Probing millicharged particles with NA64 experiment at CERN

In this paper 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 similar or equal to 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 <= m(chi) <= 500 MeV at the level Q(chi)/e less than or similar to O(10(-3)) - O(10(-2)).