Measurements of isolated-photon production inclusively and in association with jets at V8 = 13 TeV with the ATLAS detector

Abstract

This document summarises most of my work during the years of my Ph.D. It covers ATLAS Standard Model measurements with a single photon in the nal state, as well as the jet energy scale in situ measurements using +jet events and data quality inspection of the liquid argon calorimeter. The production of prompt photons in proton{proton collisions, pp ! + X, provides a testing ground for the theory of quantum chromodynamics (QCD) with a hard colourless probe not a ected by hadronisation. A quantitative understanding of the features of photon production is not only relevant for precision Standard Model measurements but also important for Higgs production and searches for resonances predicted by models beyond the Standard Model. Measurements of prompt-photon production at the Large Hadron Collider (LHC) provide a handle on the gluon density in the proton by means of the Compton scattering and can be used to constrain the gluon density function inside the proton. The constraining power of the measurements can be enhanced if they are combined with measurements at di erent centre-of-mass energies. The requirement of jets in the nal state allows to probe the dynamics of the hard-scattering process by measuring the correlations between the photon and the jets. Measurements of the photon plus one jet cross section as a function of the scattering angle in the centre-of-mass frame are sensitive to the spin of the particle exchanged in the t-channel and thus provide insight on the dominant production mechanism. When two jets are required, the production mechanisms can be studied in detail by selecting di erent regions of the phase space in which their contributions are enhanced according to whether the photon is the object with highest or lowest transverse momentum among the three objects considered in the nal state. All the measurements presented here refer to isolated photons. Isolation was achieved by limiting the transverse energy allowed inside a cone of size R = 0:4 around the photon. Jets were reconstructed using the anti-kt algorithm with jet radius R = 0:4. The results of four di erent analyses at p s = 13 TeV are presented, namely the measurement of inclusive isolated-photon cross sections, the ratio of inclusive isolatedphoton cross sections at p s = 13 and 8 TeV, the measurement of the cross section for isolated-photon plus jet production and the cross-section measurements for isolated-photon plus two jets production. Leading order (LO) Monte Carlo predictions of Pythia and Sherpa, which include di erent treatments of the production mechanisms already at the matrix-element level, were compared to the measurements. In addition, comparisons with the next-to-leading order (NLO) xed-order calculations of JetPhox corrected for non-perturbative e ects and Sherpa NLO predictions, which include matrix elements of +1 and 2 jets computed at NLO and +3 and 4 jets computed at LO supplemented with parton shower, are provided. In general, agreement was found between the measurements and QCD predictions with di erent features in the predicted cross-section normalisation and shape that depend on the analysis.