Article
Reference
Search for heavy resonances decaying to a W or Z boson and a Higgs boson in the qq¯(′)bb¯ final state in pp collisions at √s = 13 TeV
with the ATLAS detector
ATLAS Collaboration AKILLI, Ece (Collab.), et al .
Abstract
A search for heavy resonances decaying to a W or Z boson and a Higgs boson in the qq¯(′)bb¯ final state is described. The search uses 36.1 fb−1 of proton–proton collision data at s=13 TeV collected by the ATLAS detector at the CERN Large Hadron Collider in 2015 and 2016. The data are in agreement with the Standard Model expectations, with the largest excess found at a resonance mass of 3.0 TeV with a local (global) significance of 3.3 (2.1) σ . The results are presented in terms of constraints on a simplified model with a heavy vector triplet. Upper limits are set on the production cross-section times branching ratio for resonances decaying to a W ( Z ) boson and a Higgs boson, itself decaying to bb¯ , in the mass range between 1.1 and 3.8 TeV at 95% confidence level; the limits range between 83 and 1.6 fb (77 and 1.1 fb) at 95% confidence level.
ATLAS Collaboration, AKILLI, Ece (Collab.), et al . Search for heavy resonances decaying to a W or Z boson and a Higgs boson in the qq¯(′)bb¯ final state in pp collisions at √s = 13 TeV with the ATLAS detector. Physics Letters. B , 2017, vol. 774, p. 494-515
DOI : 10.1016/j.physletb.2017.09.066
Available at:
http://archive-ouverte.unige.ch/unige:97175
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Search for heavy resonances decaying to a W or Z boson and a Higgs boson in the q q ¯
()b b ¯ final state in pp 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:
Received21July2017
Receivedinrevisedform13September 2017
Accepted22September2017 Availableonlinexxxx Editor: W.-D.Schlatter
AsearchforheavyresonancesdecayingtoaW orZbosonandaHiggsbosonintheqq¯()bb¯finalstate isdescribed.Thesearchuses36.1 fb−1ofproton–protoncollisiondataat√
s=13 TeV collectedbythe ATLASdetectorattheCERNLargeHadronColliderin2015and2016. Thedataare inagreementwith theStandardModelexpectations, withthelargestexcessfoundataresonancemassof3.0TeVwitha local(global)significanceof3.3(2.1)σ.Theresultsarepresentedintermsofconstraintsonasimplified modelwithaheavyvectortriplet.Upperlimitsaresetontheproductioncross-sectiontimesbranching ratioforresonancesdecaying toaW (Z)bosonandaHiggsboson, itselfdecayingtobb,¯ inthe mass rangebetween1.1and3.8 TeV at95%confidencelevel;thelimitsrangebetween83and1.6 fb(77and 1.1 fb)at95%confidencelevel.
©2017PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
The discovery of the Higgs boson [1,2] confirms the validity of the Standard Model (SM) in the description of particle inter- actions atenergies up toa few hundred GeV. However, radiative correctionstotheHiggsbosonmassdriveitsvaluetothemodel’s validitylimit,indicatingeitherextremefine-tuningorthepresence ofnewphysics atan energyscalenot farabove the Higgsboson mass.It is natural to expect such newphysics to manifest itself through significant coupling to the Higgs boson, for example in decaysofnewparticles toa Higgsbosonandother SM particles.
ThisLetterpresentsasearchforresonancesproducedin36.1 fb−1 ofproton–proton(pp) collisiondataat√
s=13 TeV whichdecay toa W or Z bosonanda Higgsboson.Such resonancesare pre- dictedinmultiplemodelsofphysicsbeyondtheSM,e.g. composite Higgs[3,4]orLittleHiggs[5]models,ormodelswithextradimen- sions[6,7].
ThissearchisconductedinthechannelwheretheW orZ and Higgs bosons decay to quarks. The high mass region, with res- onance masses mV H >1 TeV (V =W,Z), where the V and H bosons are highly Lorentz boosted, is considered. The V and H bosoncandidates areeach reconstructed ina single jet,using jet substructuretechniques and b-tagging to suppressthe dominant backgroundfrommultijeteventsandtoenhancethesensitivityto
E-mailaddress:atlas.publications@cern.ch.
the dominant H→bb¯ decaymode. The reconstructeddijet mass distributionisusedtosearchforasignaland,initsabsence,toset boundson theproductioncross-section timesbranching ratiofor newbosonswhichdecaytoaW or Z bosonandaHiggsboson.
Theresultsareexpressedaslimitsinasimplifiedmodelwhich incorporates a heavy vector triplet (HVT) [8,9] of bosons; this choice allows the results to be interpreted in a large class of models. The new heavy vector bosons couple to the Higgs bo- son and SM gauge bosons with coupling strength cHgV and to the SM fermions with coupling strength (g2/gV)cF, where g is theSM SU(2)Lcouplingconstant.The parameter gV characterizes the interactions of the newvector bosons, while thedimension- lesscoefficientscH andcF parameterizedeparturesofthistypical strengthforinteractionswiththeHiggsandSMgaugebosonsand withfermions,respectively, andareexpectedtobeoforderunity inmostmodels.Twobenchmarkmodelsareused:inthefirst,re- ferredtoasModel A,thebranchingratiosofthenewheavyvector bosontoknownfermionsandgaugebosonsarecomparable,asin someextensionsoftheSMgaugegroup[10].InModel B,fermionic couplings to the new heavy vector boson are suppressed, as for exampleinacompositeHiggsmodel[11].TheregionsofHVTpa- rameterspacestudiedcorrespondtotheproductionofresonances withan intrinsicwidththat isnarrowrelative tothe experimen- talresolution.Thelatterisroughly8%oftheresonancemass.The sensitivityoftheanalysistowiderresonancesisnottested.Inad- dition,while theproductionrates ofthe newheavy chargedand neutralstates are relatedwithin the HVTmodel, thesearch pre- https://doi.org/10.1016/j.physletb.2017.09.066
0370-2693/©2017PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
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sentedhereassumestheproductionof onlya chargedorneutral resonanceandnotbothsimultaneously.
Searchesfor V Hresonances, V,haverecentlybeenperformed by theATLAS andCMScollaborations.The ATLAS searches(using leptonic V decays) based on data collected at √
s=8 TeV set a lowerlimit atthe95%confidencelevel(CL)onthe W (Z)mass at1.47 (1.36) TeV in HVT benchmark Model A with gV=1 [12].
Usingthe samedecaymodes,a search basedon3.2fb−1 ofdata collectedat√
s=13 TeV seta 95%CLlowerlimitonthe W (Z) massat 1.75 (1.49) TeV [13] in theHVT benchmarkModel A. For Model Bthe corresponding limitsare 2.22(1.58) TeV.Searchesby theCMSCollaborationat√
s=8 TeV inhadronicchannels,based onHVTbenchmarkModel BwithgV=3,excludeheavyresonance massesbelow 1.6 TeV (W→W H), below1.1 TeV and between 1.3 TeV and 1.5 TeV (Z→Z H), and below 1.7 TeV (combined V→V H) [14] at the 95% CL. Using the W→W H→νbb¯ channel, CMSexcludesnewheavy vector bosons withmassesup to 1.5 TeVin the same context [15]. The CMS Collaboration also carried out a search for a narrow resonance decaying to Z H in theqq¯τ+τ− finalstate,settinglimitsonthe Z productioncross- section [16]. Searches forheavy resonances in HVTmodels have alsobeen carried out in thehadronic W W/W Z/Z Z channelsby theATLASexperimentat√
s=13 TeV with3.2 fb−1 ofdata[17].
For Model B, a new gauge boson with mass below 2.6 TeV is excluded at the 95% CL. The CMS Collaboration combined [18]
diboson resonance searches at √
s=8 and 13 TeV [18], setting lower limits for W and Z singlets at 2.3 TeV and for a triplet at2.4 TeV.As thisLetterwas beingfinalized,the CMSCollabora- tionreleased[19]asearchinthesamefinalstateasstudiedinthis Letter,using36 fb−1 ofdatacollectedat√
s=13 TeV.ForModel B, aWbosonwithmassbelow2.45 TeVandbetween2.78 TeVand 3.15 TeVisexcluded atthe95% CL.Fora Zboson,massesbelow 1.19 TeVandbetween1.21 TeV and2.26 TeVare excludedatthe 95% CL.
2. ATLASdetector
TheATLAS detector[20] isa general-purpose particledetector usedtoinvestigateabroadrangeofphysicsprocesses.Itincludes innertrackingdevicessurroundedbya2.3mdiametersupercon- ducting solenoid, electromagnetic and hadronic calorimeters and amuonspectrometerwithatoroidalmagneticfield.Theinnerde- tectorconsistsofahigh-granularitysiliconpixeldetector,including theinsertableB-layer [21]installedafterRun1oftheLHC,asili- constripdetector,andastraw-tubetracker.Itisimmersedina2T axialmagneticfieldandprovidesprecisiontrackingofchargedpar- ticleswithpseudorapidity|η|<2.5.1 Thecalorimetersystemcon- sistsoffinelysegmentedsamplingcalorimetersusinglead/liquid- argon for the detection of electromagnetic (EM) showers up to
|η|<3.2,andcopperortungsten/liquid-argonforelectromagnetic and hadronic showers for 1.5<|η|<4.9. In the central region (|η|<1.7),asteel/scintillatorhadroniccalorimeterisused.Outside thecalorimeters,themuonsystemincorporatesmultiplelayers of trigger and tracking chambers within a magnetic field produced by asystemofsuperconducting toroids, enablingan independent precisemeasurementofmuontrackmomentafor|η|<2.7.A ded- icated triggersystem isused toselectevents [22]. The first-level
1 ATLASusesaright-handed coordinatesystemwith itsoriginat thenominal interactionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeam pipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis pointsupward.Cylindricalcoordinates (r,φ)areusedinthe transverseplane,φ beingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdefinedinterms ofthe polarangleθ asη= −ln tan(θ/2).Therapidityisdefined relativetothe beamaxisasy=1/2ln((E+pz)/(E−pz)).
triggeris implementedinhardwareandusesthe calorimeterand muondetectorstoreducetheacceptedrateto100kHz.Thisisfol- lowed by a software-based high-level trigger, which reduces the acceptedeventrateto1kHzonaverage.
3. Dataandsimulationsamples
Thisanalysisuses36.1 fb−1ofLHCppcollisionsat√
s=13 TeV collected in 2015and 2016. The data were collected during sta- blebeamconditionswithallrelevantdetectorsystemsfunctional.
Events wereselected usingatriggerthat requiresasingle anti-kt jet [23]withradiusparameter R=1.0 (large-R jet)withatrans- verseenergy(ET) thresholdof360(420)GeVin2015(2016).The trigger requirement is >99% efficient for events passing the of- fline selection of a large-R jet with transverse momentum (pT)
>450 GeV.
Signal processes, aswell asbackgrounds fromtt¯ and W/Z + jets production, are modelled with Monte Carlo (MC) simula- tion. While multijet MC events are used as a cross-check, the primary multijet background estimation is performed using data as described in Section 6. The signal is modelled using bench- mark Model A with gV =1. Results derived from this model can be directly applied to benchmark Model B by rescaling the relevant branching ratios. The signal was generated with Mad- graph5_aMC@NLO 2.2.2 [24] interfaced to Pythia 8.186 [25] for parton shower and hadronization, with the NNPDF2.3 next-to- leadingorder(NLO)partondistributionfunction(PDF)set[26]and a setoftuned parameterscalledtheATLASA14tune [27]forthe underlying event. The Higgs boson mass was set to 125.5 GeV, andHiggsbosondecaysto bothbb¯ andc¯c,assumingSM branch- ing ratios, were included in the simulation. The V→V H→ qq¯()(bb¯+cc¯) signal cross-section inModel B ranges from110 fb (203 fb) for neutral(charged) resonances with a mass of 1 TeV, down to0.09 fb(0.19 fb)forneutral(charged)resonanceswitha mass of3.8 TeV.Samples weregenerated insteps of100GeVor 200GeVupto4TeV.
The tt¯ background samples were generated with Powheg- Box v2 [28] with the CT10PDF set [29], interfaced with Pythia 6.428 [30] andthePerugia2012tune forthepartonshower[31]
usingtheCTEQ6L1PDFset[32].Thecross-sectionofthett¯process is normalizedtothe resultofa quantumchromodynamics(QCD) calculation at next-next-to-leading order and log (NNLO+NNLL), as implemented in Top++ 2.0 [33]. The Powheg hdamp parame- ter[34]wassettothetopquarkmass,takentobemt=172.5 GeV.
The W+jets and Z+jetsbackgroundsampleswere generatedwith Sherpa 2.1.1 [35] interfaced with the CT10 PDF set. Matrix ele- mentsofuptofourextrapartonswerecalculatedatleadingorder in QCD. Only the hadronicdecays ofthe W and Z bosons were included.Forstudieswithsimulatedmultijetevents,theMCsam- ples were generated with Pythia 8.186 [25],with the NNPDF2.3 NLOPDFandtheATLASA14tune.ThebackgroundfromSM dibo- sonandV Hproductionisnegligibleandthereforenotconsidered.
Forall simulatedevents, exceptthoseproduced usingSherpa, EvtGen v1.2.0 [36] was used to model the properties of bottom and charm hadron decays. The detector response was simulated with Geant 4 [37,38] and the events were processed with the samereconstruction softwareasthat usedfordata.Allsimulated samples include the effects due to multiple pp interactions per bunch-crossing(pile-up).
4. Eventreconstruction
Collisionverticesarereconstructedrequiringaminimumoftwo trackseachwithtransversemomentumpT>0.4 GeV.Theprimary
