Observation of electronic excitation by extremely slow protons with applications to the detection of supermassive charged particles

The sensitivity of scintillators to grand unification monopoles and other supermassive charged particles is inferred from measurements of scintillation by recoil protons with velocities as low as 2.5×${10}^{\mathrm{-}4}$c. No evidence for a response cutoff is observed at velocities extending well below the electronic-excitation threshold of 6×${10}^{\mathrm{-}4}$c expected from two-body kinematics. The relevance of these measurements to searches for supermassive relic charged particles is discussed.