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

The kinetic energy distribution of ground state muonic hydrogen atoms mu p(1S) is determined from time-of-flight spectra measured at 4, 16, and 64 hPa H-2 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 H-2 molecules. The analysis reveals a relative population of about 1%, and a pressure-dependent lifetime (e.g., 30.4(-9.7)(+21.4) ns at 64 hPa) of the long-lived mu p(2S) population, equivalent to a 2S quench rate in mu p(2S) + H-2 collisions of 4.4(-1.8)(+2.1) x 10(11) s(-1) at liquid-hydrogen density.