Monte Carlo and large angle gluon radiation

TL;DR

The paper exposes a fundamental tension in dipole-based Monte Carlo schemes: while dipole radiation captures large-angle soft-gluon effects, naively incorporating recoil in the dipole multiplication framework disrupts collinear factorization and fails to reproduce DGLAP evolution. By building a generating functional for soft emissions and deriving recoil-free evolution, the authors quantify subleading large-angle corrections and demonstrate that recoil strategies consistent with Catani–Seymour-like kinematics lead to incorrect collinear resummation when energy ordering is used. The work shows that naive recoil implementations spoil the correct energy-momentum conservation required for DGLAP, suggesting that recoil in dipole-based MC must be designed with care to preserve collinear factorization. Overall, the study highlights limitations of dipole-only schemes for full MC event generation and underscores the need for recoil prescriptions that respect both soft large-angle coherence and strict collinear resummation. This has implications for the development of more accurate parton-shower algorithms, particularly for inter-jet observables and global event properties.

Abstract

We discuss the problem of incorporating recoil effects into the probabilistic QCD evolution scheme based on the picture of colour dipoles as done in recent Monte Carlo programs. Such a scheme correctly describes subleading soft contributions to multiplicity distributions. However we find that a simple receipt for incorporating recoil effects into the dipole multiplication picture conflicts the collinear factorization and does not lead to the correct DGLAP equation.