An Unimaginably Sharp Image of Antiprotons

Researchers of the Baryon-Antibaryon-Symmetry experiment (BASE) at CERN have achieved a remarkable success: They have determined the magnetic moment of the antiproton with a previously unattained accuracy. The measurement is more precise than the best measurement for the magnetic moment of the proton.

Blicke ins BASE-Experiment am CERN.
Image: BASE Collaboration

In our everyday world we are surrounded by matter. Antimatter, however, in another matter: we have no concept of it because there is none perceptible to our senses. This is likely the reason why antimatter seems so mysterious to the non-physicist. For physicists, however, antimater has become a “normal” phenomenon. In 1928 the British physicist Paul Dirac, with the help of quantum mechanics and special relativity theory, developed a relativistic description of electrons, this today is known as the Dirac equation. This equation yields two solutions, one for negatively charged electrons, and another for positively charged electrons, which are called antielectrons or positrons. The American Carl David Anderson detected in 1932 traces of positively charged electrons in a cloud chamber, with which he examined cosmic radiation.

Term Already Used in the 19th Century

As early as the late 19th century, the existence of “antimatter” or “negative matter” – even before the development of quantum mechanics - was postulated. These old ideas, however, were all discarded. In the standard model of particle physics, there is a corresponding antiparticle for every (matter) particle, with exactly the same properties but reversed charge. Neutral particles like the photon, the Higgs, and the Z0 boson are identical to their antiparticles. Neutrinos differ from antineutrinos by their spin--neutrinos are left-turning, while antineutrinos are right-turning.

In the universe, no large masses of antimatter could be detected. It was assumed that shortly after the Big Bang, the whole of antimatter was destroyed when it came into contact with matter. Only a small imbalance in matter-antimatter symmetry remained after this mutual annihilation, in which a tiny fraction of matter material was left; from this the visible universe exits with its galaxies, stars and planets. Antimatter can be generated during particle collisions. Positrons, i.e. antielectrons, arise during the beta-decay of radioactive nuclei.

Antiparticles are More Precisely Determined as Matter Particles

Despite its fleeting nature to the human observer, antimatter is important to modern physics. This is illustrated by a recent report from the european particle physics laboratory CERN in Geneva: A team of scientists from the BASE experiment succeeded in determining a property of the antiproton with unknown accuracy. According to recent measurements, the magnetic moment of the antiproton is 2.792 847 344 1 (42) μN (where μN = eℏ / 2mp = 3.15x10-8eV / T is the nuclear magneton). The scientists involved reported the measurements in an article published on 19 October 2017 in the journal Nature.

Protons have a magnetic moment, which is also referred to as a “magnetic dipole moment.” The magnetic moment is a measure of the strength of a magnetic dipole, the elementary unit of a magnet. The magnetic moment expresses how strongly the corresponding particle reacts to the surrounding magnetic field. The new measurement result of the BASE experiment is not only remarkable for its high precision, but also because with the above-mentioned value, the magnetic moment of the antiproton is now determined more precisely than that of the proton. In this case, it is the first time ever that researchers have been able to determine the property of antimatter more precisely than that of an associated matter particle, reported the researchers of the BASE experiment in a press release.

Identical Properties of Matter and Antimatter

“The measurement of the BASE experiment is a remarkable result,” says the particle physicist Dr. Hans Peter Beck, P.D. (University of Bern / CERN). “The magnetic moment of the antiproton could be determined more precisely than that of the proton. This does not mean, however, that the two values differ from each other. In fact, the previous measurements for proton and antiproton are in accordance with the recent precise measurement,” says Beck. This corresponds to what was expected on the basis of the applicable standard model of particle physics. This accordance is also confirmed by previous measurements, which have determined the mass of the proton and the antiproton.

The BASE-collaboration is a group of researchers from Germany and Japan. With the recently published precision measurement, the BASE collaboration has surpassed its own precision record by a factor of 350, which it had set in January of this year. For the measurement, a new method was used, in which two antiprotons were made separately in two so-called Penning traps.

Author: Benedikt Vogel

Blicke ins BASE-Experiment am CERN.
Blicke ins BASE-Experiment am CERN.Image: BASE Collaboration

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