Precise measurement of HFS of positronium

TL;DR

This paper discusses a new experiment to precisely measure the hyperfine splitting of positronium, aiming to resolve a significant discrepancy between theory and previous experiments, which could indicate new physics or systematic errors.

Contribution

It introduces a novel experimental approach to measure positronium HFS with improved accuracy, addressing potential systematic errors in prior measurements.

Findings

Preliminary measurement: 203.399 ± 0.029 GHz

Discrepancy between QED prediction and experiment (3.9 sigma)

Expected future precision of about 1 ppm

Abstract

The ground state hyperfine splitting in positronium, $\Delta _{\mathrm{HFS}}$, is sensitive to high order corrections of QED. A new calculation up to $O(\alpha ^3)$ has revealed a $3.9 \sigma$ discrepancy between the QED prediction and the experimental results. This discrepancy might either be due to systematic problems in the previous experiments or to contributions beyond the Standard Model. We propose an experiment to measure $\Delta_{\mathrm{HFS}}$ employing new methods designed to remedy the systematic errors which may have affected the previous experiments. Our experiment will provide an independent check of the discrepancy. The measurement is in progress and a preliminary result of $\Delta_{\mathrm{HFS}} = 203.399 \pm 0.029 \mathrm{GHz} (143 \mathrm{ppm})$ has been obtained. A measurement with a precision of O(1) ppm is expected within a few years.