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

Abstract

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 magnetic field strength. The calculation is based on the cross sections for the atomic processes contributing to the slowing down of the positive muon and muonium , obtained from the corresponding cross sections for the proton and hydrogen by simple scaling 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 and 1 atm pressure, an amount which is unacceptably large for accurate (100 ppm or better) measurements of the muon decay parameters. It is also pointed out that at this level of accuracy a strong longitudinal magnetic field is a much less effective means of quenching the muon spin depolarization mechanism than previously thought. Effects of the electron spin polarization induced by a strong longitudinal magnetic field are also discussed.