Why Does Matter Exceed Antimatter?

In the Crab Nebula, matter and antimatter are propelled nearly to the speed of light by the Crab pulsar. Photo credit NASA

The magnetic moment of the proton has been measured more directly and with greater precision than ever before. If this sounds like the sort of thing that would only excite physicists, consider this: it could help explain how we all came to be here.

One of the greatest mysteries of physics today is why we are not constantly in danger of being wiped out by some big ball of antimatter. Most models of the universe's formation suggest that equal amounts of matter and antimatter should have been made in the big bang. However, since these two annihilate each other in with a major release of energy whenever they come in contact a universe containing equal amounts of the two would be littered with sudden explosions. And yet, here we are. While pieces of the answer have been found, few physicists think we have the full solution.

One of the important aspects of the solving the question is knowing the fundamental properties of particles such as the proton. If the same can be done for the antiproton, and subtle differences revealed, we may be able to work out why there is so much more matter in the universe than antimatter.

One of these fundamental properties is the magnetic moment, that is the extent to which it is influenced by an external magnetic field. "Protons are like tiny rod magnets. They have a magnetic moment 24 magnitudes – equal to one millionth of a billionth of a billionth – weaker than a typical compass needle,” says Andrea Mooser of Mainz University.

To measure the moment Mooser and coauthors used a double Penning trap. One trap measured the proton's spin-quantum jumps while the other conducted frequency measurements.“This is the first time we have been able to measure anything on this scale," says Mooser. The figure of 2.792847350μN has been published in Nature and is 760 times as precise as previous direct measurements and three times more precise than the best indirect measurement. 

Penning traps are used to measure the magnetic moments of electrons and positrons (anti-electrons). However, with a magnetic moment 660 times smaller than an electron, protons are a more difficult matter.

The previous best estimate of the proton's magnetic moment came from a 1972 study of the hyper-fine structure of atomic hydrogen. Since this required corrections based on theoretical models, its precision is limited.

CERN are preparing to perform matching experiments on antiprotons. The authors suggest that the use of their method could improve the current best value for antiproton spin by a factor of “at least 1000”. If the magnetic moment of the antiproton is the same as the proton the quest for an explanation of the imbalance in the universe will have to look elsewhere. However, if a difference, however small, is found we will have a crucial clue to the secret.

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