Phenomenological implications on a hidden sector from the festina lente bound

We apply the festina lente (FL) bound on a hidden sector with U(1) gauge symmetries. Since the FL bound puts a lower bound on masses of particles charged under the U(1) gauge symmetries, it is possible to constrain the hidden sector even with a tiny coupling to the Standard Model. In particular, we focus on the phenomenological implications of the FL bound on milli-charged particles, which naturally arise when kinetic mixing between the photon and the hidden photon is allowed. It turns out that the milli-charged particle with the mass M ≲ 5 meV is prohibited by the FL bound in the case of a single hidden U(1), insensitively of the value of small kinetic mixing. This bound is crucial when bosonic dark matter is taken into consideration in this framework: the fuzzy bosonic dark matter models requesting minuscule masses are ruled out by the FL bound if the longevity of dark matter is protected by the hidden gauge symmetry.