Searching for the dark sector in two-body anti-muon decay with the polarization of monochromatic positrons

TL;DR

This paper proposes a method to differentiate dark sector bosons in anti-muon decay by analyzing the polarization of emitted positrons, enhancing the ability to identify the nature of potential dark particles.

Contribution

It introduces a polarization-based technique to distinguish between different dark sector portals in anti-muon decay experiments, with high confidence levels.

Findings

Polarization analysis can differentiate dark photon from axion-like particles with few events.

Massive spin-1 dark portals are indistinguishable from axion-like cases using this method.

The approach can potentially identify a massive spin-2 dark boson.

Abstract

The $\mu^+ \to e^+ X$ decay, where $X$ is a dark sector boson, provides one of the strongest available bounds on the scale of dark sector interactions. The $X$ boson can be an axion or a dark photon. We show that the concurrent determination of the anti-muon and positron polarizations makes possible to distinguish with a confidence level of 99% between the two dark sector portals with as few as 6 observed events in the case of the massless dark photon. Instead, the massive spin-1, dimension 4 dark portal cannot be distinguished from the axion-like case. We also discuss the possibility that the $X$ boson be a massive spin-2 particle.