BASE (Baryon Antibaryon Symmetry Experiment), AD-8, is a multinational collaboration at the Antiproton Decelerator facility at CERN, Geneva. The goal of the Japanese and German BASE collaboration[1] are high-precision investigations of the fundamental properties of the antiproton, namely the charge-to-mass ratio and the magnetic moment.
The single antiprotons are stored in an advanced Penning trap system, which has a multi-trap system at its core. It consists of a reservoir trap,[2] a precision trap, an analysis trap and a cooling trap. The reservoir trap has the capability to store antiprotons for several years[3] and allows BASE to operate experiments independent from accelerator cycles. The precision trap is for high precision frequency measurements, and the analysis trap has a strong magnetic field inhomogeneity superimposed, which is used for single particle spin flipspectroscopy. By measuring the spin flip rate as a function of the frequency of an externally applied magnetic-drive, a resonance curve is obtained. Together with a measurement of the cyclotron frequency, the magnetic moment is extracted.
BASE physics
The BASE collaboration developed techniques to observe the first spin flips of a single trapped proton[4] and applied the double-trap technique to measure the magnetic moment of the proton with a fractional precision of three parts in a billion,[5] later improved to a precision of 300 parts in a trillion, [6] being the most precise measurement of this fundamental property of the proton. With the invention of a two particle/three trap technique BASE measured the antiproton magnetic moment with a fractional accuracy of 1.5 parts in a billion, which improved the previous most precise proton/antiproton comparison in that sector [7] by more than a factor of 3000. This measurement constitutes one of the most stringent tests of CPT invariance with baryons to date, and sets the most stringent limits on antimatter/dark matter interaction to date.[8]
Inspired by this work BASE has also used Penning trap detectors as axion haloscopes, and derived stringent narrow-band laboratory limits on the conversion of axions to photons.[9]
In 2022 BASE measured that the charge-to-mass ratios of protons and antiprotons are equal within a precision of 16 parts per trillion.[10][11] This measurement constitutes the most precise test of CPT invariance in the baryon sector and sets the first differential constraints on the clock weak equivalence principle using baryonic antimatter.
