S-wave elastic scattering of $o$-Ps from ${\text{H}}_2$ at low energy

Abstract

The confined variational method is applied to investigate the low-energy elastic scattering of ortho-positronium from ${\text{H}}_2$ by first-principles quantum mechanics. Describing the correlation effect with explicitly correlated Gaussians, we obtain accurate $S$-wave phase shifts and pick-off annihilation parameters for different incident momenta. By a least-squares fit of the data to the effective-range theory, we determine the $S$-wave scattering length, $A_s=2.06a_0$, and the zero-energy value of the pick-off annihilation parameter, ${}^1{\text{Z}}_{\text{eff}}=0.1858$. The obtained ${}^1{\text{Z}}_{\text{eff}}$ agrees well with the precise experimental value of $0.186(1)$ (J.\ Phys.\ B \textbf{16}, 4065 (1983)) and the obtained $A_s$ agrees well with the value of $2.1(2)a_0$ estimated from the average experimental momentum-transfer cross section for Ps energy below 0.3 eV (J.\ Phys.\ B \textbf{36}, 4191 (2003)).

5 pages