A First Determination of Parton Distributions with Theoretical Uncertainties

TL;DR

This paper introduces a method to incorporate theoretical uncertainties from missing higher order corrections into the determination of proton parton distribution functions, improving the accuracy of predictions for LHC processes.

Contribution

It presents the first approach to include missing higher order uncertainties as a covariance matrix in PDF fits, validated at NLO and applied to NNPDF3.1 NLO PDFs.

Findings

MHOU can be integrated into the covariance matrix for PDFs.

Including MHOU affects the central values and uncertainties of PDFs.

Validation shows consistency with known NNLO corrections.

Abstract

The parton distribution functions (PDFs) which characterize the structure of the proton are currently one of the dominant sources of uncertainty in the predictions for most processes measured at the Large Hadron Collider (LHC). Here we present the first extraction of the proton PDFs that accounts for the missing higher order uncertainty (MHOU) in the fixed-order QCD calculations used in PDF determinations. We demonstrate that the MHOU can be included as a contribution to the covariance matrix used for the PDF fit, and then introduce prescriptions for the computation of this covariance matrix using scale variations. We validate our results at next-to-leading order (NLO) by comparison to the known next order (NNLO) corrections. We then construct variants of the NNPDF3.1 NLO PDF set that include the effect of the MHOU, and assess their impact on the central values and uncertainties of the resulting PDFs.