Coupled-channel analysis of collisional effects on HFS transitions in antiprotonic helium atoms

Abstract

Collisions of metastable antiprotonic helium with atoms of a medium induce transitions between hyperfine structure sublevels as well as shifts and broadenings of the microwave M1 spectral lines. We consider these phenomena in the framework of a model with scalar and tensor interactions between and He atoms. S-matrix is obtained by solving coupled-channels equations involving 4 HFS sublevels (F = L ± 1/2, J = F ± 1/2) of the nL level and relative angular momenta up to l = 5 at the kinetic energy E ≲ 25 K. The inelastic cross sections are less than elastic ones by four orders of value for the single electron or antiproton spin-flip transitions (ΔJ = ±1, ΔF = 0, ± 1) and by seven orders of value for the double spin-flip transitions (ΔJ = 0, ± 2, ΔF = ±1). The cross sections show a resonance behaviour at very low energy (E ∼ 1–4 K). The results for the relaxation rate λ(F− → F+), the frequency shift and broadening of the M1 spectral lines for the favoured transitions (ΔF = ±1, ΔJ = ±1) are compatible with the recent experimental data obtained by a triple (laser–microwave–laser) resonance method at T = 6 K. The calculated values are lowered by a factor of 1.5–2 with the temperature rising up to 25 K.