Scale and isolation sensitivity of diphoton distributions at the LHC

Precision measurements of diphoton distributions at the LHC display some tension with theory predictions, obtained at next-to-next-to-leading order (NNLO) in QCD. We revisit the theoretical uncertainties arising from the approximation of the experimental photon isolation by smooth-cone isolation, and from the choice of functional form for the renormalisation and factorisation scales. We find that the resulting variations are substantial overall, and enhanced in certain regions. We discuss the infrared sensitivity at the cone boundaries in cone-based isolation in related distributions. Finally, we compare predictions made with alternative choices of dynamical scale and isolation prescriptions to experimental data from ATLAS at 8 TeV, observing improved agreement. This contrasts with previous results, highlighting that scale choice and isolation prescription are potential sources of theoretical uncertainty that were previously underestimated.

Automated summary

Precision measurements of diphoton distributions at the LHC show tension with theory predictions at next-to-next-to-leading order in QCD. This tension comes from the approximation of experimental photon isolation and the choice of scales. By comparing predictions with alternative scale choices and isolation prescriptions, improved agreement with experimental data is observed, highlighting the underestimated sources of theoretical uncertainty in scale choice and isolation prescription.