Status of the SUB-Millicharge ExperimenT (SUBMET) at J-PARC

Electric charge quantization is a long-standing question in particle physics. While fractionally charged particles (millicharged particles hereafter) have typically been thought to preclude the possibility of Grand Unified Theories (GUTs), well-motivated dark-sector models have been proposed to predict the existence of millicharged particles while preserving the possibility for unification. Such models can contain a rich internal structure, providing candidate particles for dark matter. Several experiments have searched for millicharged particles ( \(\chi \) s), but in the parameter space of the charge (Q) and mass ( \(m_\chi \) ), the region of \(m_\chi >0.1~\textrm{GeV}/c^2\) and \(Q<10^{-3}e\) is largely unexplored. SUB-Millicharge ExperimenT (SUBMET) has been proposed to search for sub-millicharged particles using 30-GeV proton fixed-target collisions at J-PARC. The detector is composed of two layers of stacked scintillator bars and photo-multiplier tubes (PMTs) and is proposed to be installed 280 m distance from the target. The main background is expected to be a random coincidence between the two layers due to dark counts in PMTs, which can be reduced significantly using the timing of the proton beam. With \(N_\textrm{POT}=5\times 10^{21}\) , the experiment provides sensitivity to \(\chi \) s with the charge down to \(7\times 10^{-5}e\) in \(m_\chi < 0.2~\textrm{GeV}/c^2\) and \(10^{-3}e\) in \(m_\chi <1.6~\textrm{GeV}/c^2\) . This is the regime largely uncovered by the previous experiments. The status of this experiment is presented in this talk.