Article
Reference
Search for resonances in the mass distribution of jet pairs with one or two jets identified as b -jets in proton–proton collisions at √s = 13 TeV
with the ATLAS detector
ATLAS Collaboration
ANCU, Lucian Stefan (Collab.), et al .
Abstract
Searches for high-mass resonances in the dijet invariant mass spectrum with one or two jets identified as b -jets are performed using an integrated luminosity of 3.2 fb−1 of proton–proton collisions with a centre-of-mass energy of s=13 TeV recorded by the ATLAS detector at the Large Hadron Collider. No evidence of anomalous phenomena is observed in the data, which are used to exclude, at 95% credibility level, excited b⁎ quarks with masses from 1.1 TeV to 2.1 TeV and leptophobic Z′ bosons with masses from 1.1 TeV to 1.5 TeV. Contributions of a Gaussian signal shape with effective cross sections ranging from approximately 0.4 to 0.001 pb are also excluded in the mass range 1.5–5.0 TeV.
ATLAS Collaboration, ANCU, Lucian Stefan (Collab.), et al . Search for resonances in the mass distribution of jet pairs with one or two jets identified as b -jets in proton–proton collisions at √s = 13 TeV with the ATLAS detector. Physics Letters. B , 2016, vol. 759, p. 229-246
DOI : 10.1016/j.physletb.2016.05.064
Available at:
http://archive-ouverte.unige.ch/unige:84165
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Search for resonances in the mass distribution of jet pairs with one or two jets identified as b-jets in 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:
Received29March2016
Receivedinrevisedform13May2016 Accepted20May2016
Availableonlinexxxx Editor:W.-D.Schlatter
Searches forhigh-mass resonancesin thedijet invariant mass spectrumwith oneortwo jetsidenti- fiedasb-jetsareperformedusinganintegratedluminosityof3.2 fb−1ofproton–protoncollisionswith acentre-of-massenergyof√s
=13 TeV recordedbythe ATLASdetectorattheLargeHadronCollider.
No evidenceofanomalousphenomenaisobservedinthedata,whichareusedtoexclude,at95% cred- ibility level, excitedb∗ quarks with massesfrom 1.1 TeV to 2.1 TeV andleptophobic Z bosons with massesfrom1.1 TeV to1.5 TeV.ContributionsofaGaussian signalshape witheffectivecross sections rangingfromapproximately0.4to0.001 pbarealsoexcludedinthemassrange1.5–5.0 TeV.
©2016PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
Fig. 1.Leading-order Feynman diagrams for the two processes considered:gb→b∗→bgandqq¯→Z→bb.¯
1. Introduction
Manyextensionsto theStandardModel(SM)predict theexis- tenceofnewmassiveparticlesthatcoupletoquarksorgluons.If producedinproton–proton(pp)collisionsattheLargeHadronCol- lider(LHC),thesenewbeyond-the-SM(BSM)particlescoulddecay into quarks (q) or gluons (g), creating resonant excesses in the two-jet (dijet) invariant massdistributions [1–6].If thenew par- ticlecouples to the b-quark and decays into bb¯,bq or bg pairs, a dedicatedsearchfordijetresonanceswithoneorbothjetsiden- tifiedasoriginatingfromab-quark(“b-jet”)couldgreatlyincrease thesignalsensitivity.
Priorresonancesearchesindijeteventscontainingb-jets were performed by the CDF [7] and CMS [8,9] experiments, probing the mass ranges 200–750 GeV and 1–4 TeV respectively. Excited heavy-flavourquarks have been investigated in alternative decay modesaswell [10].No BSMphenomenahavebeenobservedyet.
E-mailaddress:atlas.publications@cern.ch.
The increase incentre-of-mass energyofthe ppcollisions at the LHC from √
s=7 and 8 TeV to 13 TeV provides a new energy regime in which to search for such a heavy resonance. This is particularly true for heavy states coupling to b-quarks from the protonsea,whencomparedtostatesproducedbyvalencequarks.
Thepartonluminositytocreatea2 TeV objectincreasesbyanad- ditionalfactor of2–3for bb¯ andbg over qq¯ andqg pairs, when increasingthecentre-of-massenergyfrom8 TeV to13 TeV.Theto- talproductionratefordijetBSMsignalscanbecomelargeenough toallowagoodsignalsensitivityevenwitharelativelysmalldata sample. Inthispaperthesearch foranewnarrowresonancede- cayingto b-quarks withthe ATLAS detector, using 3.2 fb−1 inte- grated luminosity of proton–proton collisions at √
s=13 TeV, is reported.Themassrange1.1–5.0 TeV isprobed.
The results are interpreted in the context of two benchmark processesshowninFig. 1: anexcited heavy-flavourquark b∗ and anewgaugeboson Z.Excitedquarksareaconsequenceofquark compositeness modelsthat wereproposed to explainthe genera- tionalstructure andmasshierarchyofquarks[11,12]. The Z bo- son arisesinmanyextensionstothe SMwithan additionalU(1)
http://dx.doi.org/10.1016/j.physletb.2016.05.064
0370-2693/©2016PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
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group. Two Z models are considered, one withSM-like fermion couplingsintheSequentialStandardModel(SSM)andaleptopho- bic Zmodel[13,14].Allbenchmarkmodeldecaysareexpectedto resultinanarrow resonancesuperimposedon asmoothly falling dijet invariant mass distribution. This search divides the events into samples with one or two jets identified as b-jets to en- hancethesignalsensitivitytothebenchmarkmodelsb∗→bg and Z→bb.¯ Inaddition,theresultsare interpretedinthecontext of possibleGaussian-shapedsignalcontributionstothedijetinvariant massspectra whereone orboth jetsare identifiedasb-jets. The results,presented interms of thecross section times acceptance timesbranching ratio (σ×A×BR),are quoted forcontributions withwidthsofupto15% oftheresonancemass.
2. TheATLASdetector
The ATLAS experiment [15] at the LHC is a multi-purpose particle detector witha forward–backward symmetric cylindrical geometry and a 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 trackingdetectorcoversthepseudorapidityrange|η|<2.5.Itcon- sists of,in ascending order ofradius from the beam-line, silicon pixel, silicon microstrip, and transition radiation tracking detec- tors.Thepixeldetectorsarecrucialforb-jetidentification.Forthe second LHC data-taking period, a new inner pixel layer, the In- sertableB-Layer(IBL)[16,17],was addedatameansensorradius of 3.2 cm from the beam-line. Lead/liquid-argon (LAr) sampling calorimeters provide electromagnetic (EM) energy measurements withhighgranularity.A hadron(steel/scintillator-tile) calorimeter covers the central pseudorapidity range (|η|<1.7). The end-cap and forward regions are instrumented withLAr calorimeters for EMandhadronicenergymeasurementsupto |η|=4.9.Thefirst- leveltriggerisimplementedinhardwareandusesasubsetofthe detector information to reduce the input rate from the nominal LHC collision rate to an acceptancerate of 100 kHz. This is fol- lowedbyasoftware-basedtriggerthatreducestherateofevents recordedto1kHz.
3. Dataandsimulatedeventsamples
ThedatausedinthisanalysiswerecollectedbytheATLASde- tector in pp collisions at the LHC with a centre-of-mass energy of 13 TeV during 2015. Events were recorded using a jet-based triggerrequiringatleastonejetwithatransversemomentum pT ofatleast360 GeV.Thefulldatasetcorresponds toan integrated luminosity of3.2 fb−1 withan associateduncertainty of5%after applyingquality criteriatothe data.Themeasurement ofthe in- tegratedluminosityisderived,followingamethodologysimilarto thatdetailedinRef.[18],fromacalibrationoftheluminosityscale usingapairofx–ybeam-separationscans.
MonteCarlo(MC)simulatedeventsamplesare usedto model theexpectedsignalsandstudythecompositionofSMbackground processes. The QCD dijet process is simulated with Pythia8 [19]
usingthe A14tuned parameter set[20] forthe modellingofthe
1 ATLASusesaright-handed coordinatesystemwith itsoriginat thenominal interactionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeam pipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis pointsupwards.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φ beingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdefinedinterms ofthepolarangleθasη= −ln tan(θ/2).Angulardistanceismeasuredinunitsof R≡
(η)2+(φ)2.
parton shower, hadronization andunderlying event. The leading- order(LO)partondistributionfunction(PDF)setNNPDF2.3[21]is usedforthegenerationofevents.Therenormalizationandfactor- ization scalesare set tothe averagetransverse momentum pT of the two leading jets. The EvtGen decaypackage [22] isused for bottomandcharmhadrondecays.
ThethreesignalsamplesaregeneratedwithPythia8usingthe A14 set oftuned parameters andthe NNPDF2.3 PDF set.For the b∗model,thecompositenessscaleissettotheexcited-quarkmass and 85% ofdecays are tobg.The remaining decay modesare to a SMgauge boson(Z boson,W bosonorphoton)andab-quark.
IntheSSM Zmodel,the Z bosonhasthesamecouplingstoSM fermionsastheSM Z bosonandthebottomquarkdecaybranch- ing ratio BR(Z→bb¯) is13.8%. Theleptophobic Z model differs byhavingvanishingcouplingstoleptons.Thecorrespondingvalue ofBR(Z→bb¯)is18.9%.Forboth,onlydecaystob-quarkpairsare simulated.Theintrinsicdecaywidthis ∼0.6% oftheresonance mass for the b∗ model and ∼3% of the mass forthe SSM Z boson.
The generatedsamplesare processedwiththe ATLASdetector simulation [23],which is basedon the GEANT4 package [24].To account foradditional pp interactions fromthesame orclose-by bunch crossings, a number of minimum-bias interactions gener- ated using Pythia8 and the MSTW2008LO PDF [25] set are su- perimposedonto thehard scatteringevents.The MCsamples are re-weighted tomatch thecollisions per bunch crossing observed inthedata.
4. Eventreconstructionandselection
Jets are reconstructed fromnoise-suppressed topological clus- ters[26]ofenergydepositedinthecalorimetersusingtheanti-kt algorithm[27]witharadiusparameterof0.4.Jetenergiesanddi- rections are corrected by the jet calibrations derived from√
s= 13 TeV simulation,and ppcollisiondatatakenat√
s=8 TeV and
√s=13 TeV, asdescribed inRef. [28]. Jetsare required to have pT>50 GeV.EventswhereanyofthethreeleadingjetswithpT>
50 GeV is compatiblewithnon-collision backgroundorcalorime- ternoiseareremoved. Eventsarepreselectedinthesamewayas inthedijetanalysisofRef.[5],requiringthatthe pToftheleading jetisgreaterthan440 GeV toensurefulltriggerefficiency.Anad- ditionalrequirementisplacedonthejetpseudorapidity,|η|<2.4, to ensure trackercoverage forb-jet identification. The analysisis performed in an unbiased dijet mass range of mjj>1.1 TeV. To reduce thebackgroundfromQCD multijetprocessesandenhance s-channel processes,therapidity difference y∗=(y1−y2)/2 be- tween the two leading jets isrequired tobe |y∗|<0.6. Here y1 and y2 are the rapidities of the leading and sub-leading jet re- spectively.
To identifyjetsoriginating fromb-hadrons(b-tagging) a mul- tivariate algorithm that combines information about the impact parameters of inner detector tracks associated with the jet, the presence of displaced secondary vertices, and the reconstructed flight paths of b- and c-hadrons associated withthe jet [29,30]
is employed.The b-taggingworkingpoint with85% efficiency,as determined when integrating over all jetsin a simulatedsample oftt¯events,ischosen becauseitgivesthehighestsignalsensitiv- ity.Astheaveragejetenergiesinthisanalysisarelargerthanint¯t events andthe b-taggingefficiencydrops withjet pT, theper-jet efficiencies arebelow85% andareroughly 50% forjetswitha pT of1 TeV.
Theb-jetidentificationalgorithmisappliedtothetwoleading jets, andeventsare categorizedasinclusive, single b-tagged “1b”
ordoubleb-tagged“2b”,inordertoenhancethesensitivityofdif-
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Fig. 2.Theper-eventb-taggingefficienciesaftertheeventselectionasafunctionof thereconstructedinvariantmassforsimulatedsampleswithsixdifferentb∗andZ resonancemasses.
ferentsignalcompositions.The“1b”categoryisdefinedinclusively, includingeventsfromthe“2b”category.
Theper-event b-taggingefficiencies asfunctionsoftherecon- structed invariant mass are shown in Fig. 2. Efficiencies are for benchmark models with different b∗ and Z resonance masses, afterthe eventselection is applied. The tagging efficiencyfor Z eventsintheinclusive“1b”category ishigherthanforb∗ events becausethisprocesshasmoreb-quarksinthefinalstate.Athigh mass,thegluon fromthe decayof theb∗ hasa higherprobabil- itytoproduceabb-pair,¯ whichcausestheeventtaggingefficiency tobecomparableforthe Z andb∗.Thetaggingefficiencyinthe
“2b”categoryisabout2.5 timesloweratlowmassandafactor 10 lower at high mass compared to the inclusive “1b” category for thesame Z events.The average light-flavour jet rejectionfactor forjetspassingthekinematicselectionisapproximately30forjet transversemomentaupto∼1 TeV.
Correction factors are applied to the simulated event sam- ples to compensate for differencesbetween data and simulation in b-tagging efficiencies and mis-identification rates. These cor- rections were derived from comparisons of samples of b-quark- enrichedeventsindataandsimulation[31].Theaveragecombined signal acceptanceandefficiencyis around 20%for theb∗ bench- markin the “1b” category and drops withincreasing massfrom 9%at1.5 TeV to2%at5.0 TeV forthe Zsignalsforthe“2b”cate- gory.
5. Dijetmassspectrum
The dijet mass spectrum is predominantly composed of jets arising from QCD interactions. Fig. 3 shows the comparison be- tween data and Pythia8 multijet MC simulation. The simulated distributionsarenormalizedtothe numberofeventsobservedin the data in each category separately. The bin widths are chosen toapproximatethemjj resolutionasderivedfromsimulatedQCD processes,whichrangefrom3% at1.0 TeV to2% at5.0 TeV.Good agreementbetweentheshapesofthePythia8multijetpredictions andthedataisfound. Theinclusivedistribution,notrestrictedin theinnertrackingdetectoracceptance,wasanalysedinRef.[5].
Thedijet backgroundestimation doesnot rely onthe simula- tionasitisobtaineddirectlyfromafittothemjjdistribution.The followingparameterizationansatzisadoptedtofitthedistribution inthemjjrangefrom1.1 TeV up tothelastdatapointofthein- clusive,“1b”and“2b”massdistributionsseparately,
f(z)=p1(1−z)p2zp3, (1)
Fig. 3.Theinvariantmassdistributionoftheinclusivedijet(dots),“1b”(squares) and “2b” (triangles) categories in data. The inclusive distribution is similar to Ref.[5],butanadditionalrequirementisplacedonthejetpseudorapidity,|η|<2.4.
TheMCdistributionsarenormalizedtothedatainthethreecategoriesseparately:
asolidlineforinclusivedijets,adashedlinefor“1b”andasmalldashedlinefor
“2b”categories.ThelowerpanelsshowtheratiobetweendataandMCsimulation forallthreecategories.
wherepiarefreeparametersandz=mjj/√
s.Thisansatzwasused inprevious searches [5]andisfound toprovidea satisfactory fit to leading-order Pythia8multijetMC simulationat √
s=13 TeV.
EmployingWilks’theorem[32],alog-likelihoodstatisticisusedto confirmthat noadditionalparametersareneededtomodelthese distributionsforadatasetaslargeastheoneusedforthisanaly- sis.
TheresultsofthefitsareshowninFig. 4.Thefitsofthisansatz to the data without considering systematic uncertainties return p-values of 0.73, 0.90 and 0.66 for the inclusive, “1b” and“2b”
categoriesrespectively.The p-valuewascalculatedasagoodness- of-fitmeasure usinga χ2 test statistic determined frompseudo- experiments.
ThelowerpanelsofFig. 4showthesignificancesofbin-by-bin differencesbetweenthedataandthefit.TheseequivalentGaussian significancesarecalculatedfromthePoissonprobability,consider- ingonlystatisticaluncertainties.
The statistical significance of any localized excess in the di- jet mass distribution is quantified using the BumpHunter algo- rithm[33].Thealgorithmcomparesthebinnedmjjdistributionof thedatatothefittedbackgroundestimate,consideringcontiguous massintervalsinallpossiblelocations,fromawidthoftwobinsto one-halfofthedistribution.Foreachinterval inthe scan,itcom- putesthesignificanceofanyexcessfound.Thealgorithmidentifies the intervals 1493–1614 GeV in the “1b” and3596–3827 GeV in the “2b” sample, indicated by the two vertical lines in Fig. 4, as the mostdiscrepantintervals. The statisticalsignificance ofthese
