The Japanese/German BASE collaboration at CERN’s Antiproton Decelerator (AD) has compared the charge-to-mass ratios of the antiproton and proton with a fractional precision of 69 parts in a trillion (ppt). This high-precision measurement was achieved by comparing the cyclotron frequencies of antiprotons and negatively charged hydrogen ions in a Penning trap. The result is consistent with charge–parity–time-reversal (CPT) invariance, which is one of the cornerstones of the Standard Model of particle physics, and constitutes the most precise test comparing baryons and antibaryons performed to date.
In their experiment, the BASE collaboration has profited from techniques pioneered in the 1990s by the TRAP collaboration at the Low Energy Antiproton Ring at CERN. The advanced cryogenic Penning-trap system used in BASE consists of four traps, two of which were used in this measurement – a measurement trap and a reservoir trap (figure 1). When the experiment receives a pulse of 5.3 MeV antiprotons from the AD, they strike the degrader structure, which is designed to slow them down, and release hydrogen. Negatively charged hydrogen ions (H–) can form in the process, producing a composite cloud with the antiprotons that is shuttled to the reservoir trap. BASE has developed techniques to extract single antiprotons and negative hydrogen ions from this cloud whenever needed. Moreover, the reservoir has a lifetime of more than a year, making the BASE experiment almost independent from AD cycles.
Using this extraction technique, and taking the timing from the AD cycle, BASE prepares a single antiproton in the measurement trap, while an H– ion is held in the downstream park electrode, as shown in figure 1. The cyclotron frequency of the antiproton is then measured in exactly 120 s, which corresponds to one AD cycle. The particles are subsequently exchanged by performing appropriate potential ramps, and the cyclotron frequency of the H– ion is measured. Thus, a single comparison of the charge-to-mass ratios takes only 240 s. This is much faster than in previous experiments, enabling BASE to perform about 6500 ratio comparisons in 35 days of measurement time (figure 2). The result is a value of the ratio-comparison: (q/m)p-/(q/m)p – 1 = 1(69) × 10–12, thus confirming CPT at the level of ppt.
The high sampling rate has also enabled the first high-resolution study of diurnal variations in a baryon/antibaryon comparison, which could be introduced by Lorentz-violating cosmic-background fields. The measurement sets constraints on such variations at the level of less than 720 ppt. In addition, by assuming that CPT invariance holds, the measurement can be interpreted as a test of the weak equivalence principle using baryonic antimatter. If matter respects weak equivalence while antimatter experiences an anomalous coupling to the gravitational field, this gravitational anomaly would contribute to a possible difference in the measured cyclotron frequencies. Thus, by following these assumptions, the result from BASE can be used to set a limit on the gravitational anomaly parameter, αg: |αg – 1| < 8.7 × 10–7.
The main goal for the BASE experiment, which was approved in June 2013, is to measure the magnetic moment of the antiproton with a precision of parts per billion. Using the double Penning trap system, the collaboration recently performed the most precise measurement of the magnetic moment of the proton.
S Ulmer et al. 2015 Nature doi:10.1038/nature14861.