Article
Reference
Search for high-mass new phenomena in the dilepton final state using proton–proton collisions at √s = 13 TeV with the ATLAS detector
ATLAS Collaboration
ANCU, Lucian Stefan (Collab.), et al.
Abstract
A search is conducted for both resonant and non-resonant high-mass new phenomena in dielectron and dimuon final states. The search uses 3.2fb−1 of proton–proton collision data, collected at s=13TeV by the ATLAS experiment at the LHC in 2015. The dilepton invariant mass is used as the discriminating variable. No significant deviation from the Standard Model prediction is observed; therefore limits are set on the signal model parameters of interest at 95% credibility level. Upper limits are set on the cross-section times branching ratio for resonances decaying to dileptons, and the limits are converted into lower limits on the resonance mass, ranging between 2.74 TeV and 3.36 TeV, depending on the model. Lower limits on the ℓℓqq contact interaction scale are set between 16.7 TeV and 25.2 TeV, also depending on the model.
ATLAS Collaboration, ANCU, Lucian Stefan (Collab.), et al . Search for high-mass new phenomena in the dilepton final state using proton–proton collisions at √s = 13 TeV with the ATLAS detector. Physics Letters. B , 2016, vol. 761, p. 372-392
DOI : 10.1016/j.physletb.2016.08.055
Available at:
http://archive-ouverte.unige.ch/unige:86544
Disclaimer: layout of this document may differ from the published version.
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Search for high-mass new phenomena in the dilepton final state using proton–proton collisions at √
s = 13 TeV with the ATLAS detector
.TheATLASCollaboration
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received14July2016
Receivedinrevisedform17August2016 Accepted24August2016
Availableonlinexxxx Editor:W.-D.Schlatter
Asearchisconductedforbothresonantandnon-resonanthigh-massnewphenomenaindielectronand dimuonfinalstates.Thesearchuses3.2fb−1ofproton–protoncollisiondata,collectedat√
s=13 TeV bytheATLASexperimentattheLHCin2015.Thedileptoninvariantmassisusedasthediscriminating variable.NosignificantdeviationfromtheStandardModel predictionisobserved;thereforelimitsare setonthesignalmodelparametersofinterestat95%credibilitylevel.Upperlimitsaresetonthecross- sectiontimes branchingratio forresonances decaying todileptons,and the limits are converted into lowerlimitsontheresonancemass,rangingbetween2.74 TeV and3.36 TeV,dependingonthemodel.
Lowerlimitsontheqqcontactinteractionscalearesetbetween16.7 TeVand25.2 TeV,alsodepending onthemodel.
©2016PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Thedilepton(eeor μμ)final-statesignaturehasexcellentsen- sitivity to a wide variety of new phenomena expected in theo- riesbeyondtheStandardModel(SM).Itbenefitsfromhighsignal selection efficiencies and relatively small, well-understood back- grounds.
Models with extended gauge groups often feature additional U(1) symmetries with corresponding heavy spin-1 Z bosons whose decays would manifest themselves as narrow resonances inthedileptonmassspectrum.GrandUnifiedTheories(GUT)have inspiredmodels based onthe E6 gauge group [1,2],which, fora particularchoiceofsymmetry-breakingpattern,includestwoneu- tralgaugebosonsthatmixwithanangleθE6.Thisyieldsaphysical statedefinedby Z(θE6)=Zψ cosθE6+Zχ sinθE6,wherethegauge fields Zψ and Zχ are associated with two separate U(1) groups resultingfromthe breakingofthe E6 symmetry.All Zsignalsin thismodelaredefinedbyspecificvaluesofθE6 rangingfrom−π
to π, andthe six commonly motivatedcases are investigated in this search, namely Zψ, Zη, ZN, ZI, ZS, and Zχ. The widths of thesestatesvaryfrom0.5%to1.2%oftheresonancemass,respec- tively.In additionto the GUT-inspired E6 models, theSequential StandardModel (SSM)[2]provides a commonbenchmark model that includes a ZSSM boson with couplings to fermions identical to those of the SM Z boson. This search is also sensitive to a seriesofmodelsthat predictthepresenceofnarrowdileptonres-
E-mailaddress:atlas.publications@cern.ch.
onances;howeverconstraintsarenotexplicitlyevaluatedonthese models. Theseinclude theRandall–Sundrum (RS)model [3]with a warped extra dimension giving rise to spin-2 graviton excita- tions,thequantumblackholemodel[4],theZ*model[5],andthe minimalwalkingtechnicolourmodel[6].
SomemodelsofphysicsbeyondtheSMresultinnon-resonant deviations fromthe predictedSM dileptonmass spectrum. Com- positenessmodelsmotivatedbytherepeatedpatternofquarkand lepton generations predict new interactions involving their con- stituents.Theseinteractionsmayberepresentedasacontactinter- action (CI)betweeninitial-statequarksandfinal-stateleptons [7, 8].Othermodelsproducingnon-resonanteffects,butnotexplicitly evaluated here, are models with large extra dimensions [9] mo- tivated by the hierarchy problem. The following four-fermion CI Lagrangian[7,8] isusedtodescribe anewinteractionorcompos- itenessintheprocessqq→+−:
L= g22[ηLL(qLγμqL) (LγμL)+ηRR(qRγμqR) (RγμR) (1) +ηLR(qLγμqL) (RγμR)+ηRL(qRγμqR) (LγμL)],
whereg isacouplingconstantsettobe√
4π byconvention, is the CI scale, andqL,R and L,R are left-handedand right-handed quark andlepton fields,respectively. The symbol γμ denotes the gammamatrices, andthe parameters ηi j,where i and j are Lor R(leftorright),definethechiralstructureofthenewinteraction.
Differentchiralstructuresareinvestigatedhere,withtheleft–right (right–left) model obtained by setting ηLR= ±1 (ηRL= ±1) and allotherparameterstozero.Likewise,theleft–leftandright–right models are obtainedby settingthe corresponding parameters to http://dx.doi.org/10.1016/j.physletb.2016.08.055
0370-2693/©2016PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
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±1,andtheotherstozero.Thesignof ηi jdetermineswhetherthe interferencebetweenthe SM Drell–Yan(DY)qq→Z/γ∗→+− processandtheCIprocessisconstructive(ηi j= −1)ordestructive (ηi j= +1).
The mostsensitiveprevious searches fora Z decaying tothe dileptonfinalstatewerecarriedoutbytheATLASandCMSCollab- orations[10,11].Using20 fb−1 ofppcollisiondataat√
s=8 TeV, ATLAS seta lower limit at95% credibility level(CL) on the ZSSM pole mass of 2.90 TeV for the combined ee and μμ channels.
Similarlimitswere setbyCMS.Themoststringentconstraintson CI searches are also provided by the CMS and ATLAS Collabora- tions [11,12].The strongestlower limitson theqq CI scaleare
>21.6 TeV and >17.2 TeV at 95% CL for constructive and destructiveinterference,respectively,inthecaseofleft–leftinter- actionsandgivenauniformpositivepriorin1/ 2.Previousdilep- tonsearchesatATLAShavealsosetlowerlimitsontheresonance massinothermodelssuchas:anRSgravitonupto2.68 TeV,quan- tumblackholesat3.65 TeV,theZ*bosonat2.85 TeV,andminimal walkingtechnicolourupto2.27 TeV[10].Similarlowerlimitswere setbyCMSwhereequivalentsearcheswereperformed[11].
Inthisletter,asearchforresonantandnon-resonantnewphe- nomenaispresentedusingtheobservedeeand μμmassspectra extractedfromppcollisionswithintheATLASdetectorattheLarge HadronCollider(LHC)operatingat√
s=13TeV. The ppcollision datacorrespondtoanintegratedluminosityof3.2fb−1.Theanal- ysisandinterpretationofthesespectrarelyprimarilyonsimulated samplesofsignalandbackgroundprocesses.The Z masspeakre- gionisusedtonormalisethebackgroundcontributionandperform cross-checksofthe simulatedsamples.The interpretation isthen performedtakingintoaccounttheexpectedshapeofdifferentsig- nalsinthedileptonmassdistribution.
2. ATLASdetector
The ATLAS experiment [13,14] at the LHC is a multi-purpose particle detector witha forward–backward symmetric cylindrical geometry and near 4π coverage in solid angle.1 It consists of an inner tracking detector surrounded by a thin superconduct- ingsolenoidprovidinga 2Taxialmagneticfield,electromagnetic and hadronic calorimeters, and a muon spectrometer. The inner trackingdetector(ID)coversthepseudorapidityrange|η|<2.5.It consistsofsiliconpixel,siliconmicrostrip,andtransition–radiation tracking detectors. Lead/liquid-argon (LAr) sampling calorimeters provide electromagnetic (EM) energy measurements with high granularity. A hadronic (steel/scintillator-tile) calorimeter covers thecentral pseudorapidity range(|η|<1.7). Theendcap andfor- wardregions are instrumentedwithLArcalorimeters forEMand hadronic energy measurements up to |η|=4.9. The total thick- nessoftheEMcalorimeterismorethantwentyradiationlengths.
The muon spectrometer (MS) surrounds the calorimeters and is based on three large superconducting air-core toroids witheight coilseach.Thefieldintegralofthetoroidsrangesbetween2.0and 6.0T·mformostofthedetector.Itincludesasystemofprecision tracking chambers and fast detectors for triggering. A dedicated trigger system is used to select events. The first-level trigger is implementedinhardwareandusesthecalorimeterandmuonde- tectorsto reduce theacceptedeventratefrom40 MHzto below
1 ATLASCollaboration usesaright-handedcoordinatesystemwithitsoriginat thenominalinteractionpoint(IP)inthecentreofthedetectorandthez-axisalong thebeampipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,and they-axispointsupwards.Cylindricalcoordinates(r,φ)areusedinthetransverse plane,φbeingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdefined intermsofthepolarangleθasη= −ln tan(θ/2).Angulardistanceismeasuredin units ofR≡
(η)2+(φ)2.
100kHz.Thisisfollowedbyasoftware-basedtriggerthatreduces theacceptedeventrateto1kHzonaverage.
3. DataandMonteCarlosamples
Thedatasampleusedinthisanalysiswascollectedduring the 2015 LHCrun with pp collisions at √
s=13 TeV. Afterselecting periodswithstablebeamsandrequiringthatrelevantdetectorsys- temsarefunctional,thedatasetusedfortheanalysiscorresponds to3.2fb−1 ofintegratedluminosity.Eventquality isalsochecked to remove those events which contain noise bursts or coherent noiseinthecalorimeters.
Modellingofthevariousbackgroundsourcesreliesprimarilyon MonteCarlo(MC)simulation.Thedominantbackgroundcontribu- tionarisesfromtheDYprocess[15].Otherbackgroundsourcesare top-quark [16] and diboson(W W, W Z, Z Z) [17] production.In thecaseofthedielectronchannel,multi-jetandW+jets processes also contribute due to the misidentification of jets as electrons.
A data-drivenmethod,described inSection5,isusedtoestimate thesebackgroundcontributions.Themulti-jetandW+jets contri- butioninthedimuonchannelisnegligible.
DY events are simulated using Powheg-box v2 [18] at next- to-leading order (NLO) in Quantum Chromodynamics(QCD), and interfacedtothePythia8.186[19]partonshowermodel.TheCT10 parton distribution function (PDF) set [20] is used in the ma- trixelementcalculation. TheAZNLO[21] setoftunedparameters (“tune”)isused,withtheCTEQ6L1PDFset[22],forthemodelling ofnon-perturbativeeffects.TheEvtGenv1.2.0program[23]isused forpropertiesofthebottom andcharm hadrondecays.Photos++
version3.52[24]isusedforQuantumElectrodynamic(QED)emis- sions fromelectroweakverticesandchargedleptons.Eventyields are corrected with a mass-dependent rescaling to next-to-next- to-leading order(NNLO) intheQCD couplingconstant,computed with VRAP 0.9 [25] and the CT14NNLO PDF set [26]. The NNLO QCD corrections are a factor of ∼ 0.98 at m=3 TeV. Mass- dependent electroweak (EW) corrections are computed at NLO with Mcsanc 1.20 [27]. The NLO EW corrections are a factor of
∼ 0.86 at m=3 TeV. Those include photon-induced contribu- tions (γ γ → via t- and u-channel processes) computed with theMRST2004QEDPDFset[28].
Dibosonprocesseswithfourchargedleptons,threechargedlep- tonsandone neutrino,ortwochargedleptonsandtwoneutrinos are simulated using the Sherpa 2.1.1 generator [29]. Matrix ele- ments contain all diagramswith four electroweak vertices. They are calculated for up to one (4, 2+2ν) or no additional par- tons(3+1ν) atNLO. Dibosonprocesseswithoneofthe bosons decayinghadronicallyandtheotherleptonicallyaresimulatedus- ing the Sherpa2.1.1 generator. Theyare calculated forup to one (Z Z) or no (W W, W Z) additional partons at NLO. All are cal- culated withup to threeadditionalpartons atleading-order(LO) using the Comix [30] and OpenLoops [31] matrix element gen- erators and merged with the Sherpa parton shower [32] using the ME+PS@NLO prescription [33]. The CT10PDF set is used in conjunction with dedicated parton shower tuning developed by theSherpaauthors.TheSherpadibosonsamplecross-sectionwas scaled down to account for its use of αQED=1/129 rather than 1/132corresponding tothe useofcurrentPDGparametersasin- puttotheGμscheme.
For the generation of tt¯ and single top quarks in the W t- channel and s-channel the Powheg-box v2 generator with the CT10 PDF set in the matrix element calculations is used. EW t-channelsingle-top-quarkeventsaregeneratedusingthePowheg- boxv1 generator.Thisgeneratorusesthe four-flavourschemefor theNLOmatrixelementcalculationstogether withthefixedfour- flavourPDFsetCT10f4.Foralltop-quarkprocesses,top-quarkspin
