How to measure the spin of invisible states in $e^+e^- \to \gamma + X$

TL;DR

This paper proposes methods to determine the spin and nature of invisible particles produced in electron-positron collisions by analyzing photon angular distributions, polarization, and cross sections, aiding searches for hidden photons and axions.

Contribution

It introduces measurement strategies based on angular and polarization analyses to identify the spin and type of invisible states in collider experiments.

Findings

Strategies can distinguish between hidden photons and axions.

Sensitivity estimates for Belle II to detect axions and hidden photons.

Analysis of Standard Model background impacts detection sensitivity.

Abstract

We examine the production of an invisible state $X$ together with a photon, $e^+e^- \to \gamma +X$, at electron positron colliders and present measurement strategies that can detect the spin of the invisible state as well as the underlying production mechanism, based on the angular distribution of the final state photon, the cross sections for polarized initial states and the photon polarization. Our measurement strategy can be used to identify whether the invisible state is a hidden photon or an axion. The results are compared with a detailed analysis of the Standard Model background and we calculate the sensitivity reach for searches for axions and hidden photons at Belle II.