Uncertainties on $α_S$ in the MMHT2014 global PDF analysis and implications for SM predictions

L. A. Harland-Lang; A. D. Martin; P. Motylinski; R. S. Thorne

Paper

Uncertainties on $α_S$ in the MMHT2014 global PDF analysis and implications for SM predictions

Abstract

We investigate the uncertainty in the value of the strong coupling

when allowing it to be a free parameter in the recent MMHT global analyses of deep-inelastic and related hard scattering data that was undertaken to determine the parton distribution functions (PDFs) of the proton. The analysis uses the standard framework of leading twist fixed--order collinear factorisation in the

scheme. We study the constraints on the value of

coming from the individual data sets by repeating the NNLO and NLO global analyses at various fixed values of

about its optimum values, spanning the range

in units of

, and making all PDFs sets available. The inclusion of the measurements of the cross section for inclusive

production in the global fit allows us to explore the correlation between the values taken for the mass

of the top quark and

. We find that the best fit values are

and

at NLO and NNLO respectively, with the central values changing to

and

when the world average of

is used as a data point. We investigate the interplay between the uncertainties on

and the uncertainties on the PDFs. In particular we calculate the cross sections for key processes at the LHC and show how the uncertainties coming from the standard MMHT2014 PDFs and from

can be provided independently and can be combined.