Circular polarisation of gamma rays as a probe of dark matter interactions with cosmic ray electrons

TL;DR

This paper explores how circular polarisation of gamma rays from cosmic ray electrons scattering off dark matter could serve as a novel probe for dark matter interactions, highlighting potential signals and detection challenges.

Contribution

It introduces the concept of using circular polarisation of gamma rays as a new method to detect and characterize dark matter interactions involving parity violation.

Findings

Circular polarisation can reach up to 90% in the model.

A distinctive peak in the photon flux spectrum is predicted.

Detection is challenging but possible with future experiments like e-ASTROGAM.

Abstract

Conventional indirect dark matter (DM) searches look for an excess in the electromagnetic emission from the sky that cannot be attributed to known astrophysical sources. Here, we argue that the photon polarisation is an important feature to understand new physics interactions and can be exploited to improve our sensitivity to DM. In particular, circular polarisation can be generated from Beyond the Standard Model interactions if they violate parity and there is an asymmetry in the number of particles which participate in the interaction. In this work, we consider a simplified model for fermionic (Majorana) DM and study the circularly polarised gamma rays below 10 GeV from the scattering of cosmic ray electrons on DM. We calculate the differential flux of positive and negative polarised photons from the Galactic Center and show that the degree of circular polarization can reach up to 90%. Finally, once collider and DM constraints have been taken into account, we estimate the required sensitivity from future experiments to detect this signal finding that, although a distinctive peak will be present in the photon flux spectrum, a near future observation is unlikely. However, different sources or models not considered in this work could provide higher intensity fluxes, leading to a possible detection by e-ASTROGAM. In the event of a discovery, we argue that the polarisation fraction is a valuable characterisation feature of the new sector.