Today we report in SCIENCE on a new measurement of the proton magnetic moment in units of the nuclear magneton. The updated value gp/2=2.792 847 344 62 (82) is consistent with our previous best measurement, but improves the precision by a factor of 11. The measurement was carried out using an optimized double Penning trap technique. Compared to our 2014 measurement, a trap with higher magnetic field stability and homogeneity was implemented. Together with a significantly improved cooling system for the preparation of sub-thermal cyclotron quantum states and an optimized spin transition drive method, the 11-fold improved measurement became possible.

This new result improves, together with our recent measurement of the antiproton magnetic moment gpbar/2=2.792 847 344 1 (42), the test of the fundamental charge, parity, time invariance by a factor of two, still being CPT consistent. The result reported here is an important step towards further advancing the precision in CPT-tests, the application of the optimized double Penning trap method to the antiproton will improve direct tests of CPT invariance in the magnetic moment sector by a factor of 5.