BASE is a Japanese / German collaboration at the antiproton decelerator facility which was approved in 2013. The goal of the effort is to conduct Penning trap based high-precision investigations of the fundamental properties of protons and antiprotons. These type of experiments provide stringent tests of the fundamental charge, parity, time invariance, which is the most fundamental symmetry in the relativistic quantum field theories of the Standard Model. 

Fig. 1: BASE apparatus as installed in the antiproton decelerator facility. The central vessel is a superconducting magnet in persistent mode which provides the stable magnetic field for the Penning trap. The orange vessels are cryostats which cool the Penning trap apparatus. Cryopumping produces in the central trap volume background pressures of order 1e-18 mbar. 

The BASE collaboration developed from an effort which started at the University of Mainz, Germany, around 2007. The goal of this campaign was to use the elegant continuous Stern Gerlach effect, a method which was previously applied with great success to measure the magnetic moments of electrons and positrons, to perform the most precise measurement of the magnetic moment of the bare proton. This is a rather challenging task since, compared to the electron/positron system, the magnetic moment of the proton is by 660 times smaller. Thus, an apparatus had to be developed with an orders of magnitude higher sensitivity with respect to magnetic moments than any Penning trap experiment which has been built before. After years of development work – in 2011 – the group was able to report on a major milestone towards their physics goal; the first observation of spin flips with a single trapped proton.  Based on this success and after considerable improvements of the experimental apparatus the team reported on the observation of single-proton spin-quantum transitions and the first demonstration of the double-Penning-trap technique with a single proton. The sequence of achievements culminated in the most precise measurement of the magnetic moment of the proton with a fractional precision of 3.3 parts in a billion. 

What’s highly topical: the double-Penning-trap method which was used to achieve this goal for the proton can directly be applied to measure the magnetic moment of the antiproton with comparable precision. To date the most precise value of this fundamental antiparticle property comes from a 2013 ATRAP measurement. Using a single Penning trap method a fractional precision of 4.4 parts in a million was achieved. The double Penning trap technique developed by the Mainz scientists has the potential to improve this measurement by more than a factor of thousand. To apply this method to the antiproton the BASE collaboration was formed. Led by the Japanese RIKEN institute a novel type of precision Penning trap was developed – the four Penning-trap system shown in Fig. 2.

Fig. 2: BASE Penning trap stack which consists of four Penning traps, a reservoir trap, a double Penning trap for precision frequency measurements and a cooling trap for efficient mode cooling of the individual degrees of freedom.

It consists of a double Penning trap, a precision Penning trap for frequency measurements and an inhomogeneous analysis trap for spin state analysis. What’s new is that the BASE scientist added two additional traps, a small cooling trap for highly efficient single particle cooling and a reservoir trap. This unique device – invented by BASE – traps a cloud of antiprotons from the antiproton decelerator and stores them under background pressures of order 1e-18 mbars for arbitrarily long time. Methods were developed to extract single particles from this reservoir and to supply them to the precision Penning trap cycle. This enables BASE to conduct experiments almost independently from accelerator cycles, even during the winter shut-down when magnetic background noise in the accelerator hall is much lower than during the antiproton run. Currently, September 2016, the experiment is being operated with antiprotons which were trapped in November 2015, which constitutes a new record in antiproton trapping time.

Apart from magnetic moment measurements the BASE Penning trap techniques incorporate another highly exciting possibility – the comparison of the antiproton-to-proton charge-to-mass ratios. Such comparisons are based on cyclotron frequency comparisons of antiprotons and negatively charged hydrogen ions which serve as perfect proxies for protons. A scheme was implemented which enables such frequency comparisons in experiment cycle times of only 4 minutes, which is by about 50 times faster than in previous experiments. This fast measurement technique enables thus measurements at much higher statistics. By using this fast comparison technique BASE performed in the 2014 antiproton run the most precise comparison of the proton-to-antiproton charge-to-mass ratio with a fractional precision of 69 parts in a trillion, results are shown in Fig. 3. The measurement constitutes the to-date most precise test of CPT invariance with baryons, the measured result being consistent with CPT invariance.

Fig. 3: Results of the high precision cyclotron frequency comparisons of antiprotons and negatively charged hydrogen ions. In total 6500 frequency ratios were measured within a sampling time of about one month

Apart from a stringent test of CPT symmetry the data provide an interesting interpretation: By assuming that CPT invariance holds the proton/antiproton cyclotron frequency comparisons set a stringent constraint on hypothetically different gravitational red-shifts experienced by matter and antimatter particles, respectively. Given this assumption the BASE data set most stringent limits on the weak equivalence principle of general relativity.

The BASE effort has just started and is currently in the middle of the third antiproton run. A first benchmark measurement with 69 p.p.t. fractional uncertainty has been performed in a very first step. Further optimization of the apparatus will allow for a further improved comparison of the antiproton-to-proton charge-to-mass ratios as well as a thousand-fold improved measurement of the magnetic moment of the antiproton. This experiment – considerably more challenging than the charge-to-mass comparison – is currently being commissioned.