Narrow-width approximation accuracy

Understand

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.