Magnetic sublevel population in 1s-2p excitation of helium by fast electrons and protons

We report experimental and theoretical results for the magnetic sublevel population of the helium atom in collisions with fast (vi=3-9 au) electrons and protons. Cross sections for excitation of magnetic sublevels with M=0 and ±1 have been obtained using polarization measurements of emitted radiation in combination with differential cross sections. Calculations have been carried out using the expansion of the transition amplitude in the Born series over the projectile-target interaction through the second order. Results of calculations are in agreement with experimental data. We find that the particle-antiparticle Z± difference exceeds the statistical error of measurement up to collision velocities vi∼6 au for excitation of sublevels with M=0. (author)