Monte-Carlo Event Generators at NLO
John Collins
TL;DR
The paper develops a subtractive, region-based framework to embed arbitrarily non-leading order corrections into Monte-Carlo event generators within a tractable non-gauge theory (phi^3 in 6D). By decomposing cross sections into hard-scattering coefficients and jet factors, and by replacing hard-subgraph external legs with massless on-shell projections, Collins proves a factorization structure and constructs a practical MC algorithm with pointwise subtractions in momentum space. It establishes IR finiteness for the integrated jet factor, derives RG equations for the jet and hard pieces, and demonstrates the method through explicit one-loop and NLO calculations, including a detailed treatment of jet masses and veto functions. The work lays a rigorous foundation for extending NLO techniques to MC event generation, highlighting both the potential and the principal obstacles, such as handling soft emissions and gauge invariance in QCD frameworks. The approach promises improved accuracy and compatibility with analytic results while preserving the exclusive-event generation capability of MC methods.
Abstract
A method to construct Monte-Carlo event generators at arbitrarily non-leading order is explained for the case of a non-gauge theory. A precise and correct treatment of parton kinematics is provided. Modifications of the conventional formalism are required: parton showering is not exactly the same as DGLAP evolution, and the external line prescription for the hard scattering differs from the LSZ prescription. The prospects for extending the results to QCD are discussed.