Touch BASE

In a recent Nature article (see here), the BASE collaboration reported the most precise comparison of the charge-to-mass ratio of the proton to its antimatter equivalent, the antiproton. This result is just the beginning and many more challenges lie ahead.


CERN's AD Hall, where the BASE experiment is set-up.

The Baryon Antibaryon Symmetry Experiment (BASE) was approved in June 2013 and was ready to take data in August 2014. During these 14 months, the BASE collaboration worked hard to set up its four cryogenic Penning traps, which are the heart of the whole experiment. As their name indicates, these magnetic devices are used to trap antiparticles – antiprotons coming from the Antiproton Decelerator – and particles of matter – negative hydrogen ions produced in the system by interaction with a degrader that slows the antiprotons down, allowing scientists to perform their measurements. “We had very little time to set up the whole experiment but we eventually succeeded in taking data and we are very happy to already have such good results,” says Stefan Ulmer, spokesperson of the experiment.

For the charge-to-mass ratio measurement, BASE developed techniques to have one particle upstream of the measurement trap and another one downstream of the trap. The charge-to-mass ratio is extracted from the cyclotron frequency of the particles measured in exactly 120 seconds, which corresponds to one AD cycle. Scientists are able to perform the measurement for each particle – the antiproton and the hydrogen ion – separately and then compare the two measurements, with a very high sampling rate. Any discrepancies in the fundamental properties of matter and antimatter would provide scientists with important clues to understanding physics beyond the standard model. “Our measurement tests one of the most fundamental principles of relativistic field theory, CPT invariance [see box],” says Ulmer. “Stable matter-antimatter systems such as the proton and the antiproton or bound systems such as hydrogen and anti-hydrogen are particularly attractive, since the stability of these systems allows infinite observation times, and thus, makes high-precision investigations possible.”

The high-precision measurement performed by BASE shows no difference between the proton and the antiproton, with four times higher energy resolution than previous measurements. “This result is only the beginning of our scientific programme,” says Ulmer. “One fundamental quantity of the proton and the antiproton, the magnetic moment, has not yet been compared with high precision and the physics goal of BASE is precisely this.”

Currently, the magnetic moment of the proton is known at a level of 3.3 x10−9, as measured by the BASE collaboration, while the magnetic moment of the antiproton is known at a level of 4.4 x10−6, as measured by the ATRAP collaboration in 2012. “We are currently working on an advanced superconducting magnet system to make the magnetic field of our trap system more homogeneous and at the same time more stable,” explains Ulmer. “We plan to start measuring the magnetic moments within the 2015 antiproton run and hope to produce a ppb measurement by the start of the next long shutdown of the accelerators at the latest.” The collaboration expects to improve the current data by over a factor of 1000.

Did you know?

The final component of the BASE trap system is called the “reservoir trap”. This trap is able to store antiprotons for months on end, allowing the BASE collaboration to continue operating and measuring even without beam. The results just published in Nature are based on 13,000 measurements performed over a 35-day campaign.


CPT Invariance

All experimental observations so far confirm that the physics processes evolve in exactly the same way if, at the same time, charge is inversed (that is, if we exchange matter with the corresponding antimatter), parity is inversed (a right-handed coordinate system is changed into a left-handed one or vice versa, which is equivalent to looking at the system in a mirror), and time is reversed (which also reverses the particle momenta). The combined CPT conservation is thought to be a fundamental property of physics laws. Experimental searches for CPT violation, including the recent result from BASE, have yielded negative results.


by Antonella Del Rosso