Search for new phenomena in dijet mass and angular distributions from pp collisions at s=13 TeV with the ATLAS detector

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Search for new phenomena in dijet mass and angular distributions from pp collisions at s=13 TeV with the ATLAS detector

ATLAS Collaboration

ANCU, Lucian Stefan (Collab.), et al.

Abstract

This Letter describes a model-agnostic search for pairs of jets (dijets) produced by resonant and non-resonant phenomena beyond the Standard Model in 3.6 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. The distribution of the invariant mass of the two leading jets is examined for local excesses above a data-derived estimate of the smoothly falling prediction of the Standard Model. The data are also compared to a Monte Carlo simulation of Standard Model angular distributions derived from the rapidity of the two jets. No evidence of anomalous phenomena is observed in the data, which are used to exclude, at 95% CL, quantum black holes with threshold masses below 8.3 TeV, 8.1 TeV, or 5.1 TeV in three different benchmark scenarios; resonance masses below 5.2 TeV for excited quarks, 2.6 TeV in a W′ model, a range of masses starting from mZ′=1.5 TeV and couplings from gq=0.2 in a Z′ model; and contact interactions with a compositeness scale below 12.0 TeV and 17.5 TeV respectively for destructive and constructive interference between [...]

ATLAS Collaboration, ANCU, Lucian Stefan (Collab.), et al . Search for new phenomena in dijet mass and angular distributions from pp collisions at s=13 TeV with the ATLAS detector. Physics Letters. B , 2016, vol. 754, p. 302-322

DOI : 10.1016/j.physletb.2016.01.032

Available at:

http://archive-ouverte.unige.ch/unige:80157

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Physics Letters B

www.elsevier.com/locate/physletb

Search for new phenomena in dijet mass and angular distributions from pp collisions at √

s = 13 TeV with the ATLAS detector

.ATLASCollaboration

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

Articlehistory:

Received7December2015

Receivedinrevisedform18January2016 Accepted18January2016

Availableonline20January2016 Editor: W.-D.Schlatter

This Letter describes a model-agnostic search for pairs of jets (dijets) produced by resonant and non-resonant phenomena beyondthe Standard Model in3.6 fb⁻¹ ofproton–proton collisions witha centre-of-mass energy of√

s=13 TeV recorded bythe ATLAS detectorat theLarge HadronCollider.

The distributionof the invariant mass of the two leading jets is examined for local excesses above a data-derived estimate ofthe smoothly falling prediction ofthe Standard Model. The data are also comparedtoaMonteCarlosimulationofStandardModelangulardistributionsderivedfromtherapidity of the two jets. No evidence of anomalous phenomena is observed in the data, which are used to exclude, at95% CL, quantum black holes with threshold massesbelow 8.3 TeV, 8.1 TeV, or5.1 TeV in three different benchmarkscenarios; resonance massesbelow 5.2 TeV forexcitedquarks, 2.6 TeV in aW model, arangeofmassesstarting frommZ=¹.5 TeV andcouplingsfrom gq=⁰.2 in a Z model; andcontact interactionswithacompositenessscale below 12.0 TeV and17.5 TeV respectively for destructive and constructive interference between the new interaction and QCDprocesses. These results significantlyextendtheATLAS limitsobtainedfrom8 TeVdata. Gaussian-shapedcontributions to the mass distribution are also excluded if the effective cross-section exceedsvalues rangingfrom approximately50–300 fbformassesbelow2 TeV to2–20 fbformassesabove4 TeV.

©^{2016CERNforthebenefitoftheATLAS}Collaboration.PublishedbyElsevierB.V.Thisisanopen accessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

The centre-of-mass energyof proton–proton(pp) collisions at theLargeHadronCollider(LHC)atCERNhasbeenincreasedfrom

√s=^{8 TeV to}√

s=^{13 TeV,openinganewenergyregime toob-} servation.

NewparticlesproducedinLHCcollisionsmustinteractwiththe constituentpartonsoftheproton.Consequently,thenewparticles can alsoproduce partonsin thefinal state. Finalstates including partonsoftendominateinmodelsofnewphenomena beyondthe StandardModel (BSM).The partonsshower andhadronize,creat- ing collimated jets of particles carrying approximately the four- momenta of the partons. The total production rates for two-jet (dijet) BSMsignalscanbe large,allowing searchesforanomalous dijetproductiontotestforsuchsignalswitharelativelysmalldata sample,evenatmassesthatconstitute significantfractionsofthe totalhadroncollisionenergy.

IntheStandardModel(SM),hadroncollisionsproducejetpairs primarilyvia2→^{2 partonscatteringprocessesgovernedbyquan-} tum chromodynamics (QCD). Farabove the confinementscale of QCD (≈^{1 GeV),jets emergefrom collisions withlarge transverse}

E-mailaddress:[email protected].

momenta, pT, perpendicular tothe directionof the incident par- tons. Forthe dataanalysedhere, QCD predicts asmoothly falling dijetinvariant massdistribution,m_{j j}.Newstatesdecayingto two jetsmayintroducelocalized excessesinthisdistribution.In QCD, duetot-channelpolesinthecross-sectionsforthedominantscat- teringprocesses, mostdijetproductionoccursatsmallangles θ^∗, definedasthepolarangleinthedijetcentre-of-massframe.¹Many theoriesofBSMphysicspredictadditionaldijetproductionwitha significantpopulationofjetsproducedatlargeangleswithrespect tothebeam;forreviewsseeRefs.[1,2].Thesearchreportedinthis LetterexploitsthesegenericfeaturesofBSMsignalsinananalysis ofthemj j andangulardistributions.

As is common, a rapidity y=^ln((E+^pz)/(E−^pz))/2 is de- fined for each of the outgoing partons, where E is its energy and p_z isthecomponentofitsmomentumalong thebeamline.² Each incoming parton carries a fraction (Bjorken x) of the mo-

1 Since, experimentally,thetwo partonscannotbedistinguished,θ^∗ isalways takenbetween0andπ/2 withrespecttothebeam.

2 ATLAS usesaright-handedcoordinatesystemwithitsoriginatthe nominal interactionpoint(IP)inthecentreofthedetectorandthe z-axisalongthebeam line.Thex-axispoints fromtheIPtothecentreoftheLHCring,andthe y-axis pointsupwards.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φ beingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdefinedinterms http://dx.doi.org/10.1016/j.physletb.2016.01.032

0370-2693/©^{2016CERNforthebenefitoftheATLAS}Collaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

mentumoftheproton.Amomentumimbalancebetweenthetwo partonsbooststhecentre-of-massframeofthecollisionrelativeto thelaboratoryframealong thez directionby yB=^ln(x1/x2)/2= (y₃+^y4)/2, where y_B is the rapidity of the boosted centre-of- mass frame, x₁ and x₂ are the fractions of the proton momen- tumcarried by each partonand y3 and y4 are the rapidities of the outgoing partons inthe detectorframe. Differences between tworapidities areinvariant undersuch Lorentzboosts, hencethe followingfunctionoftherapidity difference y^∗=(y₃−^y4)/2 be- tweenthetwojets,

χ=^e2|y∗|∼¹+^cosθ^∗ 1−^cosθ^∗,

isthesameinthedetectorframeasinthepartoniccentre-of-mass frame.Inthecentre-of-massframe,thetwopartonshaverapidity

±^y∗.

Thevariable χ isconstructedsuchthatinthelimitofmassless partonscattering, andwhenonlyt-channel scatteringcontributes to the partonic cross-section, the angular distribution dN/dχ is approximatelyindependentof χ.Themeasuredshapesoftheob- served dN/dχ distributions differ fromthe parton-level distribu- tionsbecausetheobserveddistributionsconvolvetheparton-level distributionswithnon-uniformpartonmomentumdistributionsin x1 andx2.Restrictingtherangeoftwo-partoninvariant massand placinganuppercuton y_B reducesthesedifferences.

Priorsearches of dijetdistributions with lower-energy hadron collisions at the SppS¯ [3–5], the Tevatron [6,7], and the LHC at

√s=^{7–8 TeV [8–19] and recently at 13 TeV [20], did not find} BSMphenomena. This Letter presents an analysis of 3.6 fb⁻¹ of proton–protoncollisionLHC dataat√

s=13 TeV recorded bythe ATLAS detector, focusing on thedistributions ofmj j andχ with methodsbasedonthoseusedbyRefs.[17,19].

2. TheATLASdetector

TheATLASexperiment[21]attheLHCisamulti-purposeparti- cledetectorwithaforward–backwardsymmetriccylindricalgeom- etrywithlayers oftracking,calorimeter,andmuondetectorsover nearlytheentiresolidanglearoundtheppcollisionpoint.Thedi- rectionsandenergiesofhigh-pT hadronicjetsaremeasuredusing silicontrackingdetectorsandstraw tubesdetecting transitionra- diation,finelysegmentedhadronicandelectromagneticcalorime- ters,andamuon spectrometer.Asteel/scintillator-tile calorimeter provides hadronic energy measurements for the pseudorapidity range |η_|<1.7. A lead/liquid-argon (LAr) calorimeter provides electromagnetic(EM)energymeasurementswithhighergranular- itywithin theregion |η|<3.2.The end-capandforward regions areinstrumented withLArcalorimeters forEM andhadronicen- ergymeasurementsupto|η_|=4.9.Thefirst-leveltriggerisimple- mentedinhardwareandusesasubsetofthedetectorinformation to reduce the accepted rate to 100 kHz. This is followed by a software-basedtriggerthatreducestherateofeventsrecordedto 1 kHz.

3. Dataselection

Collisioneventsarerecordedusingatriggerrequiringthepres- enceofatleastonejetreconstructedinthesoftware-basedtrigger witha pT ofatleast 360 GeV.Groups ofcontiguous calorimeter cells(topologicalclusters)areformedbasedonthesignificanceof

ofthepolarangleθasη= −^{ln tan}(θ/2).Itisequivalenttotherapidityformassless particles.

the energy deposit over calorimeter noise [22]. Topological clus- tersaregroupedintojetsusingtheanti-kt algorithm[23,24]with radiusparameterR=⁰.4.Jetfour-momentaarecomputedbysum- mingoverthetopologicalclustersthatconstituteeachjet,treating the energyof each cluster asa four-momentum withzero mass.

The reconstruction efficiency for jets with pT above 20 GeV is 100%. Jet calibrations derived from √

s=13 TeV simulation, and collision datatakenat√

s=^{8 TeV and}√

s=^{13 TeV, areused to} correct the jet energies and directions to those of the particles from the hard-scatter interaction. This calibration procedure, de- scribedinRefs. [25–27],isimprovedby adata-derivedcorrection totherelativecalibrationofjetsinthecentralandtheforwardre- gions. The dijet mass resolution is2.4% and 2%, for dijetmasses of2and5 TeVrespectively.Thejetenergyscaleuncertaintyfrom 8 TeV data iscomplementedby systematicuncertaintiescovering thedifferencesbetween8 TeV and 13 TeV data. Thetotal jeten- ergy scale uncertaintyis 1% forcentral jetswith pT of 500 GeV, and3%forjetsof 2 TeV. Analysisofjet dataat13 TeV using the insitu techniques described in Ref. [28] confirms the jet calibra- tionanduncertaintyestimates.Beyondthe p_T rangeoftheinsitu techniques,forthequantitiesusedtocalibratejetsaswellasother kinematicquantities,thedataagreewithsimulationwithinquoted uncertainties.

Eventscontainingatleasttwojetsareselectedforofflineanal- ysis if the pT of the leading andsubleading jets is greater than 440 GeV and 50 GeV respectively. This requirement ensures a trigger efficiency of at least 99.5% for collisions with |^y∗|<1.7 andremoves a negligible number ofevents fromunbalanced di- jeteventsoriginatingfromadditionalinteractionswithinthesame bunchcrossingorjetresolutiontails.Eventsarediscardedfromthe search ifanyofthe threeleading jetswith pT>50 GeV is com- patiblewithnon-collisionbackgroundorcalorimeternoise[29].

4. Simulatedcollisions

Forthissearch,eventsfromQCDprocessesaresimulatedwith Pythia 8[30] usingthe A14[31] setoftuned parameters forthe underlyingeventandtheleading-orderNNPDF2.3[32] partondis- tribution functions (PDFs). The renormalization and factorization scales are setto theaverage pT of thetwo leading jets.Detector effects are simulated using Geant4 [33] within the ATLAS soft- ware infrastructure [34]. The same software used to reconstruct datawasalsousedtoreconstructsimulatedevents.Thesimulated eventsareusedtopredicttheangulardistributionfromQCDpro- cesses and forqualitative comparisons to kinematic distributions indata.

Pythia 8calculationsusematrixelementsthatareatleadingor- derintheQCDcouplingconstantwithsimulationofhigher-order contributions partially covered by the parton shower (PS) mod- elling. They also include modelling of hadronization effects. The distributions of events predicted by Pythia 8 are reweighted to the next-to-leading-order (NLO) predictions of NLOJET++ [35–37]

using mass- and χ-dependent correction factors defined as in Ref. [19].The correction factors modify the shape ofthe angular distributions atthelevelof15% atlowvaluesof χ andhighval- ues ofm_{j j}. The correction is 5% orless at the highestvalues of

χ. The Pythia 8 predictions also omit electroweak effects. These areincludedasadditionalmass- and χ-dependentcorrectionfac- tors[38] thatareunityatlowm_{j j} anddifferfromunitybyup to 3%inthemj j>3.4 TeV region.

BSMsignalsamplesofexcitedquarks[39,40],newheavyvector bosons [41–43], quantum black holes [44–46] andcontact inter- actions [47–49] are simulated andreconstructed using the same procedure asfor QCD processes.The models andthe parameters chosenforgenerationaredescribedinSection7.

Fig. 1. The reconstructed dijetmass distribution(filled points) for eventswith

|y^∗|<0.6 and pT>440(50)GeV fortheleading (subleading)jets.Thesolidline depictsthefittoEq.(1),asdiscussedinthetext.Predictionsforanexcitedquark andaquantumblackholesignalpredictedbytheBlackMaxgenerator(QBHBM) areshownabovethefit,normalizedtothepredictedcross-section.Theverticallines indicatethemostdiscrepantintervalidentifiedbytheBumpHunteralgorithm,for whichthe p-valueisstatedinthefigure.Themiddlepanelshowsthebin-by-bin significancesofthe data-fitdifferences,considering onlystatistical uncertainties.

Thelowerpanelshowstherelativedifferencesbetweenthedataandthepredic- tionofPythia 8simulationofQCDprocesses,correctedforNLOandelectroweak effects,andisshownpurelyforcomparison.Theshadedbanddenotestheexperi- mentaluncertaintyinthejetenergyscalecalibration.

5. Selectionforthemassdistributionanalysis

Them_{j j} distributionofeventswith|^y∗|<0.6 (χ<3.3)isanal- ysedforevidenceofcontributionsfromresonantBSMphenomena.

The requirementon |^y∗| ^{reducesthe backgroundfrom QCD pro-} cesses. To avoid kinematic bias from the y^∗ and p_T selections describedabove,theanalysisisconfinedtomj j>1.1 TeV.

Fig. 1 shows the observed mj j distribution for the resonance selection,overlaid withexamplesof thesignalsdescribed inSec- tion 7. The bin widths are chosen to approximate the m_{j j} res- olution as derived from the simulation of QCD processes, and therefore widen as the mass increases. The largest value of m_{j j} measuredis6.9 TeV.

ToestimatetheSMbackground,theansatz,

f(z)=^p1(1−^z)^p2z^p3, (1) wherez≡^mj j/√

s,isfittothemj j distributioninFig. 1toobtain theparameters pi.Thefitrangeis1.1–7.1 TeV.CDF,CMS,andAT- LAS dijetsearches such as those described in Refs. [6,8,13,14,17]

havefoundthat expressionssimilarto Eq.(1)describe dijetmass distributionsobserved atlower collisionenergies.The ansatzalso describes leading-order and next-to-leading order simulations of QCDdijetproductionat√

s=^{13 TeV.A}log-likelihood-ratiostatis- ticemploying Wilks’stheorem[50] was used todetermineifthe backgroundestimationwouldbesignificantly improvedby anad- ditionaldegreeof freedom.Withthe currentdataset,Eq.(1)was foundtobesufficient.

Fig. 1alsoshowstheresultofthefit.Thefitdescribestheob- serveddatawitha p-valueof0.87,usingaPoissonlikelihoodtest

statistic.Themiddlepanelofthefigureshowsthesignificancesof bin-by-bin differences betweenthe dataandthe fit.These Gaus- siansignificancesarecalculatedfromthePoissonprobability,con- sidering only statistical uncertainties. The lower panel compares the data to the prediction of Pythia 8 simulation of QCD pro- cesses, correctedforNLO andelectroweakeffects.Eventhough it is notusedinthe analysisofthem_{j j} distribution,thesimulation isshowntobeingoodagreementwiththedata.

TheuncertaintyinvaluesoftheparametersinEq.(1)isevalu- atedbyfittingthemtopseudo-datadrawnviaPoissonfluctuations around the fittedbackground model.The uncertainty inthe pre- dictionineachmj j binistakentobetherootmeansquare ofthe functionvalue forall pseudo-experimentsinthatbin.Toestimate an uncertainty due to the choice of the background parameteri- zation,aparameterizationwithoneadditionaldegreeoffreedom, z^p4logz, iscompared tothe nominalansatz, andthedifference is takenasanuncertainty.Thepredictionofthem_{j j}distributiondoes not involvesimulatedcollisions andthusisnot affectedby theo- reticalorexperimentaluncertainties.

The statistical significance of any localized excess in the m_{j j} distributionisquantifiedusingtheBumpHunteralgorithm[51,52].

Thealgorithmcomparesthebinnedmj j distributionofthedatato thefittedbackgroundestimate,consideringcontiguousmassinter- valsinallpossiblelocations,fromawidthoftwobinstoawidth of half ofthe distribution. Foreach interval inthe scan, it com- putesthesignificanceofanyexcessfound.Thealgorithmidentifies the interval 1.53–1.61 TeV,indicated by thetwo vertical linesin Fig. 1,asthe mostdiscrepantinterval. The statisticalsignificance of thisoutcomeis evaluated usingtheensemble ofpossibleout- comes across all intervalsscanned, by applying the algorithm to manypseudo-datasamplesdrawnrandomlyfromthebackground fit.Withoutincludingsystematicuncertainties,theprobabilitythat fluctuationsofthebackgroundmodelwouldproduceanexcessat least assignificant astheone observed inthe data,anywhere in the distribution,is 0.67.Thus, there isnoevidence ofa localized contributiontothemassdistributionfromBSMphenomena.

6. Selectionfortheangulardistributionsanalysis

The dN/dχ (angular) distributions of events with |^y∗|<1.7 (i.e. χ<30.0) and|^yB|<1.1 are alsoanalysed for contributions from BSM signals. Fig. 2 shows the angular distributions of the data in different m_{j j} ranges, the SM prediction for the shape of theangulardistributions,andexamplesofthesignalsdescribedin Section 7.The data with mj j<2.5 TeV are discarded to remove bias from the kinematic selections described earlier. The highest mj j measuredis7.9 TeV.TheSMpredictionisobtainedfromsim- ulation,asdescribedinSection4.Intheanalysis,thepredictionin eachm_{j j} rangeisnormalizedtomatchtheintegralofthedatain thatrange.

Theoreticaluncertaintiesinsimulationsoftheangulardistribu- tions fromQCDprocessesare estimatedasdescribed inRef. [19].

The effect on the QCD prediction of varying the PDFs is esti- mated using NLOJET++ withthree differentPDF sets: CT10 [53], MSTW2008[54] andNNPDF23 [32]. Asthe choice ofPDF largely affectsthetotalcross-sectionratherthantheshapeofthe χdistri- butions, theseuncertainties arenegligible(<1%).Theuncertainty dueto thechoice ofrenormalizationandfactorizationscaleswas estimatedusing NLOJET++ by varyingeach independentlyup and downbyafactortwo,excludingoppositevariations.Theresulting uncertainty,takenastheenvelopeofthevariationsinthenormal- ized χ distributions,dependsonbothm_{j j} and χ,risingto20%at thesmallest χvaluesathighmj jvalues.Thestatisticaluncertainty ofthesimulatedNLOcorrectionsislessthan1%.Thedominantex- perimentaluncertaintyinthepredictionsofthe χ distributionsis