Precision Positronium Spectroscopy as a Test of the Helium Ionization Energy Anomaly

J. Pérez-Ríos, D. B. Cassidy

Precise measurements of the metastable helium (1s)(2s)${}^3{S}_1$ ionization energy have revealed a 9$\sigma$ discrepancy with QED theory that persists in isotopic measurements, suggestin…