Evaluation of Photos Monte Carlo ambiguities in case of four fermion final states

TL;DR

This paper evaluates the systematic ambiguities of Photos Monte Carlo in simulating bremsstrahlung corrections for four-fermion final states, comparing its results with fixed-order calculations to assess accuracy in challenging phase space regions.

Contribution

It provides a detailed validation of Photos Monte Carlo's accuracy in complex four-fermion final states, highlighting its reliability in regions with hard, non-collinear photons.

Findings

Differences between Photos and MadGraph are within a few percent in critical phase space regions.

Photos' predictions are consistent with fixed-order calculations in non-logarithmic regions.

The study supports Photos' use for precise simulations of bremsstrahlung in complex final states.

Abstract

With the increasing precision requirements and growing spectrum of applications of Monte Carlo simulations the evaluation of different components of such simulations and their systematic ambiguities become of utmost interest. In the following, we will address the question of systematic errors for Photos Monte Carlo for simulation of bremsstrahlung corrections in final states, which can not, in principle, be identified as a decay of resonances. It is possible, because the program features explicit and exact parametrization of phase space for multi-body plus multi-photon final states. The Photos emission kernel for some processes consist of complete matrix element, in the remaining cases appropriate approximation is used. Comparisons with results of simulations, from generators based on exact phase space and exact fixed order matrix elements, can be used. For the purpose of such validations Photos provides an option to restrict emissions to single photon only. In the current work we concentrate on final state bremsstrahlung in $q \bar{q}(e^+e^-) \to l^+l^- l^+l^- \gamma$ and $\gamma\gamma \to l^+l^- \gamma$ processes. The reference distributions used as a cross-check are obtained from the fixed-order MadGraph Monte Carlo simulations. For the purpose of validation we concentrate on those phase space regions where Photos is not expected to work on the basis of its design alone. These phase space regions of hard, non-collinear photons, do not contribute to large logarithmic terms. We find that in these phase space regions the differences between Photos and MadGraph results do not surpass a few percent and these regions, in turn, contribute about 10% to the observed process rates. This is encouraging in view of the possible ambiguities for precise calculation of realistic observables.