  Solar nuclear reactions can occasionally produce sub-MeV elusive beyond the
Standard Model particles that escape the solar interior without further
interactions. This study focuses on massive spin-one particles. We construct
the general theoretical framework and identify two crucial mixing sources
involving the photon, which facilitate communication between the hidden and
visible sectors: kinetic mixing with the photon, and plasma-induced mixing due
to thermal electron loops. For both cases, we focus on the second stage of the
solar proton-proton chain and evaluate the fluxes of monochromatic 5.49~MeV
hidden vectors produced by the $p(d, ^3{\rm He})\gamma^\prime$ nuclear
reaction. We then investigate their terrestrial detection via Compton-like
scatterings. The incoming fluxes are polarized, and we evaluate the cross
sections for Compton-like scatterings for transverse and longitudinal vectors.
Finally, we apply this framework to a concrete case by investigating the
sensitivity of the forthcoming Jiangmen Underground Neutrino Observatory (JUNO)
experiment and identifying parameter space where current terrestrial bounds
will be improved.
