Article
Reference
Search for new phenomena in high-mass diphoton final states using 37 fb −1 of proton–proton collisions collected at √s = 13 TeV with the
ATLAS detector
ATLAS Collaboration AKILLI, Ece (Collab.), et al .
Abstract
Searches for new phenomena in high-mass diphoton final states with the ATLAS experiment at the LHC are presented. The analysis is based on pp collision data corresponding to an integrated luminosity of 36.7 fb −1 at a centre-of-mass energy s=13 TeV recorded in 2015 and 2016. Searches are performed for resonances with spin 0, as predicted by theories with an extended Higgs sector, and for resonances with spin 2, using a warped extra-dimension model as a benchmark model, as well as for non-resonant signals, assuming a large extra-dimension scenario. No significant deviation from the Standard Model is observed.
Upper limits are placed on the production cross section times branching ratio to two photons as a function of the resonance mass. In addition, lower limits are set on the ultraviolet cutoff scale in the large extra-dimensions model.
ATLAS Collaboration, AKILLI, Ece (Collab.), et al . Search for new phenomena in high-mass diphoton final states using 37 fb −1 of proton–proton collisions collected at √s = 13 TeV with the ATLAS detector. Physics Letters. B , 2017, vol. 775, p. 105-125
DOI : 10.1016/j.physletb.2017.10.039
Available at:
http://archive-ouverte.unige.ch/unige:98342
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Search for new phenomena in high-mass diphoton final states using 37 fb
−1of proton–proton collisions collected 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:
Received14July2017
Receivedinrevisedform19October2017 Accepted20October2017
Availableonlinexxxx Editor:L.Rolandi
Searchesfornewphenomenainhigh-massdiphotonfinalstateswiththeATLASexperimentattheLHC are presented.Theanalysisisbasedonpp collisiondatacorrespondingtoanintegratedluminosity of 36.7 fb−1atacentre-of-massenergy√s
=13 TeV recordedin2015and2016.Searchesareperformed forresonanceswithspin0,aspredictedbytheorieswithanextendedHiggssector,andforresonances withspin2,usingawarpedextra-dimensionmodelasabenchmarkmodel,aswellasfornon-resonant signals,assumingalargeextra-dimensionscenario.NosignificantdeviationfromtheStandardModelis observed.Upperlimitsareplacedontheproductioncrosssectiontimesbranchingratiototwophotons asafunctionoftheresonancemass.Inaddition,lowerlimitsaresetontheultravioletcutoffscaleinthe largeextra-dimensionsmodel.
©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Newhigh-massstatesdecayingintotwophotonsarepredicted inextensionsoftheStandardModel(SM).Thediphotonfinalstate provides aclean experimental signature with excellent invariant- mass resolution and moderate backgrounds. This Letter presents anupdateofthesearchesfornewhigh-mass statesdecayinginto two photons, usingboth 2015 and2016proton–proton(pp) col- lisiondatasets recorded at a centre-of-mass energy√
s=13 TeV by the ATLAS detector atthe CERN Large Hadron Collider (LHC), corresponding to a total integrated luminosity of 36.7 fb−1. The analysis closely follows that described in Ref. [1], and includes small improvements in the photon reconstruction, selection and energycalibration.FromthemanyextensionsoftheSM thatpre- dict new high-mass resonances decaying into two photons, the two benchmarksignal models studied inRef. [1] are considered:
aspin-0resonance(X) aspredictedintheorieswithan extended Higgssector [2–8], andthe lightest Kaluza–Klein (KK) [9]spin-2 graviton excitation (G∗) of a Randall–Sundrum [10] model with one warpedextra dimension,later referred to asRS1. The ATLAS andCMScollaborationsreportedamodestexcessinthediphoton invariant-mass spectra with respect to the SM continuum back- groundnearamassvalueof750 GeV[1,11],using3.2–3.3 fb−1of pp collision data recorded in 2015at theLHC. The ATLAS result correspondstoaglobalsignificanceof2.1 standarddeviations (σ).
E-mailaddress:atlas.publications@cern.ch.
The CMS result corresponds to a global significance of 1.6σ. No significantexcessisobservedbyCMSin12.9 fb−1 ofdatacollected in2016[12].
In addition to searching for a resonant signal, data are inter- preted using the model proposed by Arkani-Hamed, Dimopoulos and Dvali (ADD)[13]. Motivatedby the weakness ofgravity, the ADDmodelpredictstheexistenceofnextradimensionsofspace- time where only gravity can propagate. If the ultraviolet cutoff scale(MS) oftheKaluza–Kleinspectrumislowerthanthe Planck scaleinthe(4+n)-dimensionalspace-time,theextradimensions may be detected via virtual KK graviton exchange before being observed viadirect KK graviton emission. Thestrength ofgravity inthepresence ofextradimensionsistypicallyparameterizedby
ηG=F/MS4,where F isadimensionlessparameteroforderunity reflecting thedependenceofthe virtualKK graviton exchange on the number of extra dimensions. Several theoretical formalisms existin theliterature [14–16].While thedefinitionof ηG iscon- sistent, each formalism uses a different convention for F, which consequentlyleadstoadifferentdefinitionofMS.TheKKgraviton exchange creates a set of finely spaced resonances, which man- ifests itself as a non-resonant deviation from the expected SM background inthe diphotonmass distribution dueto limitedex- perimental resolution.The effectivediphoton cross section is the result of the SM and ADD amplitudes, as well as their interfer- ence. Theinterferencetermintheeffectivecrosssection islinear in ηG andthepureKK gravitonexchangetermisquadraticin ηG. Theinterferenceeffectisassumedtobeconstructiveinformalisms considered in this Letter. Previous searches for an ADD graviton https://doi.org/10.1016/j.physletb.2017.10.039
0370-2693/©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
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signalinthediphotondecaychannelwere carriedoutby theAT- LAS[17]andCMS[18]experimentsbasedonLHCRun 1data.The substantial increase in the centre-of-mass energy in LHC Run 2 greatlyenhancesthesensitivitytotheADDscenarioathighermass scales[19].
2. ATLASdetector
TheATLAS detector[20,21]isa multi-purposedetectorwitha forward–backward symmetric cylindrical geometry.1 The systems most relevant for the presented searches are the inner detec- tor (ID), immersed in a 2 T magnetic field produced by a thin superconducting solenoid, and the calorimeters. The ID consists offine-granularity silicon pixeland microstrip detectorscovering the pseudorapidity range|η|<2.5 complementedby a gas-filled straw-tubetransitionradiationtracker(TRT)atlarger radiiwhich covers the region |η|<2.0 and provides electron identification capabilities.Theelectromagnetic(EM)calorimeterisalead/liquid- argonsamplingcalorimeterwithaccordiongeometry.Itisdivided intoabarrelsectioncovering|η|<1.475 andtwoend-capsections covering 1.375<|η|<3.2. For |η|<2.5, it is divided intothree layers in depth, which are finely segmented in η and φ. A thin presampler layer, covering |η|<1.8, is used to correct for fluc- tuations in upstream energy losses. Hadronic calorimetry in the region |η|<1.7 uses steel absorbers andscintillator tiles asthe activemedium.Liquid-argon calorimetrywithcopperabsorbersis usedinthehadronicend-capcalorimeters,whichcovertheregion 1.5<|η|<3.2.A forwardcalorimeterusingcopperandtungsten absorberswithliquidargoncompletesthecalorimetercoverageup to|η|=4.9.Themuonspectrometer,locatedbeyondthecalorime- ters,consistsofthreelargeair-coresuperconductingtoroidsystems withprecision trackingchambersproviding accurate muontrack- ingfor|η|<2.7 andfastdetectorsfortriggeringfor|η|<2.4.
Events are selected using a first-level trigger implemented in customelectronics,whichreducestheeventratetoadesignvalue of atmost 100 kHz using a subset of detectorinformation [22].
Software algorithms with access to the full detector information arethenusedinthehigh-leveltriggertoyieldanaveragerecorded eventrateofabout1 kHz.
3. Dataandsimulatedeventsamples
Data were collected in 2015 and 2016using pp collisions at acentre-of-mass energyof√
s=13 TeV with abunch spacingof 25 ns. The average number of pp interactions per bunch cross- ing is 13 in 2015 and 25 in 2016, with a peak instantaneous luminosity up to 1.4×1034 cm−2s−1. Events from pp collisions arerecordedusinga diphotontriggerwithtransverseenergy(ET) thresholdsof35 GeV and25 GeV forthe ET-orderedleading and subleadingphoton candidates,respectively. Inthe high-level trig- ger, the shapesof the energy depositions in the EM calorimeter are required to match those expected forelectromagnetic show- ersinitiatedbyphotons.Thetriggerhasasignalefficiencycloseto 100%foreventsfulfillingthefinal eventselection, withan uncer- tainty below 0.4%. After applying data-quality requirements, the data sample corresponds to an integrated luminosity of 3.2 fb−1
1 TheATLASexperimentusesaright-handedcoordinatesystemwithitsoriginat thenominalinteractionpoint(IP)inthecentreofthedetectorandthez-axisalong thebeampipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,and the y-axispointsupward.Cylindricalcoordinates(r,φ)areusedinthetransverse plane,φbeingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdefined intermsofthepolarangleθasη= −ln tan(θ/2).Thetransverseenergyisdefined asET=Esin(θ ).
forthe2015dataand33.5 fb−1 forthe2016data.Themeasure- ment of theintegrated luminosity hasan uncertainty of2.1% for the 2015 data and 3.4% for the 2016 data. The uncertainties in the 2015 and2016 integratedluminosities are derived following a methodology similar to that detailed in Ref. [23], from a cali- brationoftheluminosity scaleusingax–y beam-separationscan performed inAugust 2015, and a preliminarycalibration using a scanperformedinMay2016,respectively.Thecorrelationbetween thetwoyears’luminosityuncertaintiesistakenintoaccount.
SimulatedMonteCarlo(MC)eventsareusedforoptimizingthe search strategy[23],andforthesignalandbackgroundmodelling studies detailedinSections5 and6,respectively.Interference ef- fects between the resonant signal and the background processes areneglected.
The spin-0 signal MC samples were generated using the effective-field-theory approach implemented in MadGraph5_
aMC@NLO [24] version 2.3.3 at next-to-leading order (NLO) in quantum chromodynamics (QCD). From the Higgs characteriza- tion framework [25], CP-even dimension-five operators coupling thenewresonancetogluonsandphotonswereincluded.Samples were generatedwiththeNNPDF3.0NLO partondistributionfunc- tions (PDFs) [26], using the A14 set of tuned parameters (tune) of Pythia 8.186[27,28] for the parton-showerand hadronization simulation. Simulatedsamples were produced forfixed valuesof themassandwidthoftheassumedresonance,spanningtherange 200–2400 GeV for themass,andtherangefrom4 MeV to10%of themassforthedecaywidth.Choosinganimprovedsignalmodel with an event generator different from the one used in Ref. [1]
providesadescriptionofthesignalwhichislesssensitivetomod- ellingeffects fromthe off-shellregion.Theimpact ofthischange is onlyvisible inscenarios witha large signal decaywidth, with massvaluesattheTeV scale.
Spin-2signal samplesfortheRS1modelweregeneratedusing Pythia8.186,withtheNNPDF23LOPDFset[29]andtheA14tune.
Only the lightest KK graviton excitation was generated. Its mass mG∗ was varied in the range between 500 GeV and 5000 GeV.
The dimensionlesscoupling k/MPl,where MPl=MPl/√
8π is the reducedPlanckscaleandkthecurvaturescaleoftheextradimen- sion, isassumedto be inthe range0.01 to 0.3. Fork/MPl<0.3, the KK gravitonis expectedtobe afairly narrowresonance [30], withawidthgivenby1.44(k/MPl)2mG∗.
The non-resonant KK graviton signal was simulatedusing the ADDmodelwiththerepresentationproposedbyGiudice,Rattazzi andWells(GRW)[14].TheSherpaeventgenerator(version2.1.1) wasusedtosimulateboththeSMandADDprocessesatthesame time, including their interference. The ultraviolet cutoffscale MS wasvariedbetween3500 GeV and6000 GeV inthesimulation.
Events containing two prompt photons, representing an ir- reducible background to the search, were simulated using the Sherpa [31] eventgenerator, version 2.1.1. Matrix elements were calculated with up to two additional partons at leading order (LO) in QCD and merged with the Sherpa parton-shower sim- ulation [32] using the ME+PS@LO prescription [33]. The gluon- induced box process was also included. The CT10 PDF set [34]
was used inconjunctionwitha dedicatedparton-showertune of Sherpa.Samplesofthephoton+jet reduciblebackgroundcompo- nent were also generated using Sherpa, version 2.1.1, with ma- trixelements calculatedatLOwithuptofouradditionalpartons.
The same PDF set, parton-shower tune andmerging prescription as forthe diphoton sample were used. Tostudythe dependence of data composition results (Section 4) on the event generator, Pythia8wasalsoused togenerateSM diphotonevents,basedon theLOquark–antiquarkt-channeldiagram andthegluon-induced boxprocess,andphoton+jetevents.ThesamePDFsetandparton- showertuneasfortheRS1modelsignalsampleswereused.
