The magnetic moment of the antiproton is measured at the parts-per-billion level, improving on previous measurements by a factor of about 350.
Precise comparisons of the fundamental properties of matter-antimatter conjugates provide sensitive tests of charge-parity-time (CPT) invariance1, which is an important symmetry that rests on basic assumptions of the standard model of particle physics. Experiments on mesons2, leptons3,4 and baryons5,6 have compared different properties of matter-antimatter conjugates with fractional uncertainties at the parts-per-billion level or better. One specific quantity, however, has so far only been known to a fractional uncertainty at the parts-per-million level7,8: the magnetic moment of the antiproton, Symbol. The extraordinary difficulty in measuring Symbol with high precision is caused by its intrinsic smallness; for example, it is 660 times smaller than the magnetic moment of the positron3. Here we report a high-precision measurement of Symbol in units of the nuclear magneton μN with a fractional precision of 1.5 parts per billion (68% confidence level). We use a two-particle spectroscopy method in an advanced cryogenic multi-Penning trap system. Our result Symbol = −2.7928473441(42)μN (where the number in parentheses represents the 68% confidence interval on the last digits of the value) improves the precision of the previous best Symbol measurement8 by a factor of approximately 350. The measured value is consistent with the proton magnetic moment9, μp = 2.792847350(9)μN, and is in agreement with CPT invariance. Consequently, this measurement constrains the magnitude of certain CPT-violating effects10 to below 1.8 × 10−24 gigaelectronvolts, and a possible splitting of the proton-antiproton magnetic moments by CPT-odd dimension-five interactions to below 6 × 10−12 Bohr magnetons11.