Probing millicharged particles with NA64 experiment at CERN

Abstract: In this note we estimate the sensitivity of the NA64 experiment to millicharged particles ($\chi$). That experimental facility is dedicated to the searching for dark sector particles in missing energy events at the CERN SPS. We consider missing momentum signatures in the $\simeq$ 100 GeV electron and muon beams and show that the later one allows to obtain more stringent bounds on the millicharge $Q_{\chi}$, which for the $\chi$ masses $100$~MeV $\leq m_{\chi} \leq 500$~MeV at the level $ Q_{\chi}/e\lesssim O(10^{-3}) - O(10^{-2})$.