  We revisit calculations of invisible widths of heavy mesons in the standard
model, which serve as benchmarks for the studies of production of light,
long-lived neutral particles in heavy meson decays. We challenge the common
assumption that in the standard model these widths are dominated by meson
decays into a two-neutrino final state and prove that they are dominated by
decays into four-neutrino final states. We show that current estimates of the
invisible widths of heavy mesons in the standard model underestimate the effect
by orders of magnitude. We examine currently available experimental data on
invisible widths and place constraints on the properties of dark photons. We
also comment on the invisible widths of the kaons.
