Today we report in Nature on an improved measurement of the magnetic moment of the antiproton. The new measurement outperforms our old record measurement by a factor of 350 in experimental precision. Our updated value gpbar/2=2.792 847 3441 (42) is consistent with the magnetic moment of the proton gp=2.792 847 350 (9), and thus supports the combined charge, parity, and time-reversal (CPT) invariance, an important symmetry of the Standard Model of particle physics. Remarkably, this is the first time physicists have carried out a more precise measurement on antiprotons than on protons. Together with the exciting new antihydrogen results, this milestone achievement is a demonstration of the immense progress made at CERN’s antiproton decelerator facility.
This extraordinary improvement in experimental accuracy was made possible by the invention of a novel two-particle multi-Penning-trap measurement method, which combines the non-destructive detection of the antiproton’s spin quantum state with particle-based high-resolution magnetic field measurements.
The determination of the magnetic moment of a single trapped particle is based on the measurement of two characteristic frequencies, the cyclotron frequency, which describes the particle’s revolutions per second in the magnetic field of the Penning trap, and the second, the precession frequency of the particle’s spin. Together, these allow us to access the particle’s magnetic moment. Previous antiproton measurements, such as those performed by the ATRAP collaboration in 2013 and later by BASE, used a single Penning trap with a superimposed magnetic bottle. This strong inhomogeneity in the magnetic field allows for non-destructive detection of the particle’s spin-quantum-state, a precursor to any determination of the Larmor frequency. However, such a bottle broadens the particle’s resonance lines and limits the precision of the measurement, typically to the parts per million level.
To overcome this limitation, experimentalists apply a two trap method which separates the high-precision frequency measurements to a homogeneous precision trap and the spin state analysis to a trap with the superimposed magnetic inhomogeneity. While an elegant technique, this double trap method is very challenging to implement. It took seven years of research and development work until we were able to demonstrate this double-trap method with a single trapped proton, and later applied it in a measurement of the proton magnetic moment to nine significant figures.
In the measurement reported today, we have extended the double-trap technique to a three trap / two particle scheme, in which we use a “hot” particle with an effective temperature of 300K for magnetic field measurements and a cold particle at 0.12K for spin transition spectroscopy. By alternatingly shuttling the two particles to the precision trap we were able to quasi-simultaneously sample cyclotron and Larmor frequencies by a fast adiabatic particle exchange in the same ultra-homogeneous magnetic field. However, unlike the double trap method, the two particle technique avoids time consuming resistive cooling cycles to sub-thermal temperatures, and thus, enables measurements at drastically improved frequency sampling rate, which was the major breakthrough to accomplish the goal of measuring the antiproton magnetic moment with parts per billion precision.
By combining the new 350-fold improved antiproton result with our previously measured proton result we obtain one of the most precise tests of CPT invariance in the baryon sector, which enables us to set drastically improved constraints on CPT-violating extensions of the Standard Model.
Max Planck Society: https://www.mpi-hd.mpg.de/blaum/news/index.en.html#date18102017