Antimatter doesn’t differ from charge-mass expectations

No matter how precise the measurement, matter and antimatter look stubbornly similar.

An ultrasensitive experiment comparing protons with their antimatter counterparts found no difference in the ratios of their charge to mass, researchers report in the Aug. 13 Nature. The result is consistent with the standard model of particle physics, which predicts that antiprotons are essentially protons with negative charge — the particles’ mass, spin and nearly every other property should be identical. Many physicists would love to discover even minute discrepancies, which could signal the existence of new particles and forces and help reveal why the universe is made of matter rather than antimatter.

Stefan Ulmer, a particle physicist from RIKEN in Wako, Japan, and colleagues analyzed antiprotons and negative hydrogen ions (a proton plus two electrons) one by one inside an instrument called a Penning trap. The trap’s electric and magnetic fields forced each particle to zip around in a small circle. By counting the number of revolutions per second and factoring in the strength of the trap’s magnetic field, the scientists determined the particle’s charge divided by mass. Data from the hydrogen ions were used to calculate the charge-to-mass ratio for protons.

After repeating the experiment thousands of times, the researchers achieved enough precision to say the charge-mass ratios of protons and antiprotons are equal to within 69 parts per trillion (but with an opposite sign). “They’ve done a very clean experiment,” says Klaus Jungmann, an atomic and particle physicist at the University of Groningen in the Netherlands.

After failing to find fissures in the standard model, the researchers turned their sights to the general theory of relativity but also came up empty. They found that as particles circled inside the Penning trap, gravity acted on protons and antiprotons with the same strength, just as Einstein’s theory predicts.

Ulmer’s next goal is to measure the intrinsic magnetism of the antiproton, which, like charge, should be equal but opposite that of the proton (SN: 6/28/14, p. 15). “If we detect even a small deviation,” he says, “it would have a huge effect on our entire understanding of the laws of nature.”