Long-lived $2s$ state of muonic hydrogen: population and lifetime
  Ab initio study of the density-dependent population and lifetime of the long-lived $(\mu p)_{2s}$ and the yield of $(\mu p)_{1s}$ atoms with kinetic energy 0.9 keV have been performed for the first time. The direct Coulomb $2s\to 1s$ deexcitation is proved to be the dominant quenching mechanism of the $2s$ state at kinetic energy below $2p$ threshold and explain the lifetime of the metastable $2s$ state and high-energy 0.9 keV component of $(\mu p)_{1S}$ observed at low densities. The cross sections of the elastic, Stark and Coulomb deexcitation processes have been calculated in the close-coupling approach taking into account for the first time both the closed channels and the threshold effects due to vacuum polarization shifts of the $ns$ states. The cross sections are used as the input data in the detailed study of the atomic cascade kinetics. The theoretical predictions are compared with the known experimental data at low densities. The 40% yield of the 0.9 keV$(\mu p)_{1s}$ atoms is predicted for liquid hydrogen density. 
V.P. Popov and V.N. Pomerantsev