Search for sub-millicharged particles at J-PARC

We studied the feasibility of an experiment searching for sub-millicharged particles (χs) using 30 GeV proton fixed-target collisions at J-PARC. The detector is composed of two layers of stacked scintillator bars and PMTs and is proposed to be installed 280 m from the target. The main background is a random coincidence between two layers due to dark counts in PMTs, which can be reduced to a negligible level using the timing of the proton beam. With N_POT = 10²² which corresponds to running the experiment for three years, the experiment provides sensitivity to χs with the charge down to 5 × 10⁻⁵ in m_χ< 0.2 GeV/c² and 8 × 10⁻⁴ in m_χ< 1.6 GeV/c². This is the regime largely uncovered by previous experiments. We also explored a few detector designs to achieve optimal sensitivity to χs. The photoelectron yield is the main driver, but the sensitivity does not have a strong dependence on detector configuration in the sub-millicharge regime.