The 3.5 keV X-ray line signature from annihilating and decaying dark matter in Weinberg model

TL;DR

This paper explains the 3.55 keV X-ray line as a signature of annihilating or decaying dark matter within Weinberg's model, proposing scenarios for dark matter production consistent with observed relic abundance and astrophysical constraints.

Contribution

It introduces a viable dark matter model within Weinberg's framework that accounts for the X-ray line via annihilation or decay, highlighting the dominant production mechanism and consistency with cosmological bounds.

Findings

The X-ray line can be explained by p-wave annihilation of dark matter into photons.

Only the thermal decoupling scenario matches both the X-ray signal and relic abundance.

Dark matter production via Higgs decay (freeze-in) is the dominant non-thermal process.

Abstract

Recently two groups independently observed unidentified X-ray line signal at the energy 3.55 keV from the galaxy clusters and Andromeda galaxy. We show that this anomalous signal can be explained in annihilating dark matter model, for example, fermionic dark matter model in hidden sector with global $U(1)_X$ symmetry proposed by Weinberg. There are two scenarios for the production of the annihilating dark matters. In the first scenario the dark matters with mass 3.55 keV decouple from the interaction with Goldstone bosons and go out of thermal equilibrium at high temperature ($>$ 1 TeV) when they are still relativistic, their number density per comoving volume being essentially fixed to be the current value. The correct relic abundance of this warm dark matter is obtained by assuming that about ${\cal O}(10^3)$ relativistic degrees of freedom were present at the decoupling temperature or alternatively large entropy production occurred at high temperature. In the other scenario, the dark matters were absent at high temperature, and as the universe cools down, the SM particles annihilate or decay to produce the dark matters non-thermally as in `freeze-in' scenario. It turns out that the DM production from Higgs decay is the dominant one. In the model we considered, only the first scenario can explain both X-ray signal and relic abundance. The X-ray signal arises through $p$-wave annihilation of dark matter pair into two photons through the scalar resonance without violating the constraints from big bang nucleosynthesis, cosmic microwave background, and astrophysical objects such as red giants or white dwarfs. We also discuss the possibility that the signal may result from a decaying dark matter in a simple extension of Weinberg model.