Abstract
We consider the production of prompt-photon pairs at the LHC and we report on a study of QCD radiative corrections up to the next-to-next-to-leading order (NNLO). We present a detailed comparison of next-to-leading order (NLO) results obtained within the standard and smooth cone isolation criteria, by studying the dependence on the isolation parameters. We highlight the role of different partonic subprocesses within the two isolation criteria, and we show that they produce large radiative corrections for both criteria. Smooth cone isolation is a consistent procedure to compute QCD radiative corrections at NLO and beyond. If photon isolation is sufficiently tight, we show that the NLO results for the two isolation procedures are consistent with each other within their perturbative uncertainties. We then extend our study to NNLO by using smooth cone isolation. We discuss the impact of the NNLO corrections and the corresponding perturbative uncertainties for both fiducial cross sections and distributions, and we comment on the comparison with some LHC data. Throughout our study we remark on the main features that are produced by the kinematical selection cuts that are applied to the photons. In particular, we examine soft-gluon singularities that appear in the perturbative computations of the invariant mass distribution of the photon pair, the transverse-momentum spectra of the photons, and the fiducial cross section with asymmetric and symmetric photon transverse-momentum cuts, and we present their behaviour in analytic form.
Highlights
Owing to its physics relevance, the study of diphoton production requires accurate theoretical calculations which, in particular, include QCD radiative corrections at high perturbative orders
We consider the production of prompt-photon pairs at the LHC and we report on a study of QCD radiative corrections up to the next-to-next-to-leading order (NNLO)
If we proceed to assign a perturbative uncertainty on the basis of the differences between NNLO and NLO results for differential cross sections, the NNLO uncertainty is almost doubled with respect to the NNLO scale dependence in most of the regions of figures 17 and 18 and it is further increased in limited extreme regions
Summary
2.1 Isolation criteria Hadron collider experiments at the Tevatron and the LHC do not perform inclusive photon measurements. Of the intrinsic differences between the standard and smooth cone isolation criteria, eq (2.5) implies that a reliable (theoretical or experimental) cross section result with smooth cone isolation represents a lower bound on the corresponding (i.e., with the same values of ET max and R) cross section result with standard cone isolation (this statement is valid within reliable estimates of related theoretical or experimental uncertainties). This implies that a comparison between theoretical smooth isolation results and experimental standard isolation results leads to meaningful and valuable information. We study the dependence on the isolation parameter ET max and its effects on both total cross sections and differential cross sections in various kinematical regions
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