Narrow-width approximation accuracy

TL;DR

This paper analyzes the accuracy of the narrow-width approximation (NWA) in particle physics, proving its validity for total rates and exploring off-shell corrections, with implications for MSSM scenarios and methods to enhance NWA precision.

Contribution

It provides a general proof of NWA accuracy for total rates with on-shell intermediate states and investigates off-shell effects and improvements in the approximation.

Findings

Decorrelated effects vanish for total rates of resonant processes.

Off-shell corrections can be comparable to QCD corrections.

Process-independent methods to improve NWA are explored.

Abstract

A study of general properties of the narrow-width approximation (NWA) with polarization/spin decorrelation is presented. We prove for total rates of arbitrary resonant decay or scattering processes with an on-shell intermediate state decaying via a cubic or quartic vertex that decorrelation effects vanish and the NWA is of order Gamma. Its accuracy is then determined numerically for all resonant 3-body decays involving scalars, spin-1/2 fermions or vector bosons. We specialize the general results to MSSM benchmark scenarios. Significant off-shell corrections can occur - similar in size to QCD corrections. We qualify the configurations in which a combined consideration is advisable. For this purpose, we also investigate process-independent methods to improve the NWA.