One measurement that had not been made was what is called the “magnetic moment” of the antiproton. The magnetic moment measures how a particle reacts to magnetic force. Recently, an international team of researchers at CERN used the Baryon Antibaryon Symmetry Experiment (BASE) to measure the magnetic moment of an antiproton to incredibly high precision. Their record-breaking results have now been published in the journal Nature.
What they found, however, is that the magnetic moments of both protons and antiprotons are of opposite sign, but they are otherwise identical. This has only deepened the mystery.
“All of our observations find a complete symmetry between matter and antimatter, which is why the universe should not actually exist,” said first author Christian Smorra in a statement “An asymmetry must exist here somewhere but we simply do not understand where the difference is.”
Antiprotons are artificially generated at CERN and researchers store them in a reservoir trap for experiments. BASE uses a cryogenically-cooled trap that captures the antiprotons with electrical and magnetic fields, since no physical container can hold antimatter. This team added an additional trap in an attempt to make the most precise measurements ever, to what scientists call a “parts-per-billion level of uncertainty.”
The team wrote in their paper that “the extraordinary difficulty in measuring [the magnetic moment of an antiproton] with high precision is caused by its intrinsic smallness; for example, it is 660 times smaller than the magnetic moment of the positron.”
“The measurement of antiprotons was extremely difficult and we had been working on it for ten years,” said Stefan Ulmer, spokesperson of the BASE group. “The final breakthrough came with the revolutionary idea of performing the measurement with two particles…. That created higher precision in the measurements.”
They created and captured the antiprotons in 2015 and held them for a record-breaking 405 days in the traps.
The team was able to measure the magnetic force of the antiproton to a level that is 350 times more precise than ever before. They said this is the equivalent of measuring the circumference of the earth to a precision of four centimeters.
According to their study, antiprotons have a magnetic moment measurement of −2.792847344142, while the proton’s magnetic moment is 2.7928473509. This is almost exactly the same measurement, except for the sign, and the slight difference is well within the experiment’s margin of error margin. The team said if there’s a difference between the magnetic moment of protons and antiprotons, it certainly must be smaller than the experiment can currently detect.
But with the improvements they were able to make with their study, they hope to improve them even further in the future. But for now, the mystery remains.
“Further improvements in the measurement precision of the antiproton magnetic moment using our method are possible,” they wrote in their paper. “We expect that with a technically revised apparatus, including improved magnetic shielding, an improved resistive cooling system for the cyclotron mode with lower temperature, and a precision trap with a more homogeneous magnetic field, it will be possible to achieve a tenfold improvement in the limit on… interactions from proton/antiproton magnetic moment comparisons in the future.”
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