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Measurement of the exclusive γγ → μ + μ − process in proton–proton collisions at √s = 13 TeV with the ATLAS detector
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
ATLAS Collaboration, AKILLI, Ece (Collab.), et al . Measurement of the exclusive γγ → μ + μ − process in proton–proton collisions at √s = 13 TeV with the ATLAS detector. Physics Letters. B , 2018, vol. 777, p. 303-323
DOI : 10.1016/j.physletb.2017.12.043
Available at:
http://archive-ouverte.unige.ch/unige:101030
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Measurement of the exclusive γ γ → μ+μ
− process 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:
Received15August2017
Receivedinrevisedform13December2017 Accepted13December2017
Availableonlinexxxx Editor:W.-D.Schlatter
Theproductionofexclusiveγ γ→μ+μ−eventsinproton–protoncollisionsatacentre-of-massenergy of13 TeVismeasuredwiththeATLASdetectorattheLHC,usingdatacorresponding toanintegrated luminosity of 3.2 fb−1. 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.
©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
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+μ−ppproduction(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 γ→W+W−reaction[5–7].
Theexclusive γ γ→μ+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].
E-mailaddress:atlas.publications@cern.ch.
Inordertoreproducethedata,thecalculationsofsuchphoton- inducedreactions,inparticularexclusive γ γ→μ+μ−production, 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
0370-2693/©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
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Fig. 1.Schematicdiagramsfor(a)exclusive,(b)single-protondissociativeand(c)double-protondissociativetwo-photonproductionofmuonpairsinppcollisions.Theeffect ofadditionalinteractionsbetweentheprotonsisshownin(d).
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 fb−1.
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+μ−pp 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/γ∗→μ+μ−and Z/γ∗→τ+τ−events.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/γ∗→μ+μ−, Z/γ∗→τ+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-
