Polarised cross sections for vector boson production with SHERPA

TL;DR

This paper introduces an extension to the SHERPA Monte-Carlo generator that enables simulation of polarised vector boson production, allowing detailed studies of electroweak processes and potential new physics with polarisation effects included.

Contribution

The work extends SHERPA to simulate polarised cross sections for vector bosons, including interference effects and approximate NLO QCD corrections, enabling more precise phenomenological analyses.

Findings

Polarised cross sections can be simulated for multiple bosons in a single run.

Interference between different polarisation states can be directly predicted.

Approximate NLO QCD corrections show small deviations from full predictions.

Abstract

Measurements of vector boson polarisation in vector boson production processes offer a powerful probe of the electroweak symmetry breaking mechanism, scrutinising the Standard Model and new physics scenarios alike. Since massive vector bosons can only be observed as intermediate particles, polarised cross section templates from simulation are necessary to extract their polarisation from measurable unpolarised distributions. In this work we present an extension of the SHERPA Monte-Carlo event generator allowing the simulation of polarised cross sections for vector boson production processes. Based on the narrow-width approximation, polarised cross sections of all possible polarisation combinations for an arbitrary number of intermediate vector bosons can be simulated in a single simulation run. In addition, it is possible to directly predict the interference between different intermediate polarisation states, and various differing polarisation definitions can be studied simultaneously. Besides the simulation of polarised cross sections at fixed LO and LO+PS accuracy as well as in multijet merged calculations, we also present parton-shower-matched polarised cross sections with approximate NLO QCD corrections in the vector boson production processes. We demonstrate that the differences of this approximation to full NLO QCD predictions are small and it thus opens up the possibility for fully-simulated calculations at the hadron level including polarisation information and higher-order QCD effects for the first time.