Bosonic super-WIMPs as keV-scale dark matter

TL;DR

This work investigates bosonic super-WIMPs with masses in the keV range as dark matter candidates, focusing on pseudoscalar, scalar, and vector realizations. It combines cosmological production, diffuse and galactic gamma-ray backgrounds from decays, stellar cooling limits, and direct-detection prospects via axioelectric- or V-like absorption to identify viable parameter regions. The analysis reveals that gamma-ray line searches impose stringent constraints on pseudoscalar models, while vector DM remains accessible through direct absorption signals, though both face complementary astrophysical bounds; annual modulation of the absorption signal is predicted to be negligible, around $O(10^{-5})$. These results underscore the potential of diversified direct-detection strategies to probe ultra-weakly coupled keV-scale dark matter, complementing traditional astrophysical constraints.

Abstract

We consider models of light super-weakly interacting cold dark matter, with O(10-100) keV mass, focusing on bosonic candidates such as pseudoscalars and vectors. We analyze the cosmological abundance, the gamma-background created by particle decays, the impact on stellar processes due to cooling, and the direct detection capabilities in order to identify classes of models that pass all the constraints. In certain models, variants of photoelectric (or axioelectric) absorption of dark matter in direct-detection experiments can provide a sensitivity to the superweak couplings to the Standard Model which is superior to all existing indirect constraints. In all models studied, the annual modulation of the direct-detection signal is at the currently unobservable level of O(10^{-5}).

Figures (3)

• Figure 1: Dominant decays to photons. (a) 2-photon decay of the pseudoscalar $a$, and (b) the 3-photon decay of the vector $V$.
• Figure 2: Fixing $C_\gamma$ in terms of $f_a$ and $m_a$ according to Case $C$, we plot the direct detection sensitivity to pseudoscalar DM arising from the axioelectric cross-section on Ge, assuming a fiducial sensitivity of the detector equivalent to a 1pb cross-section for a 100 GeV WIMP. We also show the constraints arising from the He-burning lifetime in HB stars, from SN cooling via a coupling to the neutron magnetic moment with $f_{aNN}=f_a$, and most significantly the monochromatic $\gamma$-background from decays in the Galaxy. The grey shaded region is excluded by the latter indirect constraints. The thick black line corresponds to the parameters required to reproduce the required dark matter abundance from thermal production with $f_{abb}=f_a$.
• Figure 3: We plot the direct detection sensitivity to vector DM from the V-electric cross-section on Ge, assuming a fiducial sensitivity of the detector equivalent to a 1pb cross-section for a 100 GeV WIMP. We also show the constraints from the He-burning lifetime in HB stars, and the $\gamma$-background from 3$\gamma$-decays in the Galaxy. The grey shaded region is again excluded by the indirect constraints, while the thick black line corresponds to the parameters required to reproduce the required dark matter abundance from thermal production.