Comments on the publication "Discrete symmetries tested at 10e-4 precision using linear polarization of photons from positronium annihilations" by P. Moskal et al

TL;DR

This paper critically examines a recent claim of high-precision tests of discrete symmetries in positronium decay, highlighting methodological flaws and clarifying the limitations of the reported measurements.

Contribution

The authors demonstrate that the measured correlation must be zero under standard assumptions and clarify misconceptions about photon polarization measurement in positronium decay experiments.

Findings

The correlation measured by Moskal et al. is necessarily zero under standard assumptions.

There is no formal basis to equate the scattering plane normal with photon polarization.

Previous measurements with proper polarimetry relate to symmetry tests.

Abstract

The authors of Ref.[1] (referred to here as "Moskal et al.") claim to have performed the most precise test of P, T and CP invariance in the decay of ortho-Positronium. In this note: 1) we demonstrate, assuming standard properties for Compton scattering, that the average value of the correlation measured by Moskal et al. must necessarily be zero, independently of any physics occurring in o-Ps decay; 2) we point out that there is no formal justification to equate the normal vector to the Compton scattering plane with the incident photon polarization, as done by Moskal et al.; 3) we observe the absence of characterization of the device as a Compton polarimeter, which is paramount in photon polarimetry; 4) we review previous measurements of the polarization of photons from o-Ps decay, properly implementing the Compton polarimetry technique, and make the connection with tests of discrete symmetries; and 5) we stress that the correlation proposed by Moskal et al. cannot be generated solely by the physics of o-Ps decay, including possible violations of discrete symmetries.