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Probing millicharged particles with NA64 experiment at CERN

In this note we estimate the sensitivity of the NA64 experiment to millicharged particles ($χ$). 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 $Q_χ$, which for the $χ$ masses $100$~MeV $\leq m_χ \leq 500$~MeV at the level $ Q_χ/e\lesssim O(10^{-3}) - O(10^{-2})$.