One-Particle Measurement of the Antiproton Magnetic Moment

Abstract

For the first time a single trapped antiproton ( " p) is used to measure the " p magnetic moment " p . The moment " p ¼ " p S=ð@=2Þ is given in terms of its spin S and the nuclear magneton ( N ) by " p = N ¼ À2:792 845 AE 0:000 012. The 4.4 parts per million (ppm) uncertainty is 680 times smaller than previously realized. Comparing to the proton moment measured using the same method and trap electrodes gives " p = p ¼ À1:000 000 AE 0:000 005 to 5 ppm, for a proton moment p ¼ p S=ð@=2Þ, consistent with the prediction of the CPT theorem.

Electron g‐factor determinations in Penning traps

“…The small size of the proton's magnetic moment, however, makes it technically more challenging. Experiments are under way at the University of Mainz [71] and at Harvard University [72], as well as at CERN for the antiproton [73,74]. Both have very recently produced first data and an improvement of the relative precision to the anticipated 10 −9 is foreseeable for the close future.…”

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

“…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, μ p . The extraordinary difficulty in measuring μ p with high precision is caused by its intrinsic smallness; for example, it is 660 times smaller than the magnetic moment of the positron 3 .…”

High-precision comparison of the antiproton-to-proton charge-to-mass ratio

“…There is a deep link between CPT invariance and Lorentz symmetry-that is, the laws of nature seem to be invariant under the symmetry transformation of spacetimealthough it is model dependent 2 . A number of high-precision CPT and Lorentz invariance tests-using a co-magnetometer, a torsion pendulum and a maser, among others-have been performed 3 , but only a few direct high-precision CPT tests that compare the fundamental properties of matter and antimatter are available [4][5][6][7][8] . Here we report high-precision cyclotron frequency comparisons of a single antiproton and a negatively charged hydrogen ion (H 2 ) carried out in a Penning trap system.…”

“…However, this model is known to be incomplete, which has inspired searches for physics beyond the standard model, such as tests of CPT invariance that compare the fundamental properties of matterto-antimatter equivalents at the lowest energies and with the greatest precision [12][13][14][15] . For leptons, for example, the magnetic anomalies of electron and positron were compared with a fractional uncertainty of about 2 parts per billion 4 , and by applying similar techniques to protons and antiprotons, the resulting g-factor (a proportionality constant which links the spin of a particle to its magnetic moment) comparison reached a precision of 4.4 parts per million 8 . We are planning to improve this measurement by at least a factor of a thousand 16,17 .…”