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 ($A^{\prime }$) produced in nuclear reactors through nuclear de-excitation following neutron capture $N^{\ast }\to N{A}^{\prime }$. Compared with the conventional Compton-like production process $\gamma e^{-}\to A^{\prime }{e}^{-}$, 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 $ϵ$ for dark photon masses in the range $0.1\mathrm{MeV}<m_{A^{\prime }}<6.9\mathrm{MeV}$. 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.

Comments: Version 2, references added and updated limits in 4-7 MeV, thanks Maxim Pospelov for pointing out the B10(n,gamma)B11 branching issue