Dijet production in $\sqrt{s}=7$ TeV $\mathit{pp}$ collisions with large rapidity gaps at the ATLAS experiment

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Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Dijet production in √

s = 7 TeV pp collisions with large rapidity gaps at the ATLAS experiment

.ATLASCollaboration

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

Articlehistory:

Received3November2015

Receivedinrevisedform10January2016 Accepted15January2016

Availableonline18January2016 Editor:W.-D.Schlatter

A 6.8 nb⁻¹ sampleof pp collision data collectedunder low-luminosityconditions at√

s=⁷ ^{TeV by} the ATLAS detector atthe Large HadronCollider isused tostudy diffractive dijetproduction. Events containingatleast two jetswith pT>20 GeV are selectedand analysedin termsofvariableswhich discriminatebetweendiffractiveandnon-diffractiveprocesses.Crosssectionsaremeasureddifferentially inη^F,thesizeoftheobservableforwardregionofpseudorapiditywhichisdevoidofhadronicactivity, and in an estimator, ˜ξ, of the fractional momentum loss of the proton assuming single diffractive dissociation (pp→^{p X).} ^Model comparisons indicate a dominant non-diffractive contribution up to moderately large η^F and small ˜ξ,withadiffractive contribution whichissigniﬁcant atthehighest

η^Fandthelowest˜ξ.Therapidity-gapsurvivalprobabilityisestimatedfromcomparisonsofthedatain thislatterregionwithpredictionsbasedondiffractivepartondistributionfunctions.

©²⁰¹⁶^CERN^for^the^benefitôf^theÂTLASCollaboration.PublishedbyElsevierB.V.Thisisanopen accessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Diffractivedissociation(e.g. pp→^{p X )}contributesalargefrac- tion of the total inelastic cross section [1] at the Large Hadron Collider (LHC). The inclusive process has been studied using the earliestLHCdatainsamplesofeventsinwhichalargegapisiden- tiﬁedintherapiditydistributionofﬁnal-statehadrons[2,3].Inthe absence ofhard scales, the understanding of thesedata isbased onphenomenologicalmethods ratherthantheestablished theory ofthestronginteraction,quantumchromodynamics(QCD).

A subset of diffractive dissociation events in which hadronic jets are produced as components of the dissociation system, X , wasfirst observedatthe SPS[4],a phenomenonwhichhassince beenstudied extensivelyatHERA [5,6] andtheTevatron[7].The jet transverse momentum provides a natural hard scale for per- turbative QCD calculations, making the process sensitive to the underlying parton dynamics of diffraction and colour-singlet ex- change.Amodel[8]inwhichthehardscatteringisfactorisedfrom acolourless componentoftheproton withits own partoniccon- tent(diffractivepartondistributionfunctions,DPDFs),correspond- ingtotheolderconceptofa pomeron[9],hasbeensuccessfulin describingdiffractivedeepinelasticscattering(ep→^{e Xp)}ât^HERA [10].TheDPDFshavebeenextractedfromfitstoHERAdatainthe frameworkofnext-to-leading-orderQCD,revealingahighlygluon- dominatedstructure[11,12].

E-mailaddress:atlas.publications@cern.ch.

The success of the factorisable approach breaks down when DPDFs from ep scattering are applied to hard diffractive cross sections in photoproduction [13,14] or athadron colliders.Teva- tron data [7] show a suppression of the measured cross section by a factor of typically 10 relative to predictions. A similar

‘rapidity-gap survival probability’ factor, usually denoted by S², was suggested by the ﬁrst results from the LHC [15]. This fac- torisation breaking is usually attributed to secondary scattering from beam remnants, also referred to as absorptive corrections, and closelyrelatedto themultiple-scattering effects which are a primary focus of underlying-event studies [16–18]. Understand- ing these effects more deeply is an important step towards a complete modelof diffractiveprocessesat hadroniccollidersand maypoint thewaytowards areconciliationofthe currentlyvery different theoretical treatments of soft and hard strong interactions.

In this paper, the ATLAS technique for finding large rapidity gaps, first introduced in Ref. [2], is developed further and applied to events in which a pair of high transverse momentum (p_T) jetsis identified. The resulting cross sections are measured as a function of the size of the rapidity gap and of an estimator of thefractional energyloss ofthe intact proton.The results are interpreted through comparisons with Monte Carlo models which incorporateDPDF-based predictions with no modelling of multiple scattering. Comparisons betweenthemeasurements and thepredictionsthusprovideestimatesoftherapidity-gapsurvival probabilityapplicabletosingledissociationprocessesatLHCener- gies.

http://dx.doi.org/10.1016/j.physletb.2016.01.028

0370-2693/©²⁰¹⁶^CERN^for^the^benefitôf^theÂTLASCollaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

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Fig. 1. Illustrationofhard single-diffractive scattering, inwhichpartons from a pomeron(P)andfromaprotonenterahardsub-process.Therapiditygapappears betweenthesystemX andtheintactproton.

2. Modelsandsimulations

MonteCarlo(MC)simulationsusingleading-order(LO)calcula- tionsinperturbativeQCDareusedinunfoldingthedatatocorrect for experimental effects andin the comparison of the measurements with theoretical models. The PYTHIA 8.165 (hereafter referred to as PYTHIA8) general-purpose LO MC generator [19] is usedto modeldijetproduction innon-diffractive (ND)events, as wellasinsinglediffractivedissociation(SD, pp→^Xp)^and^double diffractivedissociation(DD,pp→^{X Y ).}^Analternativemodelofthe SDprocess isprovidedbyPOMWIG(version2.0β)[20],whilst an alternativenext-to-leading-order(NLO)modeloftheNDprocessis providedbyPOWHEG(version 1.0)[21,22].

In both PYTHIA8 and POMWIG, hard scattering in diffractive processes takes place through the factorisable pomeron mecha- nism[8] illustrated inFig. 1. Apomeron couples toan incoming proton,acquiringafractionξ oftheproton’slongitudinalmomen- tum. The proton either scatters elastically (SD) or dissociates to formahigher-masssystem(DD).A partonfromthe pomeron(as describedbyDPDFs)thenundergoesahardscatteringwithapar- ton fromthe dissociating protonat a scale setby the transverse momentaof theresulting jets. The dissociationsystem X has an invariantmassM_X,suchthatξ=^M²_X/s ataproton–protoncentre- of-massenergy√

s.

POMWIG is based on a standard implementation of hard diffractivescattering witha factorisablepomeron, in which both the pomeron flux and the DPDFs are taken from the results of theH1 2006DPDFfit B¹ [11] andthe protonPDF set isCTEQ61 [23].Incontrast,PYTHIA8provides asimultaneousmodelofhard andsoftdiffraction [24],in whicha soft diffractivemodel inher- itedfromPYTHIA6[25]issmoothlyinterfacedtoaharddiffractive model similar to that in POMWIG. The probability of using the hard model depends on MX. The H1 2006 DPDF fit B is again used for the partonic content of the pomeron and the proton partonicstructure is takenfrom theCT10 PDFs[26].Several dif- ferentpomeronfluxparameterisationsareavailableinPYTHIA8.In additiontothedefaultSchulerandSjöstrand(S–S)model[27],al- ternativeparameterisationsbyDonnachieandLandshoff(D–L)[28]

andBergerandStreng[29,30],aswellastheMinimumBiasRocke- feller(MBR)model[31],arealsoconsideredinthisanalysis.These modelsdifferprimarily intheir predictionsforthe ξ dependence ofthe crosssection[24].TheDDprocess inPYTHIA8ismodelled similarlytotheSDprocess. Neitherofthediffractivemodelscon-

1 TheH1FitBDPDFscorrespondtothesumoftheSDprocessandthecompo- nentoftheDDprocesswherethelowerofthetwoprotondissociationmassesis smallerthan1.6GeV(seeSection6).

sideredheretakerapidity-gapdestructioneffectsintoaccount,i.e.

theysettherapiditygapsurvivalprobability S²≡^1.

An alternative for ND processes is provided by the POWHEG NLO generator. As described in Ref. [22], the ‘hardest emission cross section’ approach usedin POWHEGavoids the pathological behaviour observed in calculating cross sections with symmetric jetcutsinﬁxed-orderNLOcalculations.Here,NLOdijetproduction intheDGLAPformalismisinterfacedwithPYTHIA8toresumsoft andcollinearemissionsusingthepartonshowerapproximation.

PYTHIA8 adopts the Lund String model [32] for hadronisa- tion in each of the ND, SD and DD channels. It also contains an underlying-event modelbasedon multiplepartoninteractions (MPI). POMWIG is derived from HERWIG [33] and thus inherits itsfragmentationandcluster-basedhadronisation models.Forthe purposesofthispaper,thePOWHEGNDsimulationisinterfacedto PYTHIA8forfragmentationandhadronisation.Allconsideredmod- elsbased onthePYTHIAhadronisationmodelinclude p_T-ordered parton showering, while those based on HERWIG use angular- orderedpartonshowering.

ThedefaultMCcombinationusedforthedataunfoldingforde- tector effectsisamixtureofPYTHIA8samplesofND,SDandDD dijets,withthe“ATLASAU2-CT10”setoftunedparameters (tune) [34]fortheunderlyingevent.Inthistune,thefractionofthetotal crosssectionattributedtotheSDprocessisreducedrelativetothe defaultby10% andthat toDDby 12%,tobettermatchearlyLHC data. The Berger–Strengparameterisation, which has a very similar ξ dependence toD–L, is chosen forthe pomeron ﬂux factor.

Finally,theinteractionoftheparticles withtheATLAS detectoris simulatedusingaGEANT4-basedprogram[35,36].

3. TheATLASdetector

The ATLAS detector isdescribed indetail elsewhere [37].The beam-line is surrounded by a tracking system, which covers the pseudorapidity² range |η_|<2.5, consists of silicon pixel, silicon strip and straw tube detectors andis immersed inthe 2 T axial magneticﬁeldofasuperconducting solenoid.Thecalorimeterslie outsidethe tracking system. Ahighly segmentedelectromagnetic (EM)liquid-argonsamplingcalorimetercoverstherange|η_|<3.2.

TheEMcalorimeteralsoincludesapresamplercovering|η|<1.8.

The hadronicend-cap (HEC, 1.5<|η_|<3.2) andforward (FCAL, 3.1<|η_|<4.9)calorimetersalsouseliquidargon fortheir sensitive layers, but withreducedgranularity. Hadronic energy inthe central regionisreconstructedin asteel/scintillator-tilecalorime- ter.The shapesof thecell noise distributions inthe calorimeters are well described by Gaussian distributions, with the exception of the tile calorimeter, where the noise has extended tails, and whichisthusexcludedfromtherapiditygapﬁndingaspectsofthe analysis. Minimum-bias trigger scintillator (MBTS) detectors are mountedinfrontoftheend-capcalorimetersonbothsidesofthe interaction point andcover the pseudorapidity range2.1<|η_|<

3.8.TheMBTSisdividedintoinnerandouterrings,bothofwhich haveeight-foldsegmentation.In theanalysis,twotriggersystems areusedatLevel-1(L1),namelytheMBTSwhicheﬃcientlycollects low-pT jets,andthecalorimeter-basedtrigger(L1Calo)whichcon- centratesonhigher-pTjets.In2010,theluminositywasmeasured by monitoring the activityin forwarddetector components,with calibrationdeterminedthroughvanderMeerbeamscans[38,39].

2 IntheATLAScoordinatesystem,thez-axispointsinthedirectionoftheanti- clockwisebeamviewedfromabove.PolaranglesθandtransversemomentapTare measuredwithrespecttothisaxis.Thepseudorapidityη= −ln tan(θ/2)isagood approximationtotherapidityofaparticlewhosemassisnegligiblecomparedwith itsenergyandisusedhere,relativetothenominalz=^{0 point}^at^the^centre^of^the apparatus,todescriberegionsofthedetector.

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4. Experimentalmethod

Tostudyrapidity-gapproduction,theexperimentneeds toop- erateatverylowluminositiessuchthatthereisonaveragemuch lessthanonecollisionperbunchcrossing(i.e.negligible‘pile-up’).

This requirementhas to be balanced against the need to collect adequatenumbersofeventswithlargerapiditygaps.Theanalysis thereforeuses datafroman early 2010LHC run,witha totalin- tegratedluminosityof6.8 nb⁻¹.Theaveragenumberofcollisions perbunchcrossingis0.12.

ThejetselectionfollowsthatusedintheATLAS2010dijetanal- ysis[40].Jetswithp_T>20 GeV and|η|<4.4 arereconstructedby applying the anti-k_t algorithm [41] to topological clusters atthe standardATLASjetenergyscale.Forcomparisons,inparticle-level MCmodels,jetsareformedwiththeanti-kt algorithmfromstable (cτ>10 mm)ﬁnal-stateparticles.Theanalysisisperformedwith jetsoftwo differentradius parameters R=⁰.4 and R=⁰.6.Ap- proximatelytwiceasmanyjetsarereconstructedwiththeR=⁰.6 thanwiththe R=⁰.4 requirementinthekinematicrangecovered here.

The calorimeter-based jet trigger (‘L1Calo’) is used with the lowestavailable p_T thresholdinphase-spaceregions whereitsef- ficiencyisdeterminedtobegreaterthan60%.Thiscriterionissat- isfiedforcentraljetsatallpseudorapiditiesintherange|η|<2.9 withpT>29(34)GeV forjetswithR=⁰.4(0.6).Atlowertrans- versemomenta,orwherethejetsarebeyondtheL1Calo ηrange, the MBTS trigger isused, withthe requirementof a signal in at least one segment. The MBTS trigger is fully efficient for dijet events, but has a substantial time-dependent prescale (which is takenintoaccountin theoff-lineanalysis), reducingthe effective luminosityforforwardandlow-p_T jetsto0.303 nb⁻¹.

At least two jetsare required, withjet barycentres satisfying

|η_|<4.4 and with pT>20 GeV. These requirements correspond totheregioninwhichthejetenergyscaleandresolutionarewell knownandinwhichthejetsarefullycontainedwithinthedetec- tor.

Several sources of background were investigated. To reject contributions from beam interactions with residual gas in the beampipe, muonsfromupstream protoninteractionstravelling as a halo around the proton beam, and cosmic-ray muons, events are required to have a primary vertex constructed from at least two tracks and consistent with the beam spot position. In-time pile-up,causedby multipleinteractions inonebunch crossing,is suppressedbyrequiringthattherebenofurtherverticeswithtwo ormoreassociatedtracks.Out-of-timepile-up,causedbyoverlap- ping signals in the detector from neighbouring bunch crossings, was investigated and found to be negligible at the large bunch spacings (>5 μs)of the chosen runs. Oncean eventis triggered and the dijet selection criteria are met, the requirement on the primary vertex removes 0.3% and 0.2% of events in the L1Calo- and MBTS-triggered data, respectively, while the in-time pile-up suppression cuts remove 9.4% and 6.5%, respectively. The latter valuesareusedtoscalethecrosssectionstoaccountforthecor- respondinglosses. Residual backgroundoccursduetothe limited position resolution of the vertex reconstruction, which typically mergespairsofverticeswithz¹^{cm into}^a^single ^vertex.^The sizeofthiseffectisestimatedby extrapolationtolower valuesof the z distribution for pairs of vertices which are resolved and its inﬂuence is evaluated by randomly overlaying minimum-bias eventson theselected sample.The effectis smallerthan 0.5%in allbinsofthemeasureddistributions.Theresidualbeam-induced background isstudied using ‘unpaired’ bunch crossings inwhich onlyonebunchofprotonspassesthroughtheATLAS detectorand isfoundtobenegligible.

Eacheventis characterisedin termsofpseudorapidity regions which are devoid of hadronic activity (‘rapidity gaps’) using a method very similar to that first introduced in Ref. [2]. Rapidity gapsare definedusingthetracking(|η_|<2.5 andpT>200 MeV) and calorimetric(|η_|<4.8) information within the ATLAS detector acceptance. Full details of the track selection can be found in Ref. [42]. Following Ref. [2], the clustering algorithm accepts calorimetercellsasclusterseedsiftheirmeasuredresponseisap- proximately fivestandard deviations above the root-mean-square noiselevel,withasmalldependenceofthethresholdonpseudora- pidity.Cellsneighbouringtheseedcellareincludedinthecluster iftheir measuredenergies exceedsmallerthresholdrequirements definedbythestandardATLAS topologicalclustering method.The particle-levelgapdefinitionisdeterminedbytheregionofpseudo- rapidity with an absence of neutral particles with p>200 MeV andchargedparticleswitheitherp>500 MeV or p_T>200 MeV.

Thesemomentumandtransversemomentumrequirementsmatch the ranges over whichthe simulation indicates that particles are likelytoberecordedinthedetectors,accountingfortheaxialmag- netic ﬁeld intheinner detector.The treatmentofcalorimeterin- formationintherapidity-gapdeterminationfollowstheprocedure introduced inRef. [43],such that therequirement p_T>200 MeV forcalorimeterclustersfromthepreviousrapidity-gapanalysis[2]

is removed. Since thistransverse momentum requirementcorre- sponds to a very highmomentum atlarge pseudorapidities, the modiﬁedapproachmorecompletelyexploitsthecapabilitiesofAT- LAS to detect low-momentum particles in the calorimeters. The totalnumbersofselectedeventsintheL1CaloandMBTSsamples withR=⁰.6 are285 191and44 372,respectively.

The variable characterising forwardrapidity gaps, η^F, is de- ﬁned by the larger of the two empty pseudorapidity regions ex- tendingbetweentheedgesofthedetectoracceptanceat η=⁴.8 or

η= −⁴.8 andthe nearesttrackorcalorimeterclusterpassing the selection requirementsatsmaller|η_|.No requirementsare placed on particle production at |η_|>4.8 and no attempt is made to identifygapsinthecentralregionofthedetector.Inthisanalysis, the sizeoftherapiditygaprelative to η_{= ±}4.8 liesintherange 0< η^F<6.5.Forexampleη^F=6.5 impliesthatthereisnore- constructedparticlewith(transverse)momentumabovethreshold inoneoftheregions−⁴.8<η<1.7 or−¹.7<η<4.8.

Foreventswhichareofdiffractiveorigin,theMonteCarlostud- ies indicate that the rapidity-gap deﬁnition selects processes in which one ofthe incomingprotonseither remains intact(SD)or isexcitedtoproduceasystemwithmass M<7 GeV (DD).Inthe second case, the systemis typically restricted to a pseudorapidity region beyond the acceptance ofthe ATLAS detector. In both cases,theotherincomingprotondissociatestoproduceahadronic systemoflargerinvariant massM_X.Thegapsize,η^F,growsap- proximately logarithmically with 1/MX,the degree of correlation beinglimitedbyevent-to-eventhadronisationﬂuctuations.

In thisanalysis, measurements ofthe energydeposits ineach event are used to construct a variable, ˜ξ which is closely corre- latedwithξandissimilartothatusedinRef.[15].Neglectingany overalltransversemomentumofthesystemX ,therelation M²_X=√

s

p_Te^±^η, (1)

holds for cases where the intactproton travels in the ±^{z direc-} tion. In other words, if the forward rapidity gap starts at η= +⁴.8 (−⁴.8), the exponential function takes the positive (nega- tive) sign. Here, the sum runs over all particles constituting the system X . This relation has the attractive feature that the sum is relatively insensitive to particles in the X system travelling in thevery forwarddirection,i.e.thosewhichare producedatlarge