurvesareloweron m 1=2 byabout50G eV forallchannels.Therefore,ATLAS searchesforn 1lepton channelsaresensitiveup to M SU SY  1.4TeV orm 1=2700G eV .TheCM S experim entanalysed severalsignaturescharacteristicform SUG RA6 with thefullS& R and theeventselection criteria optim ised forSUSY.Thediscovery potentialforintegratedlum inosity of1fb  1 is presented in Fig.1B.The curves show lim its with allsystem atic uncer-tainties included. The inclusive channels,jet+ ET= and + jet+ ET= yields the best results andallow to probean existenceofSUSY to thesam elevelofM SU SY  1.5TeV asATLAS obtainedforn 1 lepton channels. O therexperim entalsignaturesin the sam e range ofparam etersm ayhelp to conrm the discovery. The param eters ofm SUG RA can be recovered later from them easurem entofseveralobservablessuch asthereconstructed m assesofSUSY particle.Four-lepton signalfrom U niversalExtra D im ensionsThe CM S collaboration studied 4-lepton signatures in the context ofUniversalExtra Dim en-sions7 (UED)m odel.Thephenom enology ofUED isvery sim ilarto thatofSUSY,although theorigin ofUED com esfrom thesub-m illim etreED m odeloftheADD8 type.In UED,allSM eldsareallowed to propagatealong EDs.Therefore,each SM particlehasK aluza-K lein (K K )excita-tionswith thesam espin contrary to SUSY particles.In them inim alscenario with only oneED:1∈MC level, L1 HLT 2 OSSF≥ × 2∈ 4 iso≥ Bveto <(70,60,40,30)GeV Tlept p ETmiss>60GeV Zveto  (fb)σCross section -310-210-110110210310410  channelµ4 ued-300 ued-500 ued-700 ued-900 Tot Bkg  tt ZZ Zbb bbbb)2 (GeV/c-1R300 400 500 600 700 800 900 (1/fb)cPSσLuminosity for 5-11011030  mUED 4-lepton channels at CMS R = 20Λ 2 = 120 GeV/cH m   ’First data’ uncertainty 4eµ 4   µ 2e2 syst. incl. A. B.Figure 2: The CM S search for m UED signalwith the 4-lepton  nalstate. A.Signalselection e ciency in the4-m uon channel.B.The m UED 4-lepton discovery potentialasa function ofthe ED size,R 1.m UED[R  1,R],whereR   1 isasizeoftheED and an eectivecut-oscale,theK K excitationsexhibithighly degenerate m asseseven afterradiative corrections.The K K -quarksdecay to thelightestand stableK K -photonsin alongchain (qK K ! Z K K q;Z K K ! lK K l ;lK K ! K K l )producing softleptonsand jetsin thedetector.Therefore,the4-lepton nalstate isconsideredthe bestto elim inate the SM background. The CM S study9 hasbeen perform ed with the fullS& R forfoursetsofm UED param eters(R= 20 and R   1= 300,500,700,900G eV )and forthreeleptonicchannels:4,4e and 2e2.Two sam e-avourO S isolated lepton pairswererequired inthe oine selection in advance ofthe b-tagging and Z-veto rejections (Fig.2A).The discoverypotentialform UED in term softhe integrated lum inosity needed to achieve signalsignicanceof5 (Fig.2B)extendsup to R  1= 600G eV for1fb  1data.Thesystem atic uncertaintiesduetoa lim ited understandingofthedetectorperform anceduringinitialphaseoftheLHC data takingm ay shiftthesensitivity ofm UED discovery up to 1fb  1.D i-lepton resonances from Z 0 bosons and R andall-Sundrum G ravitonsThe spin-0 Z 0 gauge bosons and spin-2 K K excitations of the graviton with m asses of theorderof1TeV are predicted by m any EDsand G UT’stheories. AtLHC,such resonancesareproduced directly and prom ptly decay into pairsofsam e-avourO S leptons.Theirm assescanbe m easured from peaks in the invariant m ass distribution in the tails ofthe SM backgroundprocesses.Thissignaturehasbeen studied in theCM S experim ent.Thedom inantbackground arisesfrom theDrell-Yan (D-Y)lepton pairproduction,whereascontributionsfrom ttand the vector boson pairproduction (ZZ; W Z; W W )are signicantlysm aller and are highly suppressed by selection cuts. The K -factors related to the N N LOperturbative Q C D calculations are used to correct cross-sections in function ofthe di-leptonm ass for the D-Y and the new boson production. Theoreticaluncertainties due to a choiceofthe PDF set and various experim entaluncertainties are also considered including eects ofthe m isalignm entofthe m uon system in the early (< 1fb  1)and the late (> 100fb  1)phase ofthe LHC data taking periods. The m om entum resolution ofthe detector plays a key role inseparating thesignalfrom thebackground.New reconstruction algorithm shavebeen developedto increase the lepton reconstruction eciency. For highly energetic electrons, their energydeposited as an isolated electrom agnetic super-cluster is corrected for the energy leaking intoa hadronic calorim eter and for electronics saturation eects. For very high-pT m uons, thetrack tting in the tracker and the m uon system are optim ised to detect and correct eectsoftheir energy lost. The results ofthe CM S analyses6 obtained with the fullS& R ofsignaland background are presented in Fig.3. Both,the Z 0SSM boson originated from the so-calledSequentialSM and Z 0	from one ofthe G UT’sm odels10 can be discovered using 1fb  1 ofdata(abovetheTevatron lim itof1TeV )up to M Z 0 2:6TeV (Fig.3A).G raviton excitations,G,areA. B.C.Figure 3: A.The lum inosity required for 5 discovery as a function ofZ0m ass. B.The com parison ofRSgraviton discovery reach for 10fb 1for di erentchannels. C.The RS graviton discovery lim its with system aticuncertainties(dash lines)forthe m uon channel.predicted by the Randall-Sundrum 11 (RS)m odelin which only one ED isseen by the gravitywhereas allSM elds live in the 3-dim ensionalUniverse. Couplings and the width ofG  aregiven by theparam eterc= k=M P lanck,wherek M P lanck isa m assscale factor.In Fig.3B theRS graviton discovery reach corresponding to 10fb  1 is shown for three decay channels. Them uon channelhasthelowestdiscovery potentialduetom om entum resolution eects.Although,thephoton channelhasthebranching ratio alm osttwo tim eslargerthen leptoniconesitsreachiscom parableto theelectron channeldueto Q C D and prom ptphoton irreduciblebackgrounds.From Fig.3C onecan seethat1fb  1 ofdatashould besucienttodiscoverRS gravitonsdecayingto m uons with m ass up to M G   2:4TeV . Experim entalm ethods have been proposed tom easure thespin ofsuch new resonances6.Sum m aryLeptons provide the cleanest signature for exotic searches with the rst LHC data. Thus,inclusivesearcheswith leptonsarevery prom isingforverifyingtheoreticalpredictionsforphysicsbeyond the Standard M odel. Recent studies on the discovery potentialfor SUSY perform edby the ATLAS and CM S collaborations are consistent with each other and dem onstrate thatprobing the production ofsquarksand gluinos with m asses ofthe orderof1.5TeV is possiblewith integrated lum inosity aslow as1fb  1. W ith 1fb  1 the searchesforEDscan setdiscoverylim itsatthelevelof:R  1= 600G eV form UED m odel,M G   2:4TeV fortheRS graviton,andM Z 0  2:6TeV form assive Z0bosons.R eferences1. ATLAS Collaboration,CERN-LHCC-94-43 (1994)2. CM S Collaboration,CERN-LHCC-94-38 (1994)3. S.P.M artin,hep-ph/9709356 (1997)4. L.Alvarez-G aum e,J.Polchinski,M .B.W ise,Nucl.Phys.B 221,495 (1983)5. M .L.M angano,M .M oretti,F.Piccinini,R.Pittau,A.D.Polosa,hep-ph/0206293 (2003)6. CM S Collaboration,CERN/LHCC 2006-021,CM S TDR 8.2 (2006)7. K .K ong,K .T.M atchev,hep-ph/0610057 (2006)8. N.Arkani-Ham ed,S.Dim opoulos,G .R.Dvali,Phys.Lett.B 429,263 (1998)9. M .K azana,CM S-CR/2006-062 (2006)10. J.F.G union,L.Roszkowski,H.E.Haber,Phys.Lett.B 189,409 (187)11. L.Randall,R.Sundrum ,Phys.Rev.Lett.83,3370 (1999)

Searches for New Physics with leptons in the final state at the LHC

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