Physicists Measure Magnetic Moment of Single Antimatter Particle

A research team led by Harvard University scientists has measured the magnetic moment of the antiproton more accurately than ever before.

Antiproton annihilation (P. Lubinski, Warsaw University / G. Brumfiel)

Antiproton annihilation. An antiproton collides with a neutron at the surface of a nucleus (P. Lubinski, Warsaw University / G. Brumfiel)

“That is a spectacular jump in precision for any fundamental quality of the antiproton measurements. That’s a leap that we don’t often see in physics, at least not in a single step,” said Prof Gerald Gabrielse of the Harvard University’s Department of Physics, co-author of the study published in Physical Review Letters.

The physicists were able to capture individual protons and antiprotons in a ‘trap’ created by electric and magnetic fields. By precisely measuring the oscillations of each particle, they were able to measure the magnetism of a proton more than 1,000 times more accurately than a proton had been measured before. Similar tests with antiprotons produced a 680-fold increase in accuracy in the size of the magnet in an antiproton.

“Such measurements,” Prof Gabrielse said, “could one day help scientists answer a question that seems more suited for the philosophy classroom than the physics lab – why are we here?”

“One of the great mysteries in physics is why our Universe is made of matter. According to our theories, the same amount of matter and antimatter was produced during the Big Bang. When matter and antimatter meet, they are annihilated. As the Universe cools down, the big mystery is: Why didn’t all the matter find the antimatter and annihilate all of both? There’s a lot of matter and no antimatter left, and we don’t know why.”

Making precise measurements of protons and antiprotons could begin to answer those questions by potentially shedding new light on whether the CPT (Charge conjugation, Parity transformation, Time reversal) theorem is correct. An outgrowth of the standard model of particle physics, CPT states that the protons and antiprotons should be virtually identical – with the same magnitude of charge and mass – yet should have opposite charges.

“Though earlier experiments, which measured the charge-to-mass ratio of protons and antiprotons, verified the predictions of CPT,” Prof Gabrielse said, “further investigation is needed because the standard model does not account for all forces, such as gravity, in the Universe.”

“What we wanted to do with these experiments was to say, ‘Let’s take a simple system – a single proton and a single antiproton – and let’s compare their predicted relationships, and see if our predictions are correct. Ultimately, whatever we learn might give us some insight into how to explain this mystery.”

While researchers were able to capture and measure protons with relative ease, antiprotons are only produced by high-energy collisions that take place at the extensive tunnels of the CERN laboratory in Geneva, leaving researchers facing a difficult choice.

“Last year, we published a report showing that we could measure a proton much more accurately than ever before,” Prof Gabrielese said. “Once we had done that, however, we had to make a decision – did we want to take the risk of moving our people and our entire apparatus – crates and crates of electronics and a very delicate trap apparatus – to CERN and try to do the same thing with antiprotons? Antiprotons would only be available till mid-December and then not again for a year and a half.”

Though their results still fit within the predictions made by the standard model, Prof Gabrielse said being able to more accurately measure the characteristics of both matter and antimatter may yet help shed new light on how the Universe works.

“What’s also very exciting about this breakthrough is that it now prepares us to continue down this road. I’m confident that, given this start, we’re going to be able to increase the accuracy of these measurements by another factor of 1,000, or even 10,000.”


Bibliographic information: J. DiSciacca et al. 2013. One-Particle Measurement of the Antiproton Magnetic Moment. Phys. Rev. Lett. 110, 130801; doi: 10.1103/PhysRevLett.110.130801