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

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 ((p) over bar He+)(nL) 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 less than or similar to 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 (Delta J = +/- 1, Delta F = 0, +/- 1) and by seven orders of value for the double spin-flip transitions (Delta J = 0, 2, Delta F = 1). The cross sections show a resonance behaviour at very low energy (E similar to 1-4 K). The results for the relaxation rate lambda(F- -> F+), the frequency shift and broadening of the M1 spectral lines for the favoured transitions (Delta F = +/- 1, Delta J = +/- 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.