Observation of Long-Lived Muonic Hydrogen in the2SState

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 ${\mathrm{H}}_2$ 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 ${\mathrm{H}}_2$ molecules. The analysis reveals a relative population of about 1%, and a pressure-dependent lifetime (e.g., ${30.4}_{-9.7}^{+21.4}\mathrm{ns}$ at 64 hPa) of the long-lived $\mu p(2S)$ population, equivalent to a $2S$ quench rate in $\mu p(2S)+{\mathrm{H}}_2$ collisions of ${4.4}_{-1.8}^{+2.1}\times {10}^{11}{\mathrm{s}}^{-1}$ at liquid-hydrogen density.