Letter of Intent: Search for sub-millicharged particles at J-PARC

Abstract

We propose a new experiment sensitive to the detection of millicharged particles produced at the $30$ GeV proton fixed-target collisions at J-PARC. The potential site for the experiment is B2 of the Neutrino Monitor building, $280$ m away from the target. With ${\text{N}}_{\text{POT}}=10^{22}$, the experiment can provide sensitivity to particles with electric charge $3\times 10^{-4}e$ for mass less than $0.2$ $\text{GeV}/{\text{c}}^2$ and $1.5\times 10^{-3}e$ for mass less than $1.6$ $\text{GeV}/{\text{c}}^2$. This brings a substantial extension to the current constraints on the charge and the mass of such particles.