Constraints on Light Dark Matter and U bosons, from psi, Upsilon, K+, pi0, eta and eta' decays

TL;DR

The paper investigates constraints on a light U boson and light dark matter (LDM) arising from invisible and radiative decays of ψ, Υ, K^+, π^0, η, and η'. It distinguishes axial and vector couplings of the U to quarks and dark matter, deriving strong bounds on axial couplings from radiative quarkonium decays and kaon decays, while vector couplings to quarks remain comparatively less restricted. It shows that, despite these constraints, a light U boson together with LDM remains compatible with particle physics limits and can accommodate the annihilation cross sections needed for the relic abundance and possible contemporary signals such as the 511 keV line, especially when electron couplings are favorable. The results emphasize how precision meson decays and parity-violation tests collectively shape the viable parameter space for light dark sectors, while highlighting the ongoing potential of missing-energy searches to probe these scenarios.

Abstract

Following searches for photinos and very light gravitinos in invisible decays of psi and Upsilon, we discuss new limits on Light Dark Matter and U bosons, from psi and Upsilon decays, as well as rare decays of K+ and invisible decays of pi0, eta and eta' ... . The new limits involving the vector couplings of the U to quarks turn out, not surprisingly, to be much less restrictive than existing ones on axial couplings, from an axionlike behavior of a light U boson, tested in psi --> gamma U, Upsilon --> gamma U and K+ --> pi+ U decays (or as compared to the limit from parity-violation in atomic physics, in the presence of an axial coupling to the electron). Altogether the hypothesis of light U bosons, and light dark matter particles, remains compatible with particle physics constraints, while allowing for the appropriate annihilation cross sections required, both at freeze-out (for the relic abundance) and nowadays (if e+ from LDM annihilations are at the origin of the 511 keV line from the galactic bulge).