Update of Parton Distributions at NNLO

TL;DR

This paper introduces a new set of NNLO parton distributions with improved heavy flavor treatment, resulting in significant changes to gluon and heavy quark distributions and updated predictions for collider processes.

Contribution

It provides the first essentially complete set of NNLO partons incorporating full heavy flavor treatment and uncertainty estimates.

Findings

Significant change in gluon and heavy quark distributions.

Larger value of alpha_S(M_Z^2) at NNLO.

Updated predictions for W and Z production at the LHC.

Abstract

We present a new set of parton distributions obtained at NNLO. These differ from the previous sets available at NNLO due to improvements in the theoretical treatment. In particular we include a full treatment of heavy flavours in the region near the quark mass. In this way, an essentially complete set of NNLO partons is presented for the first time. The improved treatment leads to a significant change in the gluon and heavy quark distributions, and a larger value of the QCD coupling at NNLO, alpha_S(M_Z^2) = 0.1191 +/- 0.002(expt.) +/- 0.003(theory). Indirectly this also leads to a change in the light partons at small x and modifications of our predictions for W and Z production at the LHC. As well as the best-fit set of partons, we also provide 30 additional sets representing the uncertainties of the partons obtained using the Hessian approach.