# Low-Energy Scattering Properties of Ground-State and Excited-State Positronium Collisions

## TL;DR

This paper calculates low-energy scattering properties of positronium in ground and excited states using a four-body hyperspherical coordinate approach, providing detailed scattering lengths for different spin configurations relevant to experiments.

## Contribution

It introduces a four-body hyperspherical coordinate method to compute scattering lengths for various spin states of positronium collisions, including excited states.

## Key findings

- Calculated s-wave scattering lengths for different spin configurations.
- Provided scattering lengths for Ps in ground and excited states.
- Analyzed spin-dependent scattering cross-sections.

## Abstract

Low-energy elastic and inelastic scattering in the Ps(1$s$)-Ps(2$s$) channel is treated in a four-body hyperspherical coordinate calculation. Adiabatic potentials are calculated for triplet-triplet, singlet-singlet, and singlet-triplet spin symmetries in the spin representation of coupled electrons and coupled positrons, with total angular momentum $L=0$ and parity equal to $+1$. The s-wave scattering lengths for the asymptotic Ps(1$s$)-Ps(2$s$) channel are calculated for each spin configuration. Results obtained for the s-wave scattering lengths are $a_{\mathrm{TT}}=$~$7.3(2)a_0-i0.02(1)a_0$, $a_{\mathrm{SS}}=$~$13.2(2)a_0-i0.9(2)a_0$, and $a_{\mathrm{ST}}=$~$9.7(2)a_0$ for each spin configuration. Spin recoupling is implemented to extract the scattering lengths for collisions of Ps in different spin configurations through properly symmetrized unitary transformations. Calculations of experimentally relevant scattering lengths and cross-sections are carried-out for Ps atoms initially prepared in different uncoupled spin states.