Decay of the Mediator Particle at Threshold

TL;DR

This paper investigates the behavior of a light vector mediator particle near threshold regions, revealing that threshold singularities can significantly alter its decay properties and lifetime, challenging standard perturbative predictions.

Contribution

The authors develop a new method to quantitatively describe threshold singularities for mediator particles, improving upon traditional perturbative approaches.

Findings

Threshold singularities affect mediator decay rates

The mediator lifetime can be significantly different from perturbative estimates

The method provides more accurate predictions near threshold regions

Abstract

A light mediator particle is often predicted in the dark sector scenario, which weakly interacts with the standard model (SM) particles. The weakness of the interaction is usually described by a small coupling; however, the small coupling does not always guarantee the weakness of the interaction. When the mass of the mediator particle lies in a threshold region, the so-called threshold singularity may emerge, and then the perturbative calculation fails. This singularity causes several effects, e.g., the mixing between the mediator particle and bound states, the Sommerfeld effect on the final state of the mediator particle decay, etc. Taking the minimal model of the vector mediator particle decaying mainly into the SM particles as an example, we develop a method to describe the singularity quantitatively. We also calculate some physical quantities using this method, such as the lifetime of the mediator particle, and find that those could be significantly altered compared with the result of the perturbative calculation.