  Metastable ${2S}$ muonic-hydrogen atoms undergo collisional ${2S}$-quenching,
with rates which depend strongly on whether the $\mu p$ kinetic energy is above
or below the ${2S}\to {2P}$ energy threshold. Above threshold, collisional
${2S} \to {2P}$ excitation followed by fast radiative ${2P} \to {1S}$
deexcitation is allowed. The corresponding short-lived $\mu p ({2S})$ component
was measured at 0.6 hPa $\mathrm{H}_2$ room temperature gas pressure, with
lifetime $\tau_{2S}^\mathrm{short} = 165 ^{+38}_{-29}$ ns (i.e.,
$\lambda_{2S}^\mathrm{quench} = 7.9 ^{+1.8}_{-1.6} \times 10^{12}
\mathrm{s}^{-1}$ at liquid-hydrogen density) and population
$\epsilon_{2S}^\mathrm{short} = 1.70^{+0.80}_{-0.56}$ % (per $\mu p$ atom). In
addition, a value of the $\mu p$ cascade time, $T_\mathrm{cas}^{\mu p} =
(37\pm5)$ ns, was found.
