# Measurement of the exclusive γγ → μ + μ − process in proton–proton collisions at √s = 13 TeV with the ATLAS detector

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## Reference

### ATLAS Collaboration AKILLI, Ece (Collab.), et al.

Abstract

The production of exclusive γγ→μ+μ− events in proton–proton collisions at a centre-of-mass energy of 13 TeV is measured with the ATLAS detector at the LHC, using data corresponding to an integrated luminosity of 3.2 fb −1 . The measurement is performed for a dimuon invariant mass of 12 GeV

### DOI : 10.1016/j.physletb.2017.12.043

Available at:

http://archive-ouverte.unige.ch/unige:101030

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### Physics Letters B

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## μ

### s = 13 TeVwiththeATLASdetector

.TheATLASCollaboration

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

Articlehistory:

Availableonlinexxxx Editor:W.-D.Schlatter

Theproductionofexclusiveγ γμ+μeventsinproton–protoncollisionsatacentre-of-massenergy of13 TeVismeasuredwiththeATLASdetectorattheLHC,usingdatacorresponding toanintegrated luminosity of 3.2 fb1. The measurement is performed for a dimuon invariant mass of 12 GeV<

mμ+μ<70 GeV.Theintegratedcross-sectionisdeterminedwithinaﬁducialacceptanceregionofthe ATLASdetectoranddifferentialcross-sectionsaremeasuredasafunctionofthedimuoninvariantmass.

The results are comparedto theoretical predictionsbothwith and withoutcorrections for absorptive effects.

1. Introduction

When proton–proton(pp) beams collideat the LHC, typically rarephoton–photoninduced(γ γ)interactionsoccuratperceptible rateandprovideaunique opportunitytostudyhigh-energyelec- troweakprocesses[1].Comparedtootherﬁnalstates,thedilepton production is a standard candle process of the photon-induced productionmechanism,thanks toits sizeablecross-section. Using pp collisionsatacentre-of-mass energyof

s=7 TeV,measure- mentsofpp(γ γ)μ+μppproduction(referredtoasexclusive

γ γ μ+μ) were performed by the ATLAS and CMS collabo- rations [2,3]. The exclusive γ γ e+e process was also mea- sured [3,4]. A similar experimental signature has been used to studythe γ γW+Wreaction[5–7].

Theexclusive γ γμ+μ productionprocess competeswith the two-photon interactions involving single- or double-proton dissociation due to the virtual photon exchange (Fig. 1 (a–c)).

The electromagnetic (EM) break-up of the proton typically re- sults in a production of particles at small angles to the beam direction, which can mimic the exclusive process. However, the proton-dissociative processes have signiﬁcantly different kine- matic distributions compared to the exclusive reaction, allow- ing an effective separation of the different production mecha- nisms.

Ingeneral, thephoton-inducedproductionofleptonpairscon- tributesup to afew percent tothe inclusivedileptonproduction atLHCenergies[8–10].

Inordertoreproducethedata,thecalculationsofsuchphoton- inducedreactions,inparticularexclusive γ γμ+μproduction, need totake intoaccount the protonabsorptiveeffects [3].They are mainly related to additional gluon interactions between the protons(orprotonremnants),showninFig. 1(d),whichtakeplace inaddition to theQEDprocess. The sizeof theabsorption isnot expectedto bethe sameforexclusiveanddissociativeprocesses;

itmayalsodependonthereactionkinematics.Theseeffectslead to the suppression of exclusive cross-sections (typically around 10–20%)byproducingextrahadronicactivityintheeventbesides the ﬁnal-state muons. Recent phenomenological studies suggest that the exclusive cross-sections are suppressed, with a survival factorthatdecreaseswithmass[11,12].

In thispaper, a measurement ofexclusive dimuon production in ppcollisions at

s=13 TeV is presentedformuon pairswith invariant mass12 GeV<mμ+μ<70 GeV.The differentialcross- sections,dσ/dmμ+μ,aredeterminedwithinaﬁducialacceptance region. In the region 30 GeV<mμ+μ <70 GeV, the minimum transversemomentumofeachmuonisrequiredtobe10 GeV.For 12 GeV<mμ+μ<30 GeV, the minimum muon transverse mo- mentum is reduced to 6 GeV by taking advantage of the lower triggerthresholdsavailableby makingadditionalrequirementson muon-pairtopology.Inaddition,bothmuonsaremeasuredinthe pseudorapidity range of |ημ|<2.4. The measurements are com- paredtotheoreticalpredictionsbothwithandwithoutcorrections forabsorptiveeffects.

2. ATLASdetector

TheATLAS experiment[13] attheLHC isa multi-purposepar- ticledetectorwitha forward–backwardsymmetriccylindricalge- https://doi.org/10.1016/j.physletb.2017.12.043

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ometryandnearly4π coverageinsolidangle.1 Itconsistsofinner tracking devices surrounded by a superconducting solenoid, EM and hadronic calorimeters, and a muon spectrometer. The inner detector (ID) provides charged-particle tracking in the pseudora- pidity region |η|<2.5 and vertexreconstruction. It comprises a siliconpixeldetector,asiliconmicrostriptracker,andastraw-tube transition radiation tracker. The ID is surrounded by a solenoid that produces a 2 Taxial magnetic ﬁeld. Lead/liquid-argon (LAr) samplingcalorimetersprovideEMenergymeasurementswithhigh granularity.Ahadron(steel/scintillator-tile)calorimetercoversthe central pseudorapidity range |η|<1.7.The end-capand forward regions are instrumented withLAr calorimeters forboth the EM and hadronic energy measurements up to |η|=4.9. The muon spectrometer(MS)isoperatedinamagneticﬁeldprovidedbyair- coresuperconducting toroids andincludes tracking chambers for precisemuonmomentummeasurementsupto|η|=2.7 andtrig- gerchamberscoveringtherange|η|<2.4.

Atwo-level triggersystem[14] selects theevents usedinthe analysis.Theﬁrstlevelisimplementedincustomelectronics,while thesecondtriggerlevelisaﬂexiblesoftware-basedsystem.

3. Data,simulatedeventsamplesandtheoreticalpredictions This analysis uses a data set of pp collisions collected at a centre-of-mass energy

s=13 TeV during 2015 under stable beam conditions. After applying data quality requirements, this datasamplecorrespondstoanintegratedluminosityof3.2 fb1.

Calculations of the cross-section for exclusive γ γ μ+μ

production in pp collisions are based on the Equivalent Photon Approximation(EPA)[15,16]. TheEPArelies onthe propertythat the EM ﬁelds produced by the colliding protons can be treated asa beamof quasi-realphotonswith a smallvirtuality of Q2<

0.1 GeV2. Thisﬂuxofequivalent photonsisdetermined fromthe Fourier transform of the EM ﬁeld of the proton, takinginto ac- counttheEMformfactors [17].Thecross-sectionforthereaction pp(γ γ)μ+μpp is calculated by convolving the respective photon ﬂuxeswith the elementary cross-section for the process

γ γ μ+μ.The signal events forexclusive γ γ μ+μ pro- ductionweregeneratedusingtheHerwig7.0[18,19]MonteCarlo (MC)eventgenerator,inwhichthecross-sectionfortheprocessis computedby combiningthe pp EPA withthe leading-order (LO) formula for γ γ μ+μ. It is found that the predictions for exclusive γ γ μ+μ production from Herwigare identicalto thosefromLpair4.0[20]generator.

1 ATLASusesaright-handed coordinatesystemwith itsoriginat thenominal interactionpointinthecentreofthedetectorandthez-axiscoincidingwiththe axisofthebeampipe.Thex-axispointsfromtheinteractionpointtothecentreof theLHCring,andthey-axispointsupward.Thepseudorapidityisdeﬁnedinterms ofthepolarangleθasη= −ln tan(θ/2),andφistheazimuthalanglearoundthe beampipe withrespecttothe x-axis. Theangulardistance isdeﬁnedas R= ( η)2+( φ)2.Thetransversemomentumisdeﬁnedrelativetothebeamaxis.

The dominant background, photon-induced single-dissociative (S-diss) dimuon production (Fig. 1 (b)), was simulated using Lpair 4.0 with the Brasse [21] and Suri–Yennie [22] structure functions for proton dissociation. For photon virtualities Q2 <

5 GeV2 andmassesofthe dissociating systemmN <2 GeV, low- multiplicitystatesfromtheproductionanddecaysof resonances are usuallycreated.Forhigher Q2 ormN,thesystemdecaysinto avarietyofresonances,whichproducealargenumberofforward particles. The Lpair package was interfaced to JetSet 7.408 [23], wheretheLund[24]fragmentationmodelisimplemented.

The Herwig and Lpair generators do not include any correc- tions toaccountforprotonabsorptiveeffects.Hence thenormali- sationoftheseMCsamplesisfurtherconstrainedbyadata-driven procedure,asdescribedinSection6.

Fordouble-dissociative(D-diss)reactions,Pythia8.175[25]was used withthe NNPDF2.3QED[26]set ofpartondistributionfunc- tions(PDF).TheNNPDF2.3QEDsetusesLOQEDandnext-to-next- to-leading-order (NNLO) perturbative QCD (pQCD) calculations to construct the photon PDF, starting from the initial scale Q02 = 2 GeV2. Additionally,two alternativePDF sets, CT14QED[27]and LUXqed17 [28] are considered. Depending on the multiplicity of thedissociating system,thedefaultPythia8stringormini-string fragmentationmodelwasusedforprotondissociation.Theabsorp- tive effects inD-dissMC eventsare takenintoaccount usingthe defaultmulti-partoninteractionsmodelinPythia8[29].

The NLO pQCD Powheg-Box v2 [30–33] event generator was used with the CT10 [34] PDF to generate both the Drell–Yan (DY) Z/γμ+μand Z/γτ+τevents.Itwasinterfaced to Pythia 8.210 [25] applying the AZNLO [35] set of generator- parametervalues(tune)forthemodellingofnon-perturbativeef- fects, includingtheCTEQ6L1[36] PDF set.The productionoftop- quark pair (t¯t) events was also modelled using Powheg-Box,in- terfacedtoPythia6.428 [37].Theeventgeneratorsusedtomodel Z/γμ+μ, Z/γτ+τ andtt¯ reactions were interfaced toPhotos3.52[38,39]tosimulateQEDﬁnal-stateradiation(FSR) corrections.

Multiple pp interactionsperbunch crossing(pile-up)wereac- countedforbyoverlayingsimulatedminimum-biasevents,gener- ated with Pythia 8.210 using the A2 tune [40], and reweighting the distribution oftheaverage numberof interactions per bunch crossing inMC simulation tothat observed indata.Furthermore, the simulated samples were weighted such that the z-position distribution of reconstructed pp interaction vertices matches the distribution observed in data. The ATLAS detector response was modelled using the GEANT4 toolkit [41,42] and the same event reconstructionasthatusedfordataisperformed.

The measured distributionof theexclusive γ γ μ+μ pro- cess is compared with two models of absorptive corrections in Section8.

Intheﬁnite-sizeparameterisationapproach[11],theabsorptive effectsareembeddedintheevaluationofthe γ γ luminosity,tak- ingthephotonenergyandimpactparameterdependenceintoac-

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