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Physics Letters B
www.elsevier.com/locate/physletb
Search for a right-handed gauge boson decaying into a
high-momentum heavy neutrino and a charged lepton in pp collisions with the ATLAS detector at √
s = 13 TeV
.TheATLASCollaboration
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received30April2019
Receivedinrevisedform4August2019 Accepted22August2019
Availableonline18September2019 Editor: M.Doser
Asearchforaright-handedgaugebosonWR,decayingintoaboostedright-handedheavyneutrinoNR, intheframeworkofLeft-RightSymmetricModelsispresented.Itisbasedondatafromproton–proton collisionswithacentre-of-massenergyof13 TeV collectedbytheATLASdetectorattheLargeHadron Colliderduringtheyears 2015,2016and 2017, correspondingtoanintegrated luminosityof80 fb−1. Thesearchisperformedseparatelyforelectronsand muonsinthefinalstate.Adistinguishingfeature ofthesearchistheuseoflarge-radiusjetscontainingelectrons.Selectionsbasedonthesignaltopology resultinsmallerbackgroundcomparedtotheexpectedsignal.NosignificantdeviationfromtheStandard ModelpredictionisobservedandlowerlimitsaresetintheWRandNRmassplane.Massvaluesofthe WRsmallerthan3.8–5 TeV areexcludedforNRinthemassrange0.1–1.8 TeV.
©2019TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Overthepastdecades,therehavebeenseveralimportantdevel- opments atthe theoretical andexperimental frontiers to address thequestion ofneutrinomassgeneration,whichisnot explained in the Standard Model (SM) of particle interactions. A widely adoptedapproachtoexplainsmallneutrinomassesistheso-called seesawmechanism[1],wherethelightneutrinosacquiretheirMa- jorana masses from dimension-5 operators through electroweak symmetry breaking.The simplestseesaw mechanism can be cat- egorised into a few different classes, such as the Type-I [2–4], Type-II[5–7] andType-III[5,8] seesawscenarios.Type-IandType- II models can further be embedded into a Left-Right Symmetric Model (LRSM)[9–11]. The LRSMcontains SM-singlet heavy neu- trinos NR, which are introduced as the parity gauge partnersof thecorrespondingleft-handedneutrinofields,andaright-handed gaugebosonWR.
TheLRSMframeworkprovides anaturalset-upfortheseesaw mechanismandoffers severalfeatures includingparitysymmetry at high energy, mass generation of the light and heavy neutri- nos,explanationofparityviolationintheSMandexistenceofthe right-handedchargedcurrent.Thismodelcannaturallyexplainthe small neutrino masses through the Type-I seesaw via the right- handedneutrinos,andtheType-IIseesawviaSU(2)-tripletscalars.
BoththeType-IandType-IIcontributionscancoexist.IntheLRSM,
E-mailaddress:atlas.publications@cern.ch.
left-handedneutrinos(SMneutrinos) aswell asthe right-handed neutrinosare consideredtobe Majoranaparticles(i.e.to betheir ownantiparticles).The LRSMthus featuresviolationoftheglobal lepton number symmetry of the SM. Hence, the model can be testedbyobservinglepton-number-violatingprocesses,suchasthe Keung–Senjanovi ´cprocess [12],showninFig.1.
SearchesbytheATLAS [13,14] andCMS [15–19] collaborations for signatures of LRSMs have considered the final state contain- ing two chargedleptons andtwo jetsand haveexcluded regions ofthe (mWR,mNR) parameterspaceformWR andmNR up to sev- eralTeV,wheremNR andmWR denotethemassesofNR andWR, respectively.
This search is focused on the regime where the WR is very heavy compared with the NR (mNR/mWR ≤0.1), and investigates analternativesignatureforWR→NRdecays,followingRef. [20].
Theprobedmassregimeenablesexplorationofaparameterspace complementary to theone used inprevious searches that recon- struct the NR decayintoa chargedlepton andtwo jets, later re- ferred to asthe “resolved topology”. In the probed massregime, the heavy neutrinos are produced withlarge transverse momen- tum (i.e. are highly boosted) and their decay products are very collimated. Therefore a large-radius jet (large-R jet)can be used to reconstructall orpartofthe NR.Since jetconstruction inAT- LASincludestheenergydepositionofelectronsinthecalorimeters butnomuons,theanalysisstrategy isdifferentforthetwo cases.
Intheelectronchannel,theelectronenergydepositisincludedin theconstructed large-R jet originatingfromthedecayofthe NR, and thelarge-R jet can be considered as a proxyfor the NR. In https://doi.org/10.1016/j.physletb.2019.134942
0370-2693/©2019TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
Fig. 1.DiagramoftheWRdecayviaNRintochargedleptonsandquarks.Theleptons needtobeofthesameflavour,butcanbethesameoroppositecharges.Thedashed purplelinesindicatethattheNRdecayproductscanbeinsidealarge-Rjet.
themuon channel, thefour-momentum ofthe muonis addedto thelarge-R jettoobtaintheNRfour-momentum.Thesearchisre- strictedtothescenarioswherebothleptonshavethesameflavour.
No constraint on their charge is enforced, because of the higher probabilityofchargemisidentificationforhigh-pTelectrons.
Theresultsobtainedinthissearchare alsoapplicable toother variations of the LRSM that contain a right-handed gauge boson and neutral leptons, such as inverse seesaw models [21]. Addi- tionally, thissearch isalso applicable to R-parity-violatingsuper- symmetry [22,23], wherea selectron is resonantly produced and subsequentlydecaysintoanelectronandaneutralino,andthelat- terdecaystoaleptonandquarksthroughanon-zeroλcoupling.
Whentheneutralino isboosted,its decayproducts canbe recon- structed asa single large-R jet [24], analogous to the final state probedinthisanalysis.
2. ATLASdetector
TheATLAS detector [25] at theLargeHadron Collider(LHC) is a multipurpose particle detector with a forward–backward sym- metric cylindricalgeometryand a near4π coverage in solid an- gle.1 Itconsistsofaninnertrackingdetector(ID)surroundedbya thinsuperconductingsolenoidprovidinga2 Taxialmagneticfield, electromagnetic(EM)andhadroniccalorimeters,andamuonspec- trometer(MS). The ID consistsofsilicon pixel,siliconmicrostrip, andstraw-tubetransition-radiationtrackingdetectors,coveringthe pseudorapidityrange|η|<2.5.Thecalorimetersystemcoversthe pseudorapidityrange|η|<4.9.Electromagneticcalorimetryispro- videdbybarrelandendcaphigh-granularityleadandliquid-argon (LAr) sampling calorimeters, within the region |η|<3.2. There is an additional thin LAr presampler covering |η|<1.8, to cor- rect for energy loss in material upstream of the calorimeters.
1 ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominalin- teractionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeampipe.
Thex-axispointsfromtheIPtothecentreoftheLHCring,andthey-axispoints upwards.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeingthe azimuthalanglearoundthez-axis.Thepseudorapidityisdefinedintermsofthepo- larangleθasη= −ln tan(θ/2).Theangularseparationbetweentwoobjectsisde- finedas R≡
( η)2+( φ)2,where ηand φaretheseparationsinηandφ. Therapidityisdefinedasy=12lnEE+−ppz
z,whereEistheenergyandpzisthelongi- tudinalcomponentofthemomentumalongthebeampipe.Theangularseparation betweentwoobjectsintermsofrapidityisdefined as Ry≡
( y)2+( φ)2, where yand φaretheseparationsinyandφ.Momentuminthetransverse planeisdenotedbypT.
For |η|<2.5, the LAr calorimeters are divided into three layers in depth.Hadronic calorimetry is provided by asteel/scintillator- tile calorimeter, segmented into three barrel structures within
|η|<1.7,andtwocopper/LArhadronicendcapcalorimeters,which cover the region 1.5<|η|<3.2. The forward solid angle up to
|η|=4.9 is covered by copper/LAr and tungsten/LAr calorimeter modules, which are optimised forenergy measurements of elec- trons/photons and hadrons, respectively. The muon spectrometer istheoutermostlayerofthedetector,andisdesignedtomeasure muons up to |η| of 2.7. It comprises separate trigger and high- precisiontrackingchambersthatmeasurethedeflectionofmuons inamagneticfield generatedbysuperconductingair-coretoroids.
Themuontriggerchamberscoverupto|η|of2.4.
The ATLAS detector selects events using a tiered trigger sys- tem [26]. The first level is implemented in custom electronics andreducesthe eventratefromthebunch-crossing frequencyof 40 MHz to a design value of 100 kHz. The second level is im- plemented in software, running on a general-purpose processor farmwhichprocessestheeventsandreducestherateofrecorded eventstoabout1 kHz.
3. Dataandsimulationsamples
Thisanalysisusesproton–proton(pp)collisiondataatacentre- of-massenergy√
s=13 TeV collectedin2015,2016and2017that satisfyanumberofdata-qualitycriteria.Theamountofdataused inthisanalysiscorrespondstoanintegratedluminosityof80 fb−1. Simulated signal andbackground events are used to optimise the event selection, to validate the performance of large-R jets containing an electron,evaluatethe Z+jetsbackgroundcontribu- tion,andcalculatesignalyieldsandtheirsystematicuncertainties.
Signal events were simulated at leading order (LO) in QCD us- ing MG5_aMC@NLO 2.2.2 [27], with Pythia 8.186 [28] using the NNPDF23LO [29] parton distribution function (PDF) set and the A14setoftunedparameters(tune) [30] forpartonshoweringand hadronisation. A version of a LRSM model produced with Feyn- Rules [31] wasimplemented [32] in MG5_aMC@NLO andfurther modified by theauthorsofRefs. [33,34]. Thismodelassumesthe equivalenceofleftandright-handedweakgaugecouplings,univer- salityofalltheright-handedquarksandright-handedleptons,and thesamemassesforallthreeflavoursofheavy right-handedneu- trinos.Eventsweregeneratedwithoutconstraintsonthechargeof leptons, inlinewiththeproductionofMajorananeutrinos.Signal samples were generated for different mWR and mNR hypotheses, coveringtherangeof3–6 TeV formWR and150–600 GeV formNR. The production cross-sectionsare scaled to next-to-leadingorder (NLO)inQCDfollowingRef. [35].
The background processesconsidered are top-quark pairs (tt), Z/W+jets, singletop-quark,dibosonandmultijetproduction.Ta- ble1summarisesthegeneratorconfigurationsusedtoproducethe samples. Thett sample cross-sectionsare scaled tonext-to-next- to-leading order(NNLO)inperturbativeQCD,includingsoft-gluon resummation tonext-to-next-to-leading-log(NNLL) accuracy [36], assumingatop-quarkmassmt=172.5 GeV [37].Theresummation damping parameter, hdamp in the Powhegmodel, whichcontrols the matching of matrix elements to parton showers and regu- latesthehigh-pTradiation,wassetto1.5mt.Thesingle-top-quark and W/Z+jetssamplesarescaled totheNNLO theoreticalcross- sections [38–41].
The MC samples were processed through the full ATLAS de- tector simulation [50] based on Geant4 [51], or a faster simula- tion [52] based ona parameterisationofthecalorimeterresponse andGeant4fortheotherdetectorsystems,andreconstructedand analysed using thesame procedureandsoftware asusedfor the data.The signal modellingisfound tobe consistent betweenthe
Table 1
Mainfeatures oftheMonteCarlomodelsusedtosimulatebackgroundsamples.Topquarkreferstoboththett andsingle-top processes.MEandPSrefertomatrixelementandpartonshower,whileleadingorder(LO)andnext-to-leadingorder(NLO)indicate theaccuracyofthegeneratorsinperturbativeQCD(pQCD).ForPowheg+Pythia8,differentPDFsetswereusedinMEandPS.
Process Top quark W+jets Z+jets Diboson Multijet
Generator Powheg[42–45]+Pythia8 Powheg+Pythia8 Sherpa[46] Pythia8
ME order in pQCD NLO NLO NLO LO
Version v2, 8.186 v2, 8.186 2.2.1 8.186
PDF (ME, PS) NNPDF30NLO [47], NNPDF23LO CT10 [48], CTEQ6L1 NNPDF30NNLO NNPDF23LO
PS tune A14 AZNLO [49] Default A14
full and the fast simulation, after application of dedicated cali- brationprocedures. Tosimulatetheeffectsofadditionalcollisions in the same and neighbouring bunch crossings (pile-up), addi- tionalminimum-biaseventsgeneratedusingPythia8withtheA3 tune [53] andMSTW2008 [54] PDFsetwereoverlaidontothesig- nal andbackground simulated events, with a distribution of the number of collisions matching that of the data. To account for thedifferencesinparticlereconstruction,trigger,identificationand isolationefficiencies betweensimulationanddata,correctionfac- tors are derived in dedicated measurements and applied to the simulatedevents.
4. Eventselectionandcharacterisation
The event selection is designed to select signal events, while rejecting background events, based on the signal topology. The eventsare selected ifthey contain exactlytwo same-flavour lep- tons (with no charge requirement) and at least one trimmed large-R jet [55] withlarge transverse momentum pT>200 GeV.
Thehighest-pT(leading)leptonshouldbeback-to-backinazimuth with the large-R jet, while other (subleading) lepton should be containedin the large-R jet. In Fig. 2, the reconstructed pT dis- tributionsof theleading andsubleadinglepton,aswell asofthe selected large-R jet, and the candidate NR mass are shown for four representative signal samples. The leading electron and the large-R jet are balanced in pT, withthe maximaat roughly half ofthecorrespondingmWR values.TheleadingmuonpTshowsthe samecharacteristic,butthepTofthelarge-Rjetislowerandhas abroaderdistribution,asitdoesnotcontaintheenergyfromthe subleadingmuon,andthemuon pT resolutionforhigh-pT muons isworse.ThereconstructedmassoftheNRineachcaseisconsis- tentwiththeexpectedvalue.The naturalwidthoftheresonance varieswiththemassandis100GeV formWR=3 TeV.Atthismass thewidthofthereconstructedmasspeakisabout150GeV inthe electronchannel,andabout350GeV inthemuonchannel.
Thedetailedselection criteriaarelistedinTable2 andfurther discussed below. Events with electrons and muons are analysed separately.The leading lepton is required to passa single-lepton trigger.Fordatacollected in2015,thelowest pT triggerthreshold is24 GeV and20 GeV forsingle-electronandsingle-muontriggers, respectively.For2016and2017data,thethresholdis26 GeV for both.
Electroncandidatesare reconstructed froman isolated energy depositintheelectromagneticcalorimetermatchedtoanIDtrack, within thefiducial region oftransverse energy pT>26 GeV and
|η|<2.47. Candidates within the transition region between the barrelandendcapelectromagneticcalorimeters,1.37<|η|<1.52, are excluded. Muon candidates are reconstructed by combining tracksfoundintheIDwithtracksfoundinthemuonspectrometer andare required to satisfy pT>28 GeV and |η|<2.5.Electrons and muons are required to be isolated using criteria based on tracksand calorimeterenergy deposits. Fortrack-based isolation, the discriminating variableis the scalar sumof the pT of tracks
comingfromthe primary vertex2 ina variable-sizeconearound theleptondirection(excludingthetrackidentified asthelepton), with the cone size given by the maximum of R=10 GeV/pT and R0, where pT is the pT ofthe lepton, and R0 is a constant, setto 0.2forelectrons,and0.3formuons. Forcalorimeter-based isolation, thediscriminatingvariable isthesumofthetransverse energiesoftopologicalclusters [56] aroundtheleptoninaconeof size R=0.2.
The inputstothe jetconstruction are noise-suppressed three- dimensionaltopologicalclustersofenergydepositsinthecalorime- ters,builtfromcalorimetercells [56].Theyareclassifiedaseither electromagneticorhadronic,basedon theirshape,depthanden- ergy density. The energy clusters are calibrated to the hadronic scale.The momentaofthe jetsarecorrected forenergylossesin passive material and for the non-compensating response of the calorimeter [57].Thelarge-R jetsareconstructedwiththeanti-kt algorithm [58] with a radius parameter of R=1.0, through its implementation inFastJet [59]. Theyare furthertrimmed [55] to reducethecontaminationfromsoftuncorrelatedradiation.Inthis method,theoriginalconstituentsofthejetsarereclusteredusing thekt algorithm [60] witharadiusparameter Rsub=0.2 inorder to produce a collection ofsubjets. These subjets are discarded if theycarrylessthenaspecificfraction(fcut=5%)ofthepTofthe original jet. The remaining constituents are summedto form the four-momentumofthefinaljet.
Intheelectronchannel,thelarge-R jetsarerequiredtohavea massofatleast50 GeV,whilenosuch requirementis appliedin themuonchannel.Thisisbecauseintheformercase,thelarge-R jetincludestheelectron,whileinthemuonchannel,themuon is notincludedinthelarge-R jet.
Small-radiusjets constructedwiththe anti-kt algorithm using energyclusterscalibratedto theelectromagnetic scalewitha ra- dius parameterof R=0.4 areused tocheck forpossible overlap betweenobjects,andtoperformb-tagging(describedinSection5).
Inthemuonchannel,theeventisdiscardedifeithermuonsatisfies Ry(μ,jet)<min(0.4,0.04+10 GeV/pμ
T), inorder to avoidjets formedfromenergydepositsassociatedtohighenergymuons. In theelectronchannel,fortheleadingelectron,firstallsmall-radius jets within Ry=0.2 of a selected electron are removed. Then theeventisdiscardediftheleading electroniswithin Ry=0.4 of a remaining small-radiusjet. This is referred to as the nomi- naloverlapremovalprocedureforelectrons.Amodifiedprocedure, described in Section 5, is applied for thesubleading electron as, unlikemuons,electronclusterscanoverlapwithajetandthesig- nalefficiencydropsoffifthestandardoverlapremovalapproachis followed.
Furtherrequirementsbasedonthecharacteristicsofthesignal areapplied:
2 CollisionverticesareformedfromtrackswithpT>400 MeV.Ifaneventcon- tainsmorethanonevertexcandidate,theonewiththehighest
p2Tofitsassoci- atedtracksisselectedastheprimaryvertex.
Fig. 2.Reconstructeddistributionsofthetransversemomentumoftheleadinglepton,subleadinglepton,theselectedlarge-Rjet,andtheNR candidatemassinelectron (leftcolumn)andmuon(rightcolumn)channelsforfourrepresentativesignalsamplesinthesignalregion.Theindices1and2indicateleadingandsubleadinglepton, respectively.
