Probing Dark Photons from Nuclear De-excitation in Reactor Neutrino Experiment

Abstract

Reactor neutrino experiments serve as powerful probes of light new physics. We investigate MeV-scale visible dark photons ($A'$) produced in nuclear reactors through nuclear de-excitation following neutron capture $N^*\to N A'$. Compared with the conventional Compton-like production process $\gamma e^-\to A'e^-$, the nuclear de-excitation yields on-shell dark photons with masses up to the nuclear transition energy. Using data from the TEXONO CsI(Tl) detector, we derive the new constraints on the kinetic mixing parameter $\epsilon$ for dark photon masses in the range $0.1\,\mathrm{MeV} < m_{A'} < 6.9\,\mathrm{MeV}$. We find that nuclear de-excitation not only extends the mass reach of reactor searches to higher dark photon masses but also provides a stronger limit than the Compton-like production process.