The quenching of orthopositronium by helium

Measurements of the quenching rate of orthopositronium in helium determine the parameter ¹Z_eff, the effective number of electrons per atom in a singlet state relative to the positron. This number may be calculated from the wave function for the orthopositronium-helium atom system. In this paper, this wave function is computed in the static-exchange approximation and in an approximation that allows for the long-range van der Waal's forces acting between the neutral atoms; the calculated values of ¹Z_eff at thermal energies are ¹Z_eff = 0.037 and ¹Z_eff = 0.048 respectively. These results are considerably smaller than the most recent measured values ¹Z_eff = 0.1-0.25 and the possible reasons for the discrepancy are discussed.