Decaying vector dark matter as an explanation for the 3.5 keV line from galaxy clusters

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', which couple to the photon through an effective generalized Chern-Simons coupling, g_V. V' is slightly heavier than V with a mass splitting m_V' – m_V ≃ 3.5 keV. The decay of V' to V and a photon gives rise to the 3.5 keV line. The production of V and V' takes place in the early universe within the freeze-in framework through the effective g_V coupling when m_V' < T < Λ, Λ 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' are promoted to gauge bosons of spontaneously broken new U(1)_V and U(1)_V' 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.