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6.2.1

Dataset

The majority of analysis is performed with the 2015–2016 dataset, though a small conference note was issued with updated differential distributions forpγγ_T ,N_jets≥30 GeV,|∆yγγ|,pj_T1 including the 2017 dataset. Discussion regarding the pileup and luminosity delivered of the dataset can be seen in Section 3.1. The 2015–2016 dataset contains36.1fb−1 _{and the 2015–2017 dataset is about double} the luminosity at79.8fb−1_.

Events are collected with theHLT_g35_loose_g25_loose trigger in 2015–2016 and the

HLT_g35_medium_g25_medium_L12EM20VH trigger in 2017. These triggers select events with two photons, requiring the transverse energy of the highest (second highest) ET photon to be above

35(25) GeV. The deposited electromagnetic calorimeter energy of each object is required to be

loosely (or in 2017, slight stronger) consistent with that of the shower originating from a photon.

6.2.2

Simulated Samples

Signal samples were generated for the Higgs boson production mechanisms using various Monte Carlo event generators as described in this section. The mass and width of the Higgs boson were set in the simulation tomH= 125GeV andΓH = 4.07MeV [146]. The samples are normalized with the latest available theoretical SM production cross section calculations as summarized in the Yellow

Report 4 [32] from the LHC Higgs cross section working group. The H →γγ branching ratio is

calculated to be 0.227% with _HDECAY [147, 148] and_PROPHECY4F [149–151]. The following

summarizes the simulation of different production modes:

• Higgs boson production via ggF is simulated at NNLO in QCD using the_{Powheg NNLOPS}

program [152], with the_PDF4LHC15PDF set [153]. The simulation achieves NNLO accuracy

for inclusive gg → H observables by reweighting the Higgs boson rapidity spectrum in Hj-

MiNLO [154] to that of _HNNLO [155]. The parton-level events produced by the _Powheg

NNLOPS program are sent to Pythia8[156–158] with the AZNLO parameter set tuned to

The sample is then normalized to the total cross section predicted by a N3LO QCD calculation with NLO electroweak corrections [160–163].

• Higgs boson production via VBF is generated at NLO accuracy in QCD using _Powheg-

Box [164–167] with the PDF4LHC15 PDF set. The parton-level events are passed to

Pythia8 tuned with the AZNLO parameter set to provide parton showering, hadronization

and the underlying event simulation. The VBF sample is normalized with an approximate NNLO QCD cross section with NLO electroweak corrections applied [168–170].

• Higgs boson production via V His generated at NLO accuracy in QCD throughqq/qg-initiated

production, denoted as qq¯0 → V H, and through gg → ZH production using _Powheg-

Box [171] with the PDF4LHC15 PDF set and the AZNLO parameter set. Pythia8 is

used for parton showering, hadronization and the underlying event. The samples are normal- ized with calculations at NNLO in QCD and NLO electroweak corrections forqq¯0→V H and

at NLO and next-to-leading logarithmic (NLL) accuracy in QCD forgg→ZH [172–174].

• Higgs boson production via t¯tH is generated at NLO in QCD using

MadGraph5_aMC@NLO[175] with the NNPDF3.0PDF set [176] and sent to Pythia8

to provide parton showering, hadronization and the underlying event, using the A14 param-

eter set [177]. The t¯tH sample is normalized with a calculation at NLO in QCD with NLO

electroweak corrections [178–181].

• Higgs boson production via b¯bH is simulated using_{MadGraph5_aMC@NLO}interfaced to

Pythia8 with the CT10 PDF set [182], and is normalized with the cross section calcula-

tion obtained by matching, using the Santander scheme, the five-flavor scheme cross section accurate to NNLO in QCD with the four-flavor scheme cross section accurate to NLO in QCD [183–185]. The sample includes the effect of interference with the gluon–gluon fusion production mechanism.

• Associated production of a Higgs boson with a single top-quark and aW-boson (tHW) is gen- erated at NLO accuracy, removing the overlap with the t¯tH sample through a diagram regu-

larization technique, using _{MadGraph5_aMC@NLO}interfaced to _Herwig++[186–188],

with the Herwig++ UEEE5 parameter set for the underlying event and the CT10 PDF

set using the five-flavor scheme. Simulated Higgs boson events in association with a single top-quark, a b-quark and a light quark (tHq) are produced at leading order (LO) accuracy

6. Higgs Differential and Fiducial Cross Sections in the Diphoton Channel 74

Table 6.3: Summary of the event generators, PDF sets,σh, and order of calculation used to model the signal and the diphoton background processes.

Process Generator Showering PDF set σ[pb] Order of calculation ofσ

ggF Powheg NNLOPS Pythia8 PDF4LHC15 48.52 N3_{LO(QCD)+NLO(EWK)} VBF Powheg-Box Pythia8 PDF4LHC15 3.78 NNLO(QCD)+NLO(EWK) W H Powheg-Box Pythia8 PDF4LHC15 1.37 NNLO(QCD)+NLO(EWK) q¯q0_→ZH

Powheg-Box Pythia8 PDF4LHC15 0.76 NNLO(QCD)+NLO(EWK) gg→ZH Powheg-Box Pythia8 PDF4LHC15 0.12 NLO+NLL(QCD) t¯tH MadGraph5_aMC@NLO Pythia8 NNPDF3.0 0.51 NLO(QCD)+NLO(EWK) b¯bH MadGraph5_aMC@NLO Pythia8 CT10 0.49 5FS(NNLO)+4FS(NLO)

t-channeltH MadGraph5_aMC@NLO Pythia8 CT10 0.07 4FS(LO)

W-associatedtH MadGraph5_aMC@NLO Herwig++ CT10 0.02 5FS(NLO)

γγ Sherpa Sherpa CT10

V γγ Sherpa Sherpa CT10

within the four-flavor scheme and using the A14 parameter set. ThetHW andtHq samples

are normalized with calculations accurate to NLO in QCD [189].

Some descriptions of signal production are taken from [6]. After generation, the events are

processed through aGeant4[190] simulation of the ATLAS detector [191].

The continuum γγ andV γγ background spectrum are simulated using the Sherpaevent gen-

erator [192], with the_CT10PDF set and the default parameter set for the underlying-event. The corresponding matrix elements forγγ are calculated at leading order in QCD with the real emission of up to three partons. TheV γγ matrix elements are calculated at LO in QCD as well, though only

up to the emission of two partons. The samples are merged with the _Sherpaparton shower [193]

using the_Meps@loprescription [194]. TheV γγ simulation, like the signal samples, undergoes the full detector simulation. Theγγ samples are processed through a fast detector simulation, referred to as ATLAS Fast Simulation II (AFII), based on a parameterization of the performance of the calorimeters [195].

Pileup is included in the simulation for all generated events such that thehµireproduces that ob- served in the data. The inelasticppcollisions were produced usingPythia8with theMSTW2008lo

PDF set [196] and A2 parameter set [197].