Antiprotons show no hint of unexpected matter-antimatter differences

Jan 27, 2020
Protons and their antimatter counterparts mirror one another in a new ultra-precise measurement

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The BASE experiment at the European particle physics laboratory CERN uses an electromagnetic device (shown) to trap antiprotons and electrically charged hydrogen atoms. CERN

By Emily Conover
JANUARY 5, 2022 AT 11:00 AM

It’s confirmed: Protons and antiprotons are well-matched. The two types of subatomic particles mirror each other in the ratios of their electric charge to mass, a new extremely precise experiment verifies.

Antiprotons are the antimatter counterpart of protons. Every type of matter particle has such an alter ego, with similar properties but opposite electric charge. But antimatter is an enigma: Scientists still don’t understand why matter is common in the universe while antimatter is rare (SN: 11/25/19). To investigate the origins of this asymmetry, scientists keep checking, to ever greater precision, for differences between matter and antimatter particles, which could hint at how matter came to dominate the cosmos.

The Baryon Antibaryon Symmetry Experiment, or BASE, at the European particle physics laboratory CERN, near Geneva, measures the oscillations of a single antiproton confined within an electromagnetic trap. These oscillations, which reveal the charge-to-mass ratio of the antiproton, are compared with those of a trapped hydrogen ion, comprising a proton and two electrons, which give the proton charge-to-mass ratio.

After more than 24,000 of these oscillation comparisons, BASE researchers found that the two charge-to-mass ratios mirror one another with a precision of 1.6 billionths of a percent, the team reports January 5 in Nature. That’s more than four times as precise as the previous measurement (SN: 8/12/15).

The result also tests physicists’ understanding of gravity’s effect on antimatter, says Stefan Ulmer, a spokesperson of BASE and physicist at RIKEN in Wako, Japan. The Earth’s gravitational environment changes as the planet orbits the sun, so if gravity affected protons and antiprotons differently, that effect would have surfaced during the year and a half over which the data were taken. “We have shown that antimatter and matter interact with gravity … in an exactly identical way,” to within an uncertainty of 3 percent, Ulmer says.

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M.J. Borchertet al. A 16-parts-per-trillion measurement of the antiproton-to-proton charge–mass ratio. Nature. Published online January 5, 2022. doi: 10.1038/s41586-021-04203-w.

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

Matter is anything that does not contain antileptons and antiquarks. Leptons and quarks combine to form atoms. Atoms combine to form molecules. Atoms and molecules can be named as matter. However, electrons are a type of leptons and protons and neutrons are made of quark particles. Therefore, all these definitions reinforce the idea that matter is anything having a mass and a volume and not antimatter.

Antimatter is the opposite of matter. For example, proton and antiproton are a pair of matter and antimatter, respectively. The matter and antimatter pairs have the same mass but opposite electrical charges. They have some differences in quantum properties as well. e.g. a proton is positively charged while an antiproton has a negative charge.

A collision between matter and antimatter leads to mutual annihilation. It means both matter and antimatter convert into other particles having equal energies. This annihilation can give rise to intense photons such as gamma rays, neutrinos, and some other particle-antiparticle pairs. However, the majority of the energy released from annihilation is in the form of ionizing radiation.

Similar to matter, antimatter particles can bind with each to form antimatter. For instance, a positron is the antiparticle of the electron, while antiproton is the antiparticle of a proton; these two antiparticles can bind to form an antihydrogen atom. Antimatter is denoted by using a bar sign over the particle’s symbol in order to distinguish it from matter.

The key difference between matter and antimatter is that matter and antimatter have opposite electrical charges. Antimatter is basically the opposite of matter, but they have almost identical properties other than electrical charge.