Breakthrough in rapid cooling for BASE antiprotons

To study antimatter particles, experiments must cool them to the lowest possible temperatures. The BASE experiment has just reached a new milestone in this regard. In an article published by the journal Physical Review Letters, the collaboration presents a new device that reduces the time required to cool an antiproton from 15 hours to just 8 minutes. This considerable improvement makes it possible to measure antiprotons’ properties with unparalleled precision.

BASE, located in CERN's antimatter factory, specialises in studying antiprotons by measuring their fundamental properties, such as the intrinsic magnetic moment and the charge-to-mass ratio, with the highest possible precision. By comparing these measurements with those of protons, the collaboration furthers our understanding of antimatter. One of the goals is to help resolve the fundamental question of the asymmetry between matter and antimatter in the Universe.

To determine the magnetic moments of antiprotons, the experiment measures the frequency of the spin quantum transitions (spin flips) of single antiprotons, which is in itself an amazing feat. Under the influence of a magnetic field, the antiproton’s spin changes direction, alternating between -1/2 and 1/2, its two possible values. This measurement is possible only with extremely cold antiprotons. “To get a clear measurement of an antiproton’s spin transitions, we need to cool the particle to less than 200 millikelvins,” explains Barbara Latacz, the lead author of the study.

BASE’s previous device could achieve this, but only after 15 hours of cooling. “As we need to perform 1000 measurement cycles, it would have taken us three years of non-stop measurements, which would have been unrealistic,” continues Barbara Latacz. By reducing the cooling time to 8 minutes, BASE can now obtain all of the 1000 measurements it needs – and thereby improve its precision – in less than a month. As a result, the experiment has announced that its error rate for the detection of antiproton spin transitions is three orders of magnitude lower than previously.

To perform its measurements, BASE uses antiprotons that have been decelerated by the Antiproton Decelerator (AD) and then the Extra Low Energy Antiproton ring (ELENA). It then stores around a hundred antiprotons in a Penning trap, which holds them in place using electrical and magnetic fields. An antiproton is then extracted into a system made up of two Penning traps. The first trap measures the temperature of the particle. If it is too high, the antiproton is transferred to a second trap to be cooled. The particle then goes back and forth between the two traps until the desired temperature is reached.

The key to the breakthrough announced by BASE is the improvement of the cooling trap. Its diameter has been reduced to just 3.8 mm, less than half the size of that used in previous experiments. It has been equipped with an innovative segmented-electrode system to reduce the amplitude of one of the antiproton oscillations – the cyclotron mode – more effectively. The “temperature” of the antiproton is correlated with its oscillations, which have several superimposed modes. The cyclotron mode, which is linked to the particle’s movement in a magnetic field, must be reduced to allow spin state measurement. The readout electronics have also been optimised to reduce background noise, which translates into disturbances and therefore heat for the antiproton. The time spent by the antiproton in the cooling trap during each cycle has thus been reduced from 10 minutes to 5 seconds. Further improvements to the measurement trap have also made it possible to reduce the measurement time fourfold.

With its new device, BASE intends to further improve on its own precision records. “Up to now, we have been able to compare the magnetic moments of the antiproton and the proton with a precision of one part per billion. Our new device will allow us to reach a precision of a tenth or even a hundredth of a billionth,” says BASE spokesperson Stefan Ulmer. “The slightest discrepancy could help solve the mystery of the imbalance between matter and antimatter in the Universe.”