Self annihilation of confined positronium

TL;DR

This paper investigates how confinement affects positronium's self-annihilation rate using various models, revealing increased annihilation rates under confinement conditions, contrasting with some previous theoretical and experimental findings.

Contribution

It introduces new models of confinement for positronium, including soft-wall potentials, and compares their effects on annihilation rates to prior assumptions and data.

Findings

Confinement increases annihilation rate compared to vacuum.

Rate increase diminishes exponentially with separation.

Physically motivated potentials yield higher rates than previous models.

Abstract

The effect of confinement on the self-annihilation rate of positronium is studied in three levels of approximation. Artificial restriction of the electron-positron separation leads to an increase in the annihilation rate over its vacuum value; this increase is found to diminish exponentially as the maximum separation is increased. Confinement in a hard-wall spherical cavity with the center of mass free to move throughout the cavity also increases the annihilation rate over its vacuum value; the increase depends weakly on the position of the center of mass, being larger when the center of mass is near the cavity wall. Finally, to model confinement in a pore of a microporous material, the hard wall is replaced by physically motivated electron- and positron-wall potentials; it is found that the annihilation rate is larger than its vacuum value, in contradiction to calculations of Marlotti Tanzi et al. [Phys. Rev. Lett. 116, 033401 (2016)] that assumed hard-wall confinement for the electrons, and experimental data.