Measurement of isolated-photon plus two-jet production in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

The dynamics of isolated-photon plus two-jet production in pp collisions at a centre-of-mass energy of 13 TeV are studied with the ATLAS detector at the LHC using a dataset corresponding to an integrated luminosity of 36.1 fb−1. Cross sections are measured as functions of a variety of observables, including angular correlations and invariant masses of the objects in the final state, γ+jet+jet. Measurements are also performed in phase-space regions enriched in each of the two underlying physical mechanisms, namely direct and fragmentation processes. The measurements cover the range of photon (jet) transverse momenta from 150 GeV (100 GeV) to 2 TeV. The tree-level plus parton-shower predictions from SHERPA and PYTHIA as well as the next-to-leading-order QCD predictions from SHERPA are compared with the measurements. The next-to-leading-order QCD predictions describe the data adequately in shape and normalisation except for regions of phase space such as those with high values of the invariant mass or rapidity separation of the two jets, where the predictions overestimate the data. The photon is required to have a transverse energy above 150 GeV and absolute value of pseudorapity |etaGamma|<2.37, excluding the region 1.37<|etaGamma|<1.56. The photon isolation is ensured by requiring the transverse energy around a cone of radius R=0.4 around the photon to be less than (10 + 0.0042 * ETGamma) [GeV]. At particle level it is the sum of transverse energy from all stable particles, except for muons, neutrinos and the photon itself, in a cone of size DeltaR =0.4 around the photon direction after the contribution from the underlying event is subtracted; the same subtraction procedure, based on the jet-area method, used on data is applied at the particle level (see the journal publication for details). At parton level it is the sum of transverse energy from all coloured partons in a cone of size DeltaR =0.4 around the photon direction. Particle-level jets are reconstructed with the anti-kt algorithm with R=0.4 using all stable final-state particles as input; muons, neutrinos and particles frompile-up activity are excluded. Jets are required to have a transverse momentum larger than 100 GeV in the rapidity region |yjet|<2.5 and to not be closer to the photon than DeltaRgamma_jet<=0.8 in the eta-phi plane. Information about the bin-to-bin correlation of the systematic uncertainties. The following uncertainties are to be treated as uncorrelated bin-to-bin: sysPhotonID, sysBackgroundIsolation, sysBackgroundIsolationUpperLimit, sysBackgroundID, sysIsolationMC and sysMCstats. The systematic uncertainty due to the photon energy scale and resolution is partially correlated bin-to-bin and its decomposition into independent sources is given. In order to take into account properly the correlations due to the photon energy scale and resolution, see the information provided below. The systematic uncertainty due to the photon energy scale (GES) and resolution (GER) is decomposed into 76 independent components: starting from the one labelled RESOLUTION_MATERIALCALO__1down,RESOLUTION_MATERIALCALO__1up until the one labelled PH_SCALE_LEAKAGEUNCONV__1down,PH_SCALE_LEAKAGEUNCONV__1up. Each of the 76 independent components has two variations (up and down). The uncertainties due to the up and down variations for each component are not necessarily symmetric and do not necessarily have different signs. Furthermore, the uncertainty for a given variation (up or down) of a given component can change sign bin to bin in the given observable. As a result, providing the positive and negative uncertainties as such would mean that the correlation between different observable bins and different measurements is lost. To avoid that loss and to provide the information on the correlation the following format is used for the uncertainties of each independent component in all tables: for the upper entry of the uncertainty, the systematic uncertainty of the down variation is given, which can be either positive or negative, and is fully correlated with the upper entries of the other observable bins (for the same component); for the lower entry of the uncertainty, the systematic uncertainty of the up variation is given, which can be either positive or negative, and is fully correlated with the lower entries of the other observable bins (for the same component). For example, for the first component: the upper entry corresponds to the sistematic uncertainty due to RESOLUTION_MATERIALCALO__1down (first part of the label) and the lower entry corresponds to the sistematic uncertainty due to RESOLUTION_MATERIALCALO__1up (second part of the label). The analogous strategy has been followed for the 87 components of the jet energy scale (JES).

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