We focus on a novel baryon number ($\mathcal{B}$)-violating process within neutron stars, where two neutrons convert into two dark photons ($nn\to VV$) 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 $nn\to VV$ process could give rise to nontrivial 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 redshifted 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 provide a powerful tool to constrain the parameter space of new-physics models. The study of the $nn\to VV$ process, which combines astronomical observations and particle physics models, may open new windows to the detection of the $\mathcal{B}$-violating effects and provide new insights on the study of dark matter.

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• Effective field theory
• Extensions of scalar sector
• Particle astrophysics
• Particle dark matter
• Particle decays
• Phenomenology