  The T2K experiment is one of the most powerful long-baseline experiments to
investigate neutrino oscillations. The off-axis near detector called ND280 is
installed 280 m downstream from the neutrino production target to measure the
neutrino energy spectrum. In this paper, we study the capability of the ND280
detector to search for the dark photon produced through the meson rare decay
and proton bremsstrahlung processes at the proton beam dump. We find that the
ten-year operation of T2K with the ND280 detector excludes the unexplored
parameter region for the dark photon mass and kinetic mixing. We also show that
a broader parameter region can be searched by the ND280 in the future T2K
operation for dark photon as well as U(1)$_{B-L}$ gauge boson.

High Energy Physics - Phenomenology;High Energy Physics - Experiment