Direct high-precision measurement of the magnetic moment of the proton

The magnetic moment of the proton is directly measured with unprecedented precision using a double Penning trap. An important moment for matter–antimatter symmetry Although less prominent than large synchrotron experiments, measurements of fundamental constants or atomic properties can still make valuable contributions to the search of physical laws beyond the Standard Model — if the measurement precision is high enough. In a direct measurement, Andreas Mooser et al . determine the magnetic moment of the proton with unprecedented precision. The measurement is performed using a double Penning trap, a system in which a single ion is confined and manipulated in a powerful homogeneous magnetic field. In combination with a direct measurement of the antiproton magnetic moment, this work will pave the way for a rigorous test of matter–antimatter symmetry. One of the fundamental properties of the proton is its magnetic moment, µ p . So far µ p has been measured only indirectly, by analysing the spectrum of an atomic hydrogen maser in a magnetic field 1 . Here we report the direct high-precision measurement of the magnetic moment of a single proton using the double Penning-trap technique 2 . We drive proton-spin quantum jumps by a magnetic radio-frequency field in a Penning trap with a homogeneous magnetic field. The induced spin transitions are detected in a second trap with a strong superimposed magnetic inhomogeneity 3 . This enables the measurement of the spin-flip probability as a function of the drive frequency. In each measurement the proton’s cyclotron frequency is used to determine the magnetic field of the trap. From the normalized resonance curve, we extract the particle’s magnetic moment in terms of the nuclear magneton: μ p = 2.792847350(9) μ N . This measurement outperforms previous Penning-trap measurements 4 , 5 in terms of precision by a factor of about 760. It improves the precision of the forty-year-old indirect measurement, in which significant theoretical bound state corrections 6 were required to obtain µ p , by a factor of 3. By application of this method to the antiproton magnetic moment, the fractional precision of the recently reported value 7 can be improved by a factor of at least 1,000. Combined with the present result, this will provide a stringent test of matter/antimatter symmetry with baryons 8 .