Decaying Vector Dark Matter as an Explanation for the 3.5 keV Line from Galaxy Clusters

TL;DR

This paper proposes a vector dark matter model with two vector bosons, explaining the 3.5 keV X-ray line from galaxy clusters through their decay, and details its production mechanism and underlying high-energy theory.

Contribution

It introduces a novel vector dark matter model with a specific decay process and a high-energy origin for the effective coupling, connecting astrophysical observations with particle physics.

Findings

The model explains the 3.5 keV line via vector boson decay.

Dark matter production occurs through freeze-in in the early universe.

A high-energy gauge theory with milli-charged fermions generates the effective coupling.

Abstract

We present a Vector Dark Matter (VDM) model that explains the 3.5 keV line recently observed in the XMM-Newton observatory data from galaxy clusters. In this model, dark matter is composed of two vector bosons, $V$ and $V^\prime$, which couple to the photon through an effective generalized Chern-Simons coupling, $g_V$. $V^\prime$ is slightly heavier than $V$ with a mass splitting $m_{V^\prime}-m_V\simeq 3.5$~keV. The decay of $V^\prime$ to $V$ and a photon gives rise to the 3.5~keV line. The production of $V$ and $V^\prime$ takes place in the early universe within the freeze-in framework through the effective $g_V$ coupling when $m_{V^\prime}<T<\Lambda $, $\Lambda$ being the cut-off above which the effective $g_V$ coupling is not valid. We introduce a high energy model that gives rise to the $g_V$ coupling at low energies. To do this, $V$ and $V^\prime$ are promoted to gauge bosons of spontaneously broken new $U(1)_V$ and $U(1)_{V^\prime}$ gauge symmetries, respectively. The high energy sector includes milli-charged chiral fermions that lead to the $g_V$ coupling at low energy via triangle diagrams.