Journal Article

A parts-per-billion measurement of the antiproton magnetic moment

Ulmer, S., Tanaka, T., Higuchi, T., Smorra, C., Sellner, S., Walz, J., Ospelkaus, C., Harrington, J. A., Nagahama, H., Bohman, M., Yamazaki, Y., Mooser, A., Schneider, G., Matsuda, Y., Borchert, M. J., Blaum, K., Quint, W.

2017

Overview

The magnetic moment of the antiproton is measured at the parts-per-billion level, improving on previous measurements by a factor of about 350. Magnetic moment of the antiproton Comparing the fundamental properties of normal-matter particles with their antimatter counterparts tests charge–parity–time (CPT) invariance, which is an important part of the standard model of particle physics. Many properties have been measured to the parts-per-billion level of uncertainty, but the magnetic moment of the antiproton has not. Christian Smorra and colleagues have now done so, and report that it is −2.7928473441 ± 0.0000000042 in units of the nuclear magneton. This is consistent with the magnetic moment of the proton, 2.792847350 ± 0.000000009 in the same units. Assuming CPT invariance, these two values should be the same, except for the difference in sign, so this result provides a more stringent constraint on certain CPT-violating effects. Precise comparisons of the fundamental properties of matter–antimatter conjugates provide sensitive tests of charge–parity–time (CPT) invariance 1 , which is an important symmetry that rests on basic assumptions of the standard model of particle physics. Experiments on mesons 2 , leptons 3 , 4 and baryons 5 , 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 level 7 , 8 : the magnetic moment of the antiproton, . The extraordinary difficulty in measuring with high precision is caused by its intrinsic smallness; for example, it is 660 times smaller than the magnetic moment of the positron 3 . Here we report a high-precision measurement of in units of the nuclear magneton μ N 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  = −2.7928473441(42) μ N (where the number in parentheses represents the 68% confidence interval on the last digits of the value) improves the precision of the previous best measurement 8 by a factor of approximately 350. The measured value is consistent with the proton magnetic moment 9 , μ p  = 2.792847350(9) μ N , and is in agreement with CPT invariance. Consequently, this measurement constrains the magnitude of certain CPT-violating effects 10 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 magnetons 11 .

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/766 / 639/766/36 / 639/766/419 / Antimatter / Atoms & subatomic particles / Baryons / Conjugates / Cooling / Electrodes / Experiments / Humanities and Social Sciences / letter / Magnetic fields / Magnetic moments / Magnetic properties / Magnetism / Measurement / multidisciplinary / Particle physics / Radio frequency / Science / Spectroscopy / Spectrum analysis / Splitting / Standard model (particle physics) / Statistical analysis