Collisional effects on HFS transitions of antiprotonic helium
Abstract Collisions of metastable antiprotonic helium with medium atoms induce transitions between hyperfine structure sublevels as well as shift and broadening of the microwave M1 spectral lines. We consider these effects in the framework of a simple model with scalar and tensor interactions between ( p 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 ≲10 K. The calculated spin-flip cross-sections are less than elastic ones by four orders of value. At the density N =3×10 20 cm −3 and T =6 K we obtain the relaxation times τ ( FJ → F ′ J ′ )≳160 ns, the frequency shifts of M1 spectral lines Δ ν ≲66 kHz for the favored transitions (Δ F =±1,Δ J =±1) and frequency broadening of the M1 spectral lines γ /2≲5.8 MHz. The results are compatible with the recent experimental data obtained by a laser-microwave-laser resonance method.