Reweighting and Unweighting of Parton Distributions and the LHC W lepton asymmetry data

TL;DR

This paper refines a reweighting method for updating parton distribution functions with new data, demonstrating its effectiveness with LHC W lepton asymmetry measurements that improve constraints on light quark PDFs.

Contribution

It introduces a detailed reweighting and unweighting procedure for PDFs and applies it to incorporate LHC W asymmetry data, providing new constraints on light quark distributions.

Findings

Reweighting and unweighting produce statistically equivalent PDF sets.

LHC W asymmetry data significantly reduce uncertainties in light quark PDFs.

First solid constraints on PDFs from LHC data are established.

Abstract

We develop in more detail our reweighting method for incorporating new datasets in parton fits based on a Monte Carlo representation of PDFs. After revisiting the derivation of the reweighting formula, we show how to construct an unweighted PDF replica set which is statistically equivalent to a given reweighted set. We then use reweighting followed by unweighting to test the consistency of the method, specifically by verifying that results do not depend on the order in which new data are included in the fit via reweighting. We apply the reweighting method to study the impact of LHC W lepton asymmetry data on the NNPDF2.1 set. We show how these data reduce the PDF uncertainties of light quarks in the medium and small x region, providing the first solid constraints on PDFs from LHC data.