Measurement of the production cross section of three isolated photons in $pp$ collisions at $\sqrt{s}$ = 8 TeV using the ATLAS detector

TL;DR

This ATLAS study measures the production cross section of three isolated photons in $pp$ collisions at $\sqrt{s}=8$ TeV, reporting detailed differential cross sections across photon energies, pair and triplet invariants, and angular variables, along with an inclusive fiducial cross section. Using MC simulations and NLO pQCD predictions, the analysis highlights significant underestimation by NLO calculations in the low-$E_T$ and low-mass regions, while MG5_aMC@NLO with parton showers better describes high-$E_T$ and high-mass distributions. The work employs rigorous background estimation (including $e$–$\gamma$ and jet–$\gamma$ misidentifications) and bin-by-bin unfolding to particle level, with a comprehensive treatment of experimental and theoretical uncertainties. Overall, the results underscore the need for improved triphoton modeling in MC tools and motivate higher-order calculations to better capture this rare SM process and its role as a background in BSM searches.

Abstract

A measurement of the production of three isolated photons in proton-proton collisions at a centre-of-mass energy $\sqrt{s}$ = 8 TeV is reported. The results are based on an integrated luminosity of 20.2 fb$^{-1}$ collected with the ATLAS detector at the LHC. The differential cross sections are measured as functions of the transverse energy of each photon, the difference in azimuthal angle and in pseudorapidity between pairs of photons, the invariant mass of pairs of photons, and the invariant mass of the triphoton system. A measurement of the inclusive fiducial cross section is also reported. Next-to-leading-order perturbative QCD predictions are compared to the cross-section measurements. The predictions underestimate the measurement of the inclusive fiducial cross section and the differential measurements at low photon transverse energies and invariant masses. They provide adequate descriptions of the measurements at high values of the photon transverse energies, invariant mass of pairs of photons, and invariant mass of the triphoton system.

Figures (3)

• Figure 1: Measured differential cross sections for the production of three isolated photons (dots) as functions of (a) $E_{\text{T}}^{\gamma_{1}}$, (b) $E_{\text{T}}^{\gamma_{2}}$ and (c) $E_{\text{T}}^{\gamma_{3}}$. The NLO QCD calculations from MCFM and MadGraph5_aMC@NLO are also shown. The thickness of each theoretical prediction corresponds to the theoretical uncertainty. The bottom part of each figure shows the ratios of predicted and measured differential cross sections. The red inner (black outer) error bars represent the systematic uncertainties (the statistical and systematic uncertainties added in quadrature). For most of the data points, the inner error bars are smaller than the marker size and thus not visible.
• Figure 2: Measured differential cross sections for the production of three isolated photons (dots) as functions of (a) $m^{\gamma_{1}\gamma_{2}}$, (b) $m^{\gamma_{1}\gamma_{3}}$, (c) $m^{\gamma_{2}\gamma_{3}}$ and (d) $m^{\gamma\gamma\gamma}$. The NLO QCD calculations from MCFM and MadGraph5_aMC@NLO are also shown. The thickness of each theoretical prediction corresponds to the theoretical uncertainty. The bottom part of each figure shows the ratios of predicted and measured differential cross sections. The red inner (black outer) error bars represent the systematic uncertainties (the statistical and systematic uncertainties added in quadrature). For most of the data points, the inner error bars are smaller than the marker size and thus not visible.
• Figure 3: Measured differential cross sections for the production of three isolated photons (dots) as functions of (a) $\Delta\phi^{\gamma_{1}\gamma_{2}}$, (b) $\Delta\phi^{\gamma_{1}\gamma_{3}}$, (c) $\Delta\phi^{\gamma_{2}\gamma_{3}}$, (d) $|\Delta\eta^{\gamma_{1}\gamma_{2}}|$, (e) $|\Delta\eta^{\gamma_{1}\gamma_{3}}|$ and (f) $|\Delta\eta^{\gamma_{2}\gamma_{3}}|$. The NLO QCD calculations from MCFM and MadGraph5_aMC@NLO are also shown. The thickness of each theoretical prediction corresponds to the theoretical uncertainty. The bottom part of each figure shows the ratios of predicted and measured differential cross sections. The red inner (black outer) error bars represent the systematic uncertainties (the statistical and systematic uncertainties added in quadrature). For some of the data points, the inner error bars are smaller than the marker size and thus not visible.