Search for long-lived S-2 muonic hydrogen in H-2 gas

We have searched for long-lived 2S muonic hydrogen (μ−p) in H2 gas at 0.8 atm. The state could be detected by observing delayed 1.9-keV 2P→1S photons from collisionally quenched μ−p(2S). The observed upper limit (90% confidence level) for the fraction,f2S, of μ−p(2S) with lifetime τ2S in the gas in the range, 100<τ2S<1000 ns, is f2S<2%. Our results are consistent with the view that most of the μ−p(2S) have kinetic energies greater than the 0.3-eV threshold for the inelastic collision μ−p(2S)+H2→μ−p(2P)+H2 and hence are rapidly quenched. We have also measured the relative Lyman line intensities in muonic hydrogen and muonic helium at 0.8 atm with solid-state detectors [Ge for μ−He4 and Si(Li) for μ−p]. The results are (KαKtotal)μ−p=0.46±0.06, KαKtotal)μ−He4=0.76±0.04, (KβKtotal)μ−He4=0.04±0.02. These results imply that about 7.5% of μ−p atoms cascade to the 2S state.