The proton in high definition: revisiting photon-initiated production in high energy collisions

L.A Harland-Lang

doi:10.1007/jhep03(2020)128

Abstract

We re-examine the current state of the art for the calculation of photon- initiated processes at the LHC, as formulated in terms of a photon PDF in the proton that may be determined rather precisely from the known proton structure functions. We in particular demonstrate that a by construction more precise calculation is provided by a direct application of the structure function approach, best known from the case of Higgs Boson production via vector boson fusion. This avoids any artificial scale variation uncertainties, which can otherwise be rather significant for processes calculated within the standard approach thus far. To understand the source of these, we present a detailed comparison of the structure function approach and its relation to the photon PDF. We then provide precise predictions for the photon-initiated contribution to lepton pair production at the LHC, including the lepton pair transverse momentum distribution. Thus, by a direct application of the structure function formalism we show how the contribution from initial-state photons at the LHC may for the first time be included with high precision in a universal and straightforward way, providing a high definition picture of the photon content of the proton.

Highlights

JHEP03(2020)128 with the global MMHT [14] and NNPDF [15] sets, in both cases closely following the same approach as LUXqed
Currently only a rather limited set of studies including NLO EW corrections to photon-initiated production, that is via q → qγ splittings in the initial state, are available [16,17,18]. This introduces a source of uncertainty in the predicted photon-initiated cross section that is often significantly larger than that implied by the high precision determination of the photon PDF itself
In this paper we have critically re-examined the current state of the art for the calculation of such processes, which is formulated in terms of a photon PDF in the proton that may be determined rather precisely from the known proton structure functions

Summary

Structure function calculation

As in the case of Higgs production via VBF the structure function approach only includes the factorizable contributions to the cross section, that is it excludes all corrections due to QCD or EW exchanges between the protons. The corresponding corrections for photon-initiated production processes should enter at a similar level, as should NLO EW corrections due to virtual photon/Z exchange The inclusion of such corrections would in general necessitate a departure from the structure function approach, though we note that the calculation of [30], for which the leading NNLO QCD corrections are directly proportional to the LO cross section, would in principle allow a rather straightforward inclusion of these into the structure function framework. The logarithmic contribution to (3.14) ensures that the upper limit on the Q2 integral in the total cross section is set by Q2max, that is the explicit μ dependence cancels with that coming from (3.9):

Scalar

Hadron-hadron collisions

Summary and outlook