Uncertainties on ... formula ... in the MMHT2014 global PDF analysis and implications for SM predictions

摘要

We investigate the uncertainty in the strong coupling αS(MZ2) 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 MS¯ scheme. We study the constraints on αS(MZ2) coming from individual data sets by repeating the NNLO and NLO fits spanning the range 0.108 to 0.128 in units of 0.001, making all PDFs sets available. The inclusion of the cross section for inclusive tt¯ production allows us to explore the correlation between the mass mt of the top quark and αS(MZ2). We find that the best-fit values are αS(MZ2)=0.1201±0.0015 and 0.1172 ± 0.0013 at NLO and NNLO, respectively, with the central values changing to αS(MZ2)=0.1195 and 0.1178 when the world average of αS(MZ2) is used as a data point. We investigate the interplay between the uncertainties on αS(MZ2) and on the PDFs. In particular we calculate the cross sections for key processes at the LHC and show how the uncertainties from the PDFs and from αS(MZ2) can be provided independently and be combined.