Constraints on light dark matter and $U$ bosons, from $\psi$, ${\rm Y}$, $K^{+}$, $\pi °$, $\eta$ and ${\eta }^{\prime }$ decays

Following searches for photinos and very light gravitinos in invisible decays of $\psi$ and ${\rm Y}$, we discuss new limits on Light Dark Matter and $U$ bosons, from $\psi$ and ${\rm Y}$ decays, as well as rare decays of $K^{+}$ and invisible decays of $\pi °$, $\eta$ and ${\eta }^{\prime }\dots$. 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 $\psi \to \gamma U$, ${\rm Y}\to \gamma U$ and $K^{+}\to {\pi }^{+}U$ 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).