We present recent work performed in ATLAS on techniques used to reconstruct
the decays of SUSY particles at the LHC. We concentrate on strategies to be
applied to the first fb-1 of LHC data.Comment: ICHEP0

Robichaud-Veronneau, Andree

English

arXiv

We present recent work performed in ATLAS on techniques used to reconstruct the decays of SUSY particles at the LHC. We concentrate on strategies to be applied to the first fb-1 of LHC data

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arXiv:0810.2381v1  [hep-ex]  14 Oct 200834th InternationalConferenceon High Energy Physics,Philadelphia,2008Exclusive measurements for SUSY events with the ATLAS detector at theLHCA. Robichaud-Ve´ronneau, on behalf of the ATLAS collaborationUniversite´ de Gene`ve, Geneva, Switzerland (email: andree.robichaud-veronneau@cern.ch)W e present recent work perform ed in ATLA S on techniques used to reconstruct the decays ofSU SY particles at theLH C.W e concentrate on strategies to be applied to the rstfb  1 ofLH C data.1. INTRODUCTIONThe Large Hadron Collider(LHC)hasstarted operation very recently and soon itwilldeliverp  p collisionsata center-of-m assenergy of14 TeV.The ATLAS detectorwillbe used to search forevidence forphysicsbeyond theStandard M odel(SM ).Am ong the m any extensionsto the SM thatpredictwhatthisphysicsm ightbe,supersym m etry (SUSY)with R-parity conservation isa very attractiveone.Itprovidesa candidateparticlefordark m atter,thelightestneutralino,and predictsa lightHiggsboson,in agreem entwith electroweak precision m easurem ents.Thefollowingworkislim ited tothestudyofm SUG RA m odels.A listofpredened points[1]in theparam eterspaceisused.Sincethereisno LHC data yet,eventsaregenerated with Isajetand Herwig,and passed through a realisticsim ulation oftheATLAS detector.In thesem odels,pairproduction ofSUSY particlesisassum ed,each decaying ina cascadeto thelightestsupersym m etricparticle(LSP),which can only bedetected by a m issing transverseenergysignature.2. EDGE MEASUREMENTSEndpointm easurem entsare used when one particle islostin the decay orcannotbe m easured. In thiscase,theLSP isonly detected by itsm issing energy signature.To study this,the following decay chain isused:eqL ! e02q(!ellq)! e01l+l q (1)Thisdecay chain providesa largesignalto background ratio dueto itsnalstate.In thecaseofthe\Bulk" point(SU3),the decay ofthe neutralino goes through an extra step involving sleptons,since elR and e1 are lighter thane02.Forthe \Low M ass" point(SU4),theneutralino decaysdirectly to a lepton pairand theLSP sincesleptonsareheavier.Them SUG RA param etersforSU3 arem 0 = 100 G eV,m 1=2 = 300 G eV,A 0 = -300 G eV,tan = 6, > 0,and forSU4,m 0 = 200 G eV,m 1=2 = 160 G eV,A 0 = -400 G eV,tan = 10, > 0.The NLO cross-section forSU3is27.68 pb while forSU4,itis402.19 pb.2.1. Dilepton edgesBy considering the lepton pair produced in eq.1,it is possible to obtain insights about the m asses involved inthe decay.In the SU3 case,we havemedgell= me02  me01and forSU4,the expression ism ore com plex,asshown ineq. 2. The Eventswith two orthree isolated leptons(electronsorm uons)are selected. O pposite sign (O S)leptonpairsare required in the two-lepton eventsand allpossible com binationsofopposite sign leptonsare considered inthe three-lepton events. Lepton pairswith opposite-avour(O F)are subtracted from the sam e-avour(SF)pairs,and cutsareperform ed on transversem issing energy (Em issT ),transversem om enta ofthe fourleading jets,the ratiobetween Em issT and the eective m assand the transversesphericity.134th InternationalConferenceon High Energy Physics,Philadelphia,2008medgell= me02vuut 1  m ~lme02! 2vuut1  me01m ~l! 2(2)The invariantm assdistribution istted with a triangularfunction sm eared with a G aussian forthe SU3 casefor1 fb  1 asshown in gure 1.The endpointvalue obtained from the tis(99.7  1.4  0.3)G eV,where the quotederrorsarerespectivelythestatisticalerror,thesystem aticerroron thelepton energy scaleand thesystem aticerroronthe param eter[1].TheSU4 caserequiresa 3-body decay theoreticaldistribution [2]sm eared fortheexperim entalresolution.Thetgivesan endpointof(52.7 2.4 0.2)G eV for0.5fb  1.The\Coannihilation"point(SU1)showsa doubleedgein thesam einvariantm assdistribution,dueto both left-and right-handed sleptonsbeing lighterthane02. The edgescannotbe tted with 1 fb  1 although an excessisvisible,while with 18 fb  1,a tcan be obtainedwith a loweredgeat(55.8  1.2  0.2)G eV and a upperedgeat(99.3  1.3  0.3)G eV.Allresultsareconsistentwith the calculated valuesof100.2 G eV (SU3),53.6 G eV (SU4)and 56.1 and 97.9 G eV (SU1).m(ll) [GeV]0 20 40 60 80 100 120 140 160 180 200-1Entries/4 GeV/ 1 fb-1001020304050 / ndf 2χ 40.11 / 45Prob   0.679Endpoint  1.399± 99.66 Norm.     0.02563± -0.3882 Smearing  1.339± 2.273 ATLASm(ll) [GeV]0 20 40 60 80 100 120 140 160 180 200-1Entries/4 GeV/ 0.5 fb-40-20020406080100120 / ndf 2χ   10.5 / 16Prob   0.839Norm      8.971± 70.07 M1+M2     8.267± 67.71 M2-M1     2.439± 52.68 ATLASFigure 1: Invariant m ass distribution for SU3 (left) and SU4 (right). The points show the sum ofthe SM and the SUSYcontributions,the line histogram isthe SM contribution only.The resultofthe tissuperim posed and the dashed line showthe expected position ofthe endpoint.2.2. Jet + Lepton edgesAs can be seen in eq.1, allm asses can be reconstructed using the jets in the nalstate to obtain endpointm easurem ents. Three new quantitiescan be used: m llq (edge and threshold),m lq(high) and m lq(low ),which are thehighestand lowestvalueofm lq in an eventusing thesam ejetasm llq.Two straightlines,with a G aussian sm earingfora sm ooth transition between them ,aretted to a sm allrangeofdata pointsin them llq distribution,forboth theedgesand the thresholds.The endpointsareexplicitely tted.Theresultsofthe tsareshown in Table I.Endpoint SU3 truth SU3 m easured SU4 truth SU4 m easuredmm axllq 501 517  30  10  13 340 343  12  3  9mm inllq 249 265  17  15  7 168 161  36  20  4mm axlq(low ) 325 333  6  6  8 240 201  9  3  5mm axlq(high) 418 445  11  11  11 340 320  8  3  8TableI:Endpointpositionsfrom tsforSU3(1fb  1)and SU4(0.5fb  1),in G eV.Errorsarerespectively statistical,system aticand jetenergy scale uncertainty.2.3. Tau signaturesIn the previoussections,leptonswere considered to be only electronsorm uons. Tau leptonshave to be treatedseparately.Forthe decay e02 ! e1 ! e01+   ,the branching ratio is10 tim eshigherthan forotherleptons(forSU1 or SU3 scenarios). Also,since the 1is involved in this decay and the neutralino m assescan be determ ined234th InternationalConferenceon High Energy Physics,Philadelphia,2008from other m easurem ents,it is possible to determ ine the 1m ass. Since the decay of involvesneutrinos in thenalstate,itisnotpossibleto geta sharp edgeatthe m axim um kinem atic value.Invariantm assdistributions are plotted forSU1 and SU3 m odels,where the sam e-sign (SS) distribution is sub-tracted from the opposite-sign (O S)one.Specialcareisneeded concerning the tresultsdue to polarization eectson the  invariantm assdistribution,which can considerably shiftthe position ofthe endpoint.2.4. Right-handed squark pairsThe decay chain presented in eq.1 holds forleft-handed squark decay. In the case ofright-handed squarks,thedecay goesdirectly to the LSP and quark: eqR ! e01q. In this case,a new variable is introduced,the \stransversem ass" m T 2 [1].Assum ing them assoftheLSP isknown from previousm easurem ents,m T 2 can beused to determ inethe eqR m ass.A lineartisapplied to a rangeofdata pointsaround the edge ofthe m T 2 distribution to determ inetheendpointforSU3 and SU4 m odels.Theresultsofthetare591 + 13  6 (sys) 13 (stat)G eV forSU3 and 407+ 10  3(sys) 12 (stat)G eV forSU4 These should be com pared with the known values: 637 G eV forSU3 and 405 G eVforSU4.2.5. Light stopIn the particularcase ofSU4,allSUSY m assesare relatively lightand so isthe et1,with a m assof206 G eV.Asit is always decaying to the sam e channel,we can study the following: eg ! et1t ! e1bt The upper endpoint ofthe tb invariant m ass depends on allm asses involved in the decay. O nly the hadronic top decays are included inthe distribution.A tisperform ed on the invariantm assdistribution (afterW background issubtracted using thesideband m ethod)using a triangularfunction sm eared with a G aussian.Itgivesa valuefortheendpointof297  9G eV (fora 5-param etert)for200 pb  1,in agreem entwith the calculated valueof 300 G eV.3. HIGGS IN SUSY EVENTSThe Higgsboson can be produced in m any waysatthe LHC.M ostcom m only,itislooked forin SM interactions(e.g. g  g fusion),butitcan also occurin the decay ofsparticleswhich were produced by the initialinteraction,like here forthe neutralino in the SU9 m odel(\Bulk" pointwith enhanced Higgsproduction): e02 ! e01h ! e01bb.Requiring signicantm issing transverse energy suppressesthe Q CD background,enabling the observation ofHiggsdecay to b quarks.4. MASS AND PARAMETERS MEASUREMENTThe dierent endpoint m easurem entsobtained above for m any m SUG RA m odels can be used to determ ine theSUSY m assspectra and tscan be perform ed to constrain the param etersofthe given m odels.In som e cases(likethe dilepton edges),an analyticalform ula is known to describe the invariantm assshape and obtain the m ass. Inthe other cases,a 2m inim ization procedure is used to obtain the sparticle m asses from severalendpoint values.Param etersofthe m SUG RA m odels were obtained using 500 toy ts for both values ofsign(). For each t,theobservablesare sm eared using the fullcorrelation m atrix. The resultscan be found in [1]. Forthe m asses,there isagreem entbetween theoreticaland experim entalvalues,butthe parabolicerrorforthe m inim ization arestilllarge.Asforthe m SUG RA param eters,M 0 and M 1=2 can be determ ined reliably,while in the case oftan  and A0,onlythe orderofm agnitudecan be obtained forthe SU3 and SU4 m odelsin a data sam plecorresponding to 1 fb  1.334th InternationalConferenceon High Energy Physics,Philadelphia,2008AcknowledgmentsThiswork isa collaborativeeortofthe ATLAS SUSY W orking G roup and m any thanksgo to them .References[1]ATLAS Collaboration,\ExpectedPerform anceoftheATLAS Experim ent,Detector,Triggerand Physics",CERN-O PEN-2008-020,G eneva,2008,to appear.[2]U.De Sanctis,T.Lari,S.M ontesano,C.Troncon,Eur.Phys.J.C 52 (2007)743.4

Exclusive measurements for SUSY events with the ATLAS detector at the LHC

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