The Production of Pairs of Isolated Photons at Higher Orders in QCD

In this thesis, we consider the corrections to the production of a pair of isolated photons at the Large Hadron Collider (LHC) which arise at Next-to-Next-to-Leading-Order (NNLO) in QCD, and Next-to-Leading-Order (NLO) in the electroweak theory. These corrections are calculated through the antenna subtraction formalism, and implemented in the parton-level Monte Carlo program NNLOJET. This calculation is then applied to a study of the theoretical and phenomenological issues which drive the apparent tension between prior theoretical predictions at this order, and LHC data taken with the ATLAS detector at 8 TeV. In particular, we focus upon the issue of photon isolation, presenting the first calculation of the diphoton process with ‘hybrid isolation’, a compromise between the theoretical and experimental constraints upon predictions and measurements of photonic final-states. We further consider the consequences of another theoretical choice, the renormalisation and factorisation scales at which the calculation is made. We find that these two theoretical choices act in concert to generate the tension between prediction and data, and show that reasonable alternatives can lead to excellent agreement. We conclude with an application of the same approach to preliminary 13 TeV ATLAS data, again finding excellent agreement.