Measurements of integrated and differential cross sections for isolated photon pair production in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

TL;DR

The paper presents a precision measurement of isolated photon-pair production in pp collisions at √s = 8 TeV using the ATLAS detector, delivering integrated and differential fiducial cross sections with a data-driven background subtraction and two robust signal-extraction methods. The measurement employs Bayesian unfolding to particle level within a carefully defined fiducial region and compares results to state-of-the-art QCD predictions, finding fixed-order calculations to underpredict the data while Sherpa-based predictions, including NLO with parton showering, provide good agreement across most observables. The work highlights the importance of soft-gluon resummation and parton-shower effects in describing low-pT diphoton kinematics, and it provides critical benchmarks for Higgs diphoton analyses and new physics searches. Overall, the results offer a stringent test of pQCD in a clean final state and improve background understanding for precision Higgs measurements at the LHC.

Abstract

A measurement of the production cross section for two isolated photons in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=8$ TeV is presented. The results are based on an integrated luminosity of 20.2 fb$^{-1}$ recorded by the ATLAS detector at the Large Hadron Collider. The measurement considers photons with pseudorapidities satisfying $|η^γ|<1.37$ or ${1.56<|η^γ|<2.37}$ and transverse energies of respectively $E_{\mathrm{T,1}}^γ>40$ GeV and $E_{\mathrm{T,2}}^γ>30$ GeV for the two leading photons ordered in transverse energy produced in the interaction.The background due to hadronic jets and electrons is subtracted using data-driven techniques. The fiducial cross sections are corrected for detector effects and measured differentially as a function of six kinematic observables. The measured cross section integrated within the fiducial volume is $16.8 \pm 0.8$ pb. The data are compared to fixed-order QCD calculations at next-to-leading-order and next-to-next-to-leading-order accuracy as well as next-to-leading-order computations including resummation of initial-state gluon radiation at next-to-next-to-leading logarithm or matched to a parton shower, with relative uncertainties varying from 5% to 20%.

Figures (5)

• Figure 1: Distributions of the calorimeter transverse isolation energy for the (a) leading and (b) subleading photon candidates. Also shown are the projections of the signal and various background components used in the two-dimensional template fit and the full model, corresponding to their sum, after fit. At the bottom of each plot, the ratio of the data to the model after the fit is shown. A gray uncertainty band on the full model, including the systematic uncertainties discussed in the text, is shown in each case whereas the uncertainty in data is statistical only.
• Figure 2: Distributions of the reconstructed diphoton observables, together with the yields associated with the various components estimated using the two-dimensional template fit method.
• Figure 3: Measured fiducial cross section compared to the predictions from Sherpa 2.2.1, Diphox, Resbos and 2$\gamma$NNLO. The estimation of the uncertainties in the theoretical predictions are described in the text. Only the central value is shown for Resbos. The green (yellow) band represents the one- (two-)standard deviation uncertainty, including both the statistical and systematic uncertainties in the measurement added in quadrature.
• Figure 4: Differential cross sections as functions of the various observables compared to the predictions from Diphox and Resbos. At the bottom of each plot, the ratio of the prediction to the data is shown. The bars and bands around the data and theoretical predictions represent the statistical and systematic uncertainties, estimated as described in the text. Only the central values are shown for Resbos. Negative cross-section values are obtained with Diphox in the first (last) bin of $a_{\mathrm T}$ and $\phi^{*}_{\eta}$ ($\Delta\phi_{\gamma\gamma}$) and therefore are not shown (see text).
• Figure 5: Differential cross sections as functions of the various observables compared to the predictions from Sherpa 2.2.1 and 2$\gamma$NNLO. At the bottom of each plot, the ratio of the prediction to the data is shown. The bars and bands around the data and theoretical predictions represent the statistical and systematic uncertainties, estimated as described in the text. Negative cross-section values are obtained with 2$\gamma$NNLO when varying the renormalisation scale in the first two bins of $\phi^{*}_{\eta}$ and therefore are not shown (see text).