S
-wave elastic scattering of 
o
-Ps from 
H2
 at low energy
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)).
Four-body Calculation of Inelastic Scattering Cross Sections of Positronium–Antihydrogen Collision
We overview our recent calculations of scattering cross sections of positronium–antihydrogen collisions. We discuss the cross sections calculated in the vicinity of the four-body resonances of H¯Ps\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{\mathrm {H}}\mathrm {Ps}$$\end{document} and above the energy threshold for the formation of the antihydrogen positive ions (H¯+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{\mathrm {H}}^+$$\end{document}). In the former energy region where a Rydberg series of resonances dominated by the H¯+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{\mathrm {H}}^+$$\end{document}-e-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {e}^-$$\end{document} interaction appears, we show a good agreement between the resonant profiles in the cross sections and the resonance energies calculated by the complex coordinate rotation method. H¯+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{\mathrm {H}}^+$$\end{document} production cross sections near the threshold energy are reported for the positronium being initially in the (n, l) states with n≤3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$n\le 3$$\end{document}, together with all competing elastic/inelastic cross sections. Comparison of our results with the latest continuum distorted wave final state calculation is presented. A qualitative discrepancy concerning the dependence of cross sections on the angular momentum of Ps is indicated.
Creating the Virtual Universe
Simulation software is important to our understanding of the universe. The intrinsic multiphysics aspects are spiced with a range of temporal scales and spatial scales, both of which cover more digits than are available in the standard hardware. This, together with the intrinsic chaotic nature of many physical processes, poses quite a challenge. To meet this challenge, researchers developed the Astronomical Multipurpose Environment (AMUSE). Instead of writing a suite of multiphysics solvers from scratch, AMUSE's developers coupled existing solvers for each physical ingredient. The result is a highly inhomogeneous collection of dedicated solvers with a homogeneous protocol that scales to supercomputers.