A posteriori inclusion of PDFs in NLO QCD final-state calculations

TL;DR

This paper introduces a method for efficiently including PDFs in NLO QCD calculations by creating a grid of weights that can be combined with any PDF, improving accuracy and usability in collider data analysis.

Contribution

The main novelty is the use of higher-order interpolation for the PDF grid, significantly enhancing accuracy and memory efficiency in NLO QCD calculations.

Findings

Achieved 0.01% accuracy in inclusive cross-section calculations.

Method enables efficient inclusion of collider data in PDF fits.

Facilitates more sensitive tests of the Standard Model at the LHC.

Abstract

Any NLO calculation of a QCD final-state observable involves Monte Carlo integration over a large number of events. For DIS and hadron colliders this must usually be repeated for each new PDF set, making it impractical to consider many `error' PDF sets, or carry out PDF fits. Here we discuss ``a posteriori'' inclusion of PDFs, whereby the Monte Carlo run calculates a grid (in x and Q) of cross section weights that can subsequently be combined with an arbitrary PDF. The procedure is numerically equivalent to using an interpolated form of the PDF. The main novelty relative to prior work is the use of higher-order interpolation, which substantially improves the tradeoff between accuracy and memory use. An accuracy of about 0.01% has been reached for the single inclusive cross-section in the central rapidity region |y|<0.5 for jet transverse momenta from 100 to 5000 GeV. This method should facilitate the consistent inclusion of final-state data from HERA, Tevatron and LHC in PDF fits, thus helping to increase the sensitivity of LHC to deviations from standard Model predictions.