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

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 as a high-acceptance photon detector to study CPT-prohibited angular correlation in ortho-positronium (o-Ps) annihilations. We record an unprecedented range of kinematical configurations of o-Ps annihilations into three photons. Tomographic reconstruction of the annihilation points in a large medium allows single-event estimation of positronium spin orientation and, consequently, determination of the complete spectrum of angular correlation between the annihilation plane orientation and positronium spin, the non-vanishing expectation value of which would manifest CPT-violating effects. We find no violation at the precision level of 10^{−4}, with a fourfold improvement on the previous measurement. This work enables application of positronium-imaging techniques to study discrete symmetries in positronium decays.

Perspectives on translation of positronium imaging into clinics

“…In this opinion article, we discussed the current state of development of newly invented positronium imaging requiring high sensitivity and high-time-resolution multi-photon PET systems [1][2][3][4][5][6][7]. By reviewing the latest advances in the development of total-body and multi-photon PET systems, we have shown that the current PET technology is ready for effective positronium imaging in clinics with the spatial resolution comparable to that of the current PET images.…”

“…The first positronium images have recently been demonstrated using the J-PET multi-photon scanner prototype [2,7]. J-PET tomograph was enabled to reconstruct positronium images for both three-photon annihilations [7] and two-photon pick-off annihilations of ortho-positronium [2], showing that both positronium lifetime imaging and imaging of the three-photon to two-photon rate ratios are feasible. Figure 1C compares the sensitivity of two-photon positronium imaging with standard two-photon PET imaging.…”

“…The spatial resolution of the direct positronium image reconstruction depends predominantly on the time resolution of the PET system. The very first positronium image recently demonstrated using a multi-photon J-PET tomograph was reconstructed with the coincidence resolving time resolution (CRT) of about 500 ps, corresponding to a spatial resolution of 75 mm [2,7]. It is worth noting that the time resolution of PET systems is constantly improving, and there is also the ongoing rapid development of new materials with improved timing properties [41,42] and new detectors for preclinical [43] and clinical [2,8,[44][45][46] studies.…”

“…In this article, we advocate the opinion that total-body PET systems, thanks to their high imaging sensitivity and high time resolution, open up the prospect of translating positronium imaging into clinics. KEYWORDS positronium imaging, biomarkers, medical imaging, J-PET, multi-photon PET, total-body PET, theranostics, hypoxia Positronium imaging is a newly invented diagnostic method [1][2][3][4][5][6][7] with promising potential to improve specificity in the assessment of tissue pathology in vivo at the molecular level based on information available from the data collected by the upcoming generation of multi-photon (multi-gamma) PET systems [2,4,7,8].…”