Neutrino-Antineutrino Conversion from Ultralight Vector Dark Matter

We show that Majorana neutrinos convert into antineutrinos in a background of ultralight vector dark matter coupled to lepton number, such as the gauge boson of $\text{U}(1{)}_{B-L}$ or $\text{U}(1{)}_{L_i-L_j}$ with $i,j=e,\mu ,\tau$. This effect is suppressed by the small neutrino mass, but the enhancement by long astrophysical baselines can enable future searches for solar and supernova neutrinos to explore uncharted parameter space. For instance, for $\text{U}(1{)}_{B-L}$ dark matter, the observation of a supernova neutrino burst at DUNE, Hyper-Kamiokande, and JUNO could probe gauge couplings as small as $e^{\mathrm{\prime }}\sim {10}^{-32}-{10}^{-25}$ for dark matter masses of $m_{A^{\mathrm{\prime }}}\sim {10}^{-22}\text{ eV}-{10}^{-14}\text{ eV}$, beyond the capability of other future probes.