Dark photons from dineutron decays in neutron stars

We focus on a novel baryon-number ($\mathcal{B}$) violating process within neutron stars, where two neutrons convert into two dark photons ($nn \rightarrow VV$) via new Higgs-like scalar bosons. This process is believed to be greatly suppressed at low energies but could be highly amplified in a dense neutron environment like neutron stars. The $nn \rightarrow VV$ process could give rise to non-trivial effects that are distinct from similar processes in previous studies and could alter the properties of neutron stars, such as orbital periods, collapse thresholds, stability conditions, cooling rates, gravitational wave emissions, etc. The emitted dark photons may serve as dark-matter candidates and exhibit special red-shifted energy spectra mainly linked to the compactness of the neutron star. We point out that the dark photons emitted from neutron stars may yield detectable signals in future experiments. We also show that the precision pulsar-timing data provides a powerful tool to constrain the parameter space of new-physics models. The study of the $nn \rightarrow VV$ process, which combines astronomical observations and particle physics models together, may open new windows into the detection of the $\mathcal{B}$-violating effects and may also provide new insights on the study of dark matter.