Speaker
Description
The SUB-Millicharge ExperimenT (SUBMET) is designed to search for sub-millicharged particles produced in proton fixed-target collisions at J-PARC. The detector, positioned 280 meters downstream of the target, consists of two layers of stacked scintillator bars coupled to photomultiplier tubes (PMTs). The dominant background arises from random coincidences between the two detector layers, primarily due to PMT dark counts and environmental radiation. This background can be significantly mitigated using the precise timing structure of the proton beam. With a projected exposure of $N_{POT}=5\times10^{21}$, SUBMET aims to probe charges as low as $8\times 10^{−5}e$ for $m_\chi<0.2~\rm{GeV}/\rm{c}^2$, and down to $10^{−3}e$ for $m_\chi<1.6 ~\rm{GeV}/\rm{c}^2$—a parameter space largely inaccessible to previous experiments. The detector was installed in the spring of 2024, and beam data were collected in June and December of the same year. This presentation will provide an overview of the detector operation, studies conducted to characterize the detector response and background sources, and the prospects of the experiment.
