• Tiada Hasil Ditemukan

Search for single production of vector-like quarks decaying into a b quark and a W boson in proton–proton collisions at s = 13 TeV

N/A
N/A
Protected

Academic year: 2022

Share "Search for single production of vector-like quarks decaying into a b quark and a W boson in proton–proton collisions at s = 13 TeV"

Copied!
23
0
0

Tekspenuh

(1)

Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Search for single production of vector-like quarks decaying into a b quark and a W boson in proton–proton collisions at √

s = 13 TeV

.TheCMS Collaboration

CERN,Switzerland

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received28January2017

Receivedinrevisedform21June2017 Accepted10July2017

Availableonline15July2017 Editor:M.Doser

Keywords:

CMS Physics

A searchispresented foraheavy vector-likequark,decayingintoab quarkand aWboson,whichis producedsinglyinassociationwithalightflavorquarkandabquark.Theanalysisisperformedusinga datasampleofproton–protoncollisionsatacenter-of-massenergyof

s=13TeV collectedattheLHCin 2015.Thedatasetusedintheanalysiscorrespondstoanintegratedluminosityof2.3 fb1.Thesearchis carriedoutusingeventscontainingoneelectronormuon,atleastoneb-taggedjetwithlargetransverse momentum,atleastonejetintheforwardregionofthedetector,andmissingtransversemomentum.No excessoverthestandardmodelpredictionisobserved.Upperlimitsareplacedontheproductioncross sectionofheavyexoticquarks:aTquarkwithachargeof2/3,andaYquarkwithachargeof4/3.For Yquarkswithcouplingof0.5andB(YbW)=100%,theobserved(expected)lowermasslimits are 1.40(1.0) TeV.ThisisthemoststringentlimittodateonthesingleproductionoftheYvector-likequark.

©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.

1. Introduction

The standard model (SM) of particle physics has been ex- ceptionallysuccessful in describing phenomena atthe subatomic scale. The observationof a Higgs boson witha mass of 125 GeV and with properties consistent with the SM expectations [1–3]

completed the SM. However, in the absence of enormousorder- dependent cancellations, also known as fine-tuning, large SM quantumcorrectionswouldshiftthebareHiggsbosonmasstoval- uesfar beyondtheelectroweak scale.New physics isrequiredto stabilizetheHiggsbosonmassnaturallyattheelectroweakscale, i.e.withoutinvokingfine-tuning.

ManynaturalextensionsoftheSM havebeenproposedin re- cent decades. Some of these models postulate the existence of vector-like quarks (VLQs)[4–6], whichare colored fermions with left- andright-handedchiralstatesbothtransforminginthesame way under the gauge group SU(3)C×SU(2)L×U(1)Y. The VLQs donotacquire massesthroughtheYukawacouplingtothe Higgs field,andcouldcancelloopcorrectionsfromtheSM topquarkto theHiggsbosonmass.

SearchesforVLQshavealreadybeenperformedinvariousde- cay modes using proton–proton collisions at

s=8TeV. These searches were primarily focused on the pair production mecha- nism andthey ruled out VLQs withmassesup to approximately

E-mailaddress:cms-publication-committee-chair@cern.ch.

0.90 TeV[7–10].TheVLQsingleproductionmechanismiscoupling- dependent,anditcouldbecomethedominantcontributiontothe cross section at high VLQ masses. The strength of the VLQ-b-W coupling can be approximately characterized by a single dimen- sionlessparameterthatvariesfrom0to

2[11],wherethelatter wouldcorrespondtoacouplingoffullelectroweakstrength.

Inthispaper,wepresentasearchforthesingleproductionofa heavyvector-like quarkthatdecaysintoabquarkandaW boson usingthe 2015LHC dataset.Thissignaturecan arisefromeither a YoraTquarkwithacharge of4/3 or2/3,respectively,pro- ducedinassociation witha lightflavor quark anda bquark. The leading orderFeynman diagram for Y andT quark production is shown in Fig. 1. The outgoing light flavor quark q inthe upper part of the diagram produces a jet in the forward region of the detector,whichisadistinctsignatureofsingleproduction.

TheYquarkisexpectedtodecaywithabranchingfraction(B) of 100% into a b quark anda W boson [12], while the Tquark canalsodecayintotHandtZviaaflavorchangingneutralcurrent.

Searches with the 2015 LHC data set for single production of a vector-likeTquarkdecayingtotHandtZhavebeenperformedby theCMSCollaboration[13–15].Ifthe Tquark isasinglet,then it isexpectedtodecayintobW50%ofthetime.

The ATLASCollaboration publisheda search forsingleproduc- tionofYandTquarksdecayingintobWusing8 TeV proton–proton collisions [16].Theanalysispresentedhereisthefirstsuchsearch using13 TeV proton–protondata,andsetsthemoststringentlimits to dateontheproductioncrosssectionfora singleY orTquark.

http://dx.doi.org/10.1016/j.physletb.2017.07.022

0370-2693/©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.

(2)

Fig. 1.Leading order Feynman diagram for singly produced Y or T quarks.

Thesearchiscarriedoutbasedoneventscontaining oneelectron ormuon,atleastoneb-taggedjet withlargetransverse momen- tum(pT),atleastonejetintheforwardregionofthedetector,and missingtransversemomentum.

2. CMSdetectorandeventsamples

TheessentialfeatureoftheCMSdetectoristhesuperconduct- ingsolenoid,6 m indiameterand13 m inlength,whichprovides an axial magnetic field of 3.8 T. Within the solenoid volume a multi-layeredsiliconpixelandstriptrackerisusedtomeasurethe trajectoriesofchargedparticleswithpseudorapidity|η|<2.5.Out- sideofthe trackersystem, anelectromagnetic calorimeter(ECAL) madeoflead tungstatecrystals andahadron calorimeter(HCAL) made of brass and scintillators cover the region |η|<3.0. The region3.0<|η|<5.0 iscoveredbytheforwardhadroniccalorime- ter,whichismadeprimarilyofsteelandquartzfibers.Muonsare measuredingas-ionization detectorsembedded inthe steelflux- return yoke of the solenoid, and covering the region |η|<2.4.

A moredetaileddescriptionof theCMSdetector,together witha definition of the coordinate system used and the relevant kine- maticvariables,canbefoundinRef.[17].

Thedatausedforthisanalysiswererecordedduringthe2015 datatakingperiodinproton–protoncollisions ata center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 2.3 fb1. The electron data sample was collected using a trigger thatrequiredatleastoneisolatedelectron with|η|< 2.5and pT

> 27 GeV. The muon data sample was collected using a trigger thatrequiredatleastoneisolatedmuon with|η|<2.1and pT >

20 GeV.

The VLQ signal efficiencies and background contributions are estimated using Monte Carlo (MC) samples. They are validated using background enriched data samples. The tt+jets, t- and tW-channel single top-quark production and the WW processes are simulated using powheg v2 [18–20]. Single top quark pro- duction via s-channel and the WZ process are simulated with MadGraph5_amc@nlov2[21].Inclusivebosonproduction(W+jets andZ+jets)issimulatedwithMadGraphv5[22].pythia8.212[23, 24]isusedforpartonshowerdevelopmentandhadronizationand tosimulateQCDmultijetevents.

TheVLQprocessesconsideredinthispaperaregeneratedusing thetree-levelMCeventgeneratorMadGraphv5forVLQmassesin therangefrom0.70to1.80 TeV,instepsof100 GeV.TheVLQwidth issetto10 GeV forallmasses.TheNNPDF3.0[25]partondistribu- tionfunctions(PDFs)areusedforbothsignalandSMMCprocesses tomodelthe momentumdistributionof thecollidingpartonsin- sidetheprotons.

The cross sections used to normalize the SM processes are calculated to next-to-leading order (NLO) or to next-to-next-to- leadingorder(NNLO),wherethelatterisavailable[26–28].Forthe

signal,theNLOcrosssectionsaretakenfromRefs.[29,30].Forthe tt+jets,tW-channelsingletop-quark,andWW SMprocesses,NNLO crosssectionsareused,whileNLOcrosssectionsareappliedtothe remainingprocesses.

All generated events are processed through the CMS detec- tor simulation based on Geant4 [31]. Additional minimum bias events, generated with pythia 8.212, are superimposed on the hard-scatteringeventstosimulatemultipleproton–protoninterac- tions (pileup) within the neighboring bunch crossings. The sim- ulated events are weighted to reproduce the distribution of the numberofpileupinteractions,20onaverage,observedindata.

3. Eventreconstruction

All physics objects in the event are reconstructed using a particle-flow(PF)algorithm [32,33],whichuses informationfrom all subsystems to reconstruct photons, electrons, muons, and chargedand neutralhadrons. Charged particletracks are usedto reconstruct the interaction vertices. The vertex with the highest sumofsquaredpT ofallassociatedtracksistakenastheprimary vertex of the hard collision. Filters are applied to reject events whereelectronicnoiseorproton-beambackgroundsmimicenergy depositsinthedetector.

Electroncandidates are reconstructedby combiningthe track- ing information with energy deposits in the ECAL in the range

|η|<2.5 (excluding the range 1.4442<|η|<1.566, which is a transition region between endcapand barrel calorimeters). Tight identificationcriteriaareappliedtoselectwell-reconstructedelec- tron candidates. Candidates are identified [34] using information onthe shower-shape,thetrackqualityandthespatial matchbe- tween the track and the electromagnetic cluster, the fraction of total clusterenergyin theHCAL,andthe resultinglevelof activ- ityinthesurroundingtrackerandcalorimeterregions.Theenergy resolutionforelectronswith pT>40GeV,measuredusingZee decays,isonaverage1.7%intheECALcentralregionofthedetec- tor[34].

Muon candidates are identified using track segments recon- structedseparatelyfromhitsinthesilicontrackingsystemandin themuonsystem.Toidentifymuoncandidates,thetracksegments mustbeconsistentwithmuonsoriginatingfromtheprimary ver- texandsatisfyingtight identificationrequirements.The matching ofthemuonandsilicontracksegmentsresultsinarelativepTres- olutionof1.3–2.0%inthecentralregionofthedetectorformuons with20<pT<100GeV, andformuonswith pT upto 1 TeV the resolutionis10%orbetter[35].

Lepton(electron ormuon)reconstruction and triggerefficien- cies are evaluated asa function of pT and |η| in both data and simulation, using a “tag-and-probe” method [36] with recorded andsimulatedsamplesofdileptonicZevents.

An isolation variable is employed to suppress leptons origi- nating fromQCD processes. We define a relative isolation as the sum of the pT of particle tracks found in the tracker and en- ergy deposits found in the calorimeters within a cone R=

(η)2+(φ)2= 0.3(0.4) aroundthetrajectory ofthe electron (muon), divided by the lepton pT. Relative isolation is corrected forthe effects ofpileup, andisrequired to be lessthan 0.15for muons,andlessthan0.4(0.6)forelectronsinthebarrel(endcap) region.

Particles reconstructed by the PF algorithm are clustered into jetsbyusing thedirectionofeachparticleattheinteraction ver- tex.ChargedhadronsfoundbythePFalgorithmthatareassociated withpileupverticesarenotconsidered.Particlesthatareidentified asisolatedleptons areremovedfromthejetclustering procedure.

Jets are reconstructed with the anti-kT algorithm [37,38] witha

(3)

distance parameter of 0.4. An event-by-event jet-area-based cor- rection[39,40]isappliedtoremove,on astatisticalbasis, neutral pileup contribution that is not already removed by the charged- hadron subtractionproceduredescribed above. Jet energycorrec- tionsareappliedtoeachjet,asafunctionof pT and η,tocorrect forthecalorimeterresponse[41].

The missingtransverse momentum is defined asthe negative vector sumof the transverse momenta ofall the particles found by the PF algorithm, and its magnitude is referred to as ETmiss. Thedecayofaheavy quark intoaleptonicallydecaying W boson anda b quark is expectedto exhibit genuine missingtransverse momentum because ofthe undetected neutrino fromthe W de- cay. A missing transverse momentum thresholdis appliedto the selected events,andthe missingtransverse momentum vector is usedinthemassreconstruction.

Toidentifyjetsoriginatingfroma bquark (b-taggedjets),the combined secondary vertex(CSV) algorithm is used [42,43]. This taggingalgorithmcombinesvariablesthatcandistinguishbquark jets from those originating from light flavors, such as informa- tionontrackimpactparameter significanceandsecondary vertex properties. The variables are combined using a likelihood ratio technique to compute ab taggingdiscriminator. We use theCSV mediumoperatingpoint[42],whichachievesabtaggingefficiency ofapproximately70% anda mistagrateof1%.Data-to-Simulation efficiencyandmistagratescalefactorsaccountforthesmalldiffer- encesobservedbetweendata andsimulation.We usethesescale factors as a function of jet pT and η [42] to correct simulated events.

4. Eventselectionandsearchstrategy

The signal event selection requires exactly one lepton with pT>40GeV and |η|<2.1.Events withadditionalleptons having pT>10GeV and |η|<2.5 and passing relatively loose isolation andidentificationrequirementsarerejectedtosuppressdileptonic events.

Eventsarerequiredtohaveatleasttwojets,oneinthecentral andoneintheforwardregionofthedetector.Thecentraljetisre- quiredtohavepT>200GeV and|η|<2.4 andbeb-tagged.When thereismorethanonecentraljetsatisfyingtheabovecriteria,the leadingcentraljetisusedtoreconstructthemassoftheVLQ.The forwardjet(2.4<|η|<5.0)musthavepT>30GeV.

InthedecayofasinglyproducedVLQ,thebquark andtheW boson tend to be produced with the transverse momenta point- inginoppositedirections.Hence,theazimuthalanglebetweenthe centralb jetandthe lepton isrequired tosatisfy φ (,b)>2.

In addition,the lepton is requiredto be separated fromany jets with pT>40GeV producedinthe event. When ahadronic jetis found within R(,jet)<1.5, theeventisrejected. Since W bo- son originating from heavy VLQ decayhas significant pT, events are requiredto havesubstantial EmissT (>50GeV)due tothe un- detected neutrinofromthe Wboson decay.The transverse mass, MT,formedby the leptonand EmissT systemisrequiredto satisfy MT<130GeV tosuppresstt dileptonevents,whichcanmimicthe signalwhenoneoftheleptonsescapesdetection.

Finally,eventsarerequiredto have ST>500GeV,where ST is definedasthescalar sumofthetransverse momentaof thelep- ton,theleadingcentraljet,andthemissingtransversemomentum.

Thisrequirementreducesthesignalefficiencybylessthan10%for theVLQmassrangeconsideredinthispaper.

Theinvariantmassoftheheavyquarkcandidate,Minv,isrecon- structed from its decay products: the lepton, the leading central jet, and the neutrino, where the x,y-components of the neu- trino momentum are given by the missing transverse momen- tum, while the z-component is determined by constraining the

invariant massof the lepton andneutrino to the W boson mass value. The solution with the smallest value is considered as the z-component.Thismethodisused onlywhenthe solutionofthe relevant quadraticequation isreal,otherwisethe z-componentis settozero.

The single VLQ productionY/TbW would resultin a peak intheMinvdistributionatthemassoftheVLQ.Theexperimental massresolutionis12–15%andisindependentoftheVLQmass.

5. Backgroundmodeling

Thedominantbackgroundprocessesinthissearcharethepro- ductionoftt andW+jetsevents.Themodelingoftheseprocesses isvalidatedbystudyingbackground-enrichedsamples.

Toverifythemodelingofthett process,weselecteventswith the lepton and EmissT fulfilling the signal selection criteria, and at least 2 b-tagged jetswith the leading (sub-leading) jet satis- fying the requirement of pT>70(30)GeV. We also remove the R(,jet), φ (,b) and forward jet requirements to enrich the samplewithtt events.

The top quark pT spectrum from the tt simulation is known to bemismodeledandisreweightedusingtheempiricalfunction describedinRef.[44].Afterthiscorrection,thedatapointsatlarge valuesofallrelevantkinematicdistributionsareconsistentwithin systematicuncertainties.DistributionsofSTandtheinvariantmass ofthebWsysteminthett sampleareshowninFig. 2.

TheW+jets-enrichedcontrolsamplerequirementisidenticalto the signal event selection except that events with b-tagged jets are vetoed.We observethatin theW+jetssimulatedsample, the numberofeventsatlarge jet pT distributions isoverestimatedas compared with the distributions measured in data. We derive a correction fortheW+jetssimulationasafunctionofthe HT vari- able, definedasthescalar sumofthe transversemomenta ofall jetswithpT>30GeV.ThedatatosimulationratiooftheHTdistri- butioniswelldescribedbya2-parameterlinearfitwithanegative slope. A correction to the modeling of the W+jets HT spectrum is made using the results of the fit. After the correction is ap- plied, goodagreement inthe modeling ofall kinematicvariables isobserved.DistributionsofST andtheinvariantmassofthebW systemintheW+jetssampleareshowninFig. 3.

6. Systematicuncertainties

Wedividethesystematicuncertaintiesintotwocategories:un- certaintiesthatimpactonlytherateofbackgroundandsignalpre- dictions,anduncertaintiesthat affectboththerateandtheshape of the fitted Minv spectra. The shape uncertainties affecting the Minv distributionaremodeledbyvaryingthenuisanceparameters thatcharacterizetheassociatedsystematiceffectsupanddownby onestandarddeviation.

The uncertainty in the integratedluminosity is2.7% [45]. We assign the uncertainties for the normalization of the SM back- groundprocessesastheuncertaintiesoncorrespondingCMScross section measurementsat13 TeV,whichare5.6%fortt[46],14.7%

for single top quark [47], and 9.2% for W+jets [48], where in the last casewe also account separately foruncertainties in the W+heavy-flavorcontributions[49,50].

To account for the MC mismodeling correction in the W+jets sample,wederiveatwo-sideduncertaintybandusingtheHTcor- rectionprocedure. Toaccountforthe MCmismodeling correction inthett sample,wederiveatwo-sideduncertaintybandusingthe top pTreweightingprocedure.Onesideofthebandisobtainedby removing thecorrection,andtheotherside isobtainedby apply- ingtheproceduretwice.Theuncertaintiesduetothesecorrections increasewiththeriseofthetopquark pT andHT,whichleadsto

Rujukan

DOKUMEN BERKAITAN

a) clone the GRA2 and GRA5 gene fragments into prokaryotic vector, pRSET B and eukaryotic vector, pcDNA 3.1C followed by recombinant protein expression using prokaryotic

―Preparation and Characterization of Proton Exchange Membrane from Sulfonated Polystyrene and Zeolite for Microbial Fuel Cell Applications.‖ International Conference

Practically, we notice that the modified memory consideration improves the speed of convergence of the basic HSA as well as reduces the selection pressure of the basic

Undergraduate and postgraduate students are the groups of individuals who has potential high rate of mobile devices usage. This group is also known for its faster acceptance of

Many frequency domain methods have been proposed for digital image watermarking, and it is well known that transform domain watermarking schemes have better

In previous study, NiV glycoprotein (NiV-G) gene fragment was cloned into the Pichia expression vector (yeast system) for the production of a recombinant

Proton nuclear magnetic resonance ( 1 HNMR) of TEHA tosylate / water/ isopropanol (aqueous rich phase) spectra... Proton nuclear magnetic resonance ( 1 HNMR) of

Molecular classification of head and neck squamous cell carcinomas using patterns of gene expression.. Head and Neck Cancer (Baltimore,, Williams