Matter-Antimatter Symmetry: CERN Experiment Achieves Sixfold Improvement In Measurement Of Antiproton Magnetic Moment

One of the biggest unanswered questions of cosmology is — why does anything exist at all? In other words, where is all the antimatter that was created along with matter in the Big Bang 13.8 billion years ago?

If the Big Bang did create matter and antimatter in equal quantities — something that our current theoretical models tell us happened, why does only matter now remain? Why isn’t the present-day universe a wasteland of residual energy, given that matter and antimatter particles get annihilated when they come in contact?

Scientists believe that there must be a fundamental difference between matter and antimatter — one that gave the former an edge over the latter in the race for survival. Either significantly more matter was created by the Big Bang, or there is an as-of-yet-undiscovered asymmetry between matter particles and their antimatter counterparts.

That is why scientists have been hunting for violations of what is known as charge, parity and time reversal (CPT) symmetry — a central tenet of the Standard Model of particle physics, which states that the laws of physics remain unchanged even if a particle is replaced with its antiparticle, which has the opposite charge, and even if its spatial coordinates are inverted.

In November, researchers associated with the ASACUSA collaboration at the European Organization for Nuclear Research (CERN) announced that they had made a new precision measurement of the mass of the antiproton — the antimatter equivalent of a proton — relative to that of the electron. And now, scientists working with the Baryon Antibaryon Symmetry Experiment (BASE) have achieved a sixfold improvement in accuracy of the magnetic moment of the antiproton.

The magnetic moment of a proton arises due to its spin, which is an intrinsic form of angular momentum. Scientists believe that measuring the magnetic moments of antiparticles and comparing them to those of the corresponding particles could reveal violations in CPT symmetry.

In order to make their measurements, the researchers used protons generated by CERN’s antiproton decelerator and placed them in Penning traps, which use a strong magnetic field to trap particles. This allowed them to isolate individual antiprotons that were then cooled down to near absolute zero.

Their measurements revealed that magnetic moments of protons and antiprotons are identical, apart from their opposite signs, within the experimental uncertainty of 0.8 parts per million. This result improves the precision of the previous best measurement by the ATRAP collaboration — also at CERN — by a factor of six.

“We see a deep contradiction between the standard model of particle physics, under which the proton and antiproton are identical mirror images of one another, and the fact that on cosmological scales, there is an enormous gap between the amount of matter and antimatter in the universe,” BASE Collaboration spokesman Stefan Ulmer said in a statement. “Our experiment has shown, based on a measurement six times more precise than any done before, that the standard model holds up, and that there seems in fact to be no difference in the proton/antiproton magnetic moments at the achieved measurement uncertainty. We did not find any evidence for CPT violation.”

The researchers are now aiming to refine their measurements even further in order to enable a precision at the level of a few parts per billion. If future experiments do reveal hints of a CPT violation, it would provide answers to what is perhaps the most fundamental question of cosmology — why is there something rather than nothing in the universe?