In a paper published today in Nature, we present the most precise comparison to date of a fundamental property of protons and antiprotons. By analysing approximately 24,000 cyclotron frequency measurements of protons and antiprotons collected over 18 months, we have determined that their charge-to-mass ratios are identical to within an experimental uncertainty of just 16 parts per trillion (see Fig.).
This result represents the most stringent test so far of the combined charge, parity, and time reversal (CPT) symmetry — a cornerstone of modern physics and, to date, the only exact symmetry observed in nature among the discrete symmetry transformations.
Any deviation in mass between matter and antimatter counterparts would suggest the existence of physics beyond the Standard Model, potentially involving higher-dimensional and non-local phenomena. Our measurement of the antiproton-to-proton charge-to-mass ratio,
R = 1.000 000 000 003(16),
corresponds to an energy sensitivity of 2 × 10⁻²⁷ GeV, within which no such deviation is observed. This places strong constraints on potential violations of fundamental principles such as Lorentz invariance and micro causality.
Detecting a difference between matter and antimatter properties could help address one of the biggest open questions in cosmology: why the universe is dominated by matter despite theories predicting equal creation of matter and antimatter in the Big Bang. The known differences predicted by the Standard Model are far too small to explain this imbalance, highlighting the importance of ever-more sensitive experimental tests like ours.