Muon spin depolarization in noble gases during slowing down in a longitudinal magnetic field

The spin depolarization of the positive muon during slowing down in He, Ne and Ar has been calculated as a function of gas pressure and longitudinal m agnetic field strength. The calculation is based on the cross sections for the atomic processes contributing to the slowing down of the positive muon (mu(+)) and muonium (Mu = mu(+)e(-)), obtained from the corresponding cross sections for the proton (p(+)) and hydrogen (H = p(+)e(-)) by simple scali ng schemes. The depolarization mechanism taking place in the subpicosecond time regime is discussed in the context of recent experiments to determine the muon decay parameters precisely and accurately in the search for right- handed neutrinos. It has been shown that the polarization loss in He is 1% at B = 4 T and 1 atm pressure, an amount which is unacceptably large for ac curate (100 ppm or better) measurements of the muon decay parameters. It is also pointed out that at this level of accuracy a strong longitudinal magn etic field is a much less effective means of quenching the muon spin depola rization mechanism than previously thought. Effects of the electron spin po larization induced by a strong longitudinal magnetic field are also discuss ed.