Observation of long-lived muonic hydrogen in the 2S state.

The kinetic energy distribution of ground state muonic hydrogen atoms mup(1S) is determined from time-of-flight spectra measured at 4, 16, and 64 hPa H2 room-temperature gas. A 0.9 keV component is discovered and attributed to radiationless deexcitation of long-lived mu p(2S) atoms in collisions with H2 molecules.