Testing CPT symmetry in ortho-positronium decays with positronium annihilation tomography

TL;DR

This study uses positronium annihilation tomography to test CPT symmetry, achieving a threefold improvement in measurement precision and finding no violation at the 10^{-4} level.

Contribution

It introduces tomographic reconstruction techniques to estimate ortho-positronium spin without magnetic fields, enabling more precise CPT symmetry tests.

Findings

No CPT violation detected at 10^{-4} precision

Threefold improvement over previous measurements

Innovative use of PET scanner for positronium spin estimation

Abstract

Charged lepton system symmetry under combined charge, parity, and time-reversal transformation (CPT) remains scarcely tested. Despite stringent quantum-electrodynamic limits, discrepancies in predictions for the electron-positron bound state (positronium atom) motivate further investigation, including fundamental symmetry tests. While CPT noninvariance effects could be manifested in non-vanishing angular correlations between final-state photons and spin of annihilating positronium, measurements were previously limited by knowledge of the latter. Here, we demonstrate tomographic reconstruction techniques applied to three-photon annihilations of ortho-positronium atoms to estimate their spin polarisation without magnetic field or polarised positronium source. We use a plastic-scintillator-based positron-emission-tomography scanner to record ortho-positronium (o-Ps) annihilations with single-event estimation of o-Ps spin and determine the complete spectrum of an angular correlation operator sensitive to CPT-violating effects. We find no violation at the precision level of 10^{-4}, with an over threefold improvement on the previous measurement.