Subtraction method for NLO corrections in Monte-Carlo event generators for leptoproduction

TL;DR

This paper addresses incorporating NLO corrections into Monte-Carlo event generators for leptoproduction, focusing on gluon-fusion in diffractive DIS where gluon densities dominate. It introduces a subtraction framework that combines LO parton-model showering with a photon-gluon fusion contribution, carefully subtracting double-counted regions. A BS kinematics-based implementation yields non-linear, density-dependent NLO terms, motivating a new parton-kinematics algorithm that produces simpler, predominantly linear corrections and easier MSbar matching. It discusses the practical issue of negative weighted events, proposes a tunable cut-off to mitigate them, and outlines future work to generalize the method to broader processes and to integrate with standard PDFs.

Abstract

In the case of the gluon-fusion process in deep-inelastic leptoproduction, I explicitly show how to incorporate NLO corrections in a Monte-Carlo event generator by a subtraction method. I calculate the parton densities to be used by the event generator in terms of MSbar densities. The method is generalizable. A particular motivation for treating the gluon-fusion process is to treat diffractive deep-inelastic scattering properly, since in diffractive scattering the gluon density dominates the quark densities. I also propose a modified algorithm for treating parton kinematics in event generators; the new algorithm results in much simpler formulae for the NLO corrections. A disadvantage of the new method is that some of the generated events may have negative weights. However, an adjustable cut-off function is present in the formalism, and this permits a renormalization-group-like transformation that can be used to at least reduce the proportion of events with negative weights.