Search for sub-millicharged particles at J-PARC

Abstract

We studied the feasibility of an experiment searching for sub-millicharged particles (chi s) 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 = 10(22) which corresponds to running the experiment for three years, the experiment provides sensitivity to chi s with the charge down to 5 x 10(-5) in m(chi)< 0.2 GeV/c(2) and 8 x 10(-4) in m(chi)< 1.6 GeV/c(2). This is the regime largely uncovered by previous experiments. We also explored a few detector designs to achieve optimal sensitivity to chi s. The photoelectron yield is the main driver, but the sensitivity does not have a strong dependence on detector configuration in the sub-millicharge regime.