Measurement of the Initial Population and Decay Rate of the(μHe4)2S+System in a Helium Target at 50 atm

We have measured the fraction and lifetime of the ${{\left(\mu {}_{}{}^4{}_{}{}^{}\mathrm{He}\right)}_{2S}}^{+}$ systems formed by stopping negative muons in gaseous helium at 50 atm. An estimate of the external Auger effect, which contributes to depopulate the ${{\left(\mu \mathrm{He}\right)}_{2S}}^{+}$ level, is given. First experimental evidence of the two-photon decay of such a level is presented.