New solar X-ray constraints on keV Axion-Like Particles

TL;DR

This paper uses solar X-ray observations to set new constraints on axion-like particles, improving existing limits by considering photon coalescence production and absorption effects within the Sun.

Contribution

It introduces a novel approach by considering photon coalescence for ALP production and analyzes absorption effects, leading to more stringent solar X-ray constraints on ALPs.

Findings

Improved constraints on ALP parameters by an order of magnitude.

Identification of two regimes based on ALP-electron coupling $g_{ae}$.

Constraints are primarily governed by photon coupling $g_{a\gamma\gamma}$ outside the transitional region.

Abstract

The decay of Axion-Like Particles (ALPs) trapped in the solar gravitational field would contribute to the observed solar X-ray flux, hence constraining ALP models. We improve by one order of magnitude the existing limits in the parameter space $(g_{a\gamma\gamma}, m)$ by considering ALPs production via photon coalescence. For $g_{ae} \neq 0$, we demonstrate that trapped ALPs can be Compton-absorbed while crossing the Sun, resulting in two regimes in the exclusion limits, with a transition triggered by $g_{ae}$. Out of the transitional region, the solar X-ray constraints on ALPs are exclusively governed by $g_{a\gamma\gamma}$.