Signatures of leptophilic t-channel dark matter from active galactic nuclei

TL;DR

This paper investigates indirect photon signatures of leptophilic dark matter from Centaurus A, deriving strong constraints, calculating photon fluxes and polarisation asymmetries, and fitting DM models to observed gamma-ray excesses.

Contribution

It introduces a model of Majorana fermion dark matter interacting with electrons, derives new constraints from Centaurus A, and explores polarisation signals and fits to gamma-ray excesses.

Findings

Constraints on annihilation cross section are 7 orders of magnitude stronger than Galactic Center measurements.

Photon flux from DM-electron scattering is below background, but polarisation asymmetry can reach 100%.

Best fit DM mass is 123 GeV with a coupling of 0.018.

Abstract

In this work, we study indirect photon signatures of leptophilic dark matter (DM) coming from Centaurus A, where a DM density spike is believed to have survived to date contrary to the case of our galaxy. We consider a model where DM is a Majorana fermion which interacts with right-handed electrons via a scalar mediator. Assuming that the photons measured from the core of Cen A are coming from SM processes, we derive constraints on the average annihilation cross section which are 7 orders of magnitude stronger than the ones from measurements of the Galactic Center. Focusing on the allowed parameter space range, we calculate the flux of photons coming from the radiative DM-electron scattering in the AGN jets and the circular polarisation asymmetry of these photons. We find that this flux is two orders of magnitude lower than the background but its circular polarisation asymmetry can reach values close to $100\%$, indicating the need to experimentally exploit the high fraction of circular polarisation in order to detect these interactions. Since the origin of the photons in the GeV-TeV range from Cen A is not completely clear and an exotic origin is compatible with the measurements as well, we also consider the scenario in which synchrotron radiation can only partially explain the photon flux and we fit the excess with signals coming from DM annihilation, finding a best fit for a DM candidate with a mass $m_{\tilde{\chi}}= 123$ GeV and a coupling $a_R = 0.018$.