Can a proton survive on its own?

At the core of the antimatter puzzle

An extremely accurate measurement of the magnetic moment of a proton could help explain the excess of matter in the universe

Fractions of a second after the Big Bang, matter and antimatter were created in equal quantities - in order to annihilate each other again. But a small excess of matter survived and shaped the universe we know today. The cause of this small excess is one of the greatest puzzles in physics. A precise comparison of the properties of matter and antimatter could help solve it. One of these properties is the magnetic moment of the proton, which a scientific cooperation has now determined more precisely than ever before. Researchers from the Max Planck Institute for Nuclear Physics in Heidelberg were involved. Next, the scientists want to measure the magnetic moment of the antiproton.

It's a tiny leap in the cosmic mirror to which we owe our existence. In the hot phase of the birth of the universe, matter and its mirror image, the antimatter, were created in almost equal proportions. Since it was very close in the hot baby universe, the opposing brothers of matter met and radiated away. The echo of this huge explosion reverberates in the cosmic background radiation to this day. If the destruction of matter had been perfectly symmetrical at that time, our universe would have turned into a bubble made of pure radiation, which expands and cools down without any further exciting events. But the fact that there are galaxies, stars, planets and ourselves is due to a small mistake in cosmic bookkeeping. A tiny deviation from the perfect mirror symmetry between matter and antimatter could have ensured the survival of the small excess of matter.

The question of what caused this tiny jump in the cosmic mirror is one of the great as yet unsolved questions in physics. For decades, different physical disciplines with different strategies have been looking for a solution. A promising approach is to compare the fundamental properties of building blocks of matter with their antimatter mirror images. Attractive candidates for such a comparison program are the proton and the antiproton. The former is - next to the neutron - one of the building blocks of the atomic nucleus. Together with an electron, it also forms hydrogen, the simplest and most common element in the universe.

A new precision measurement 42 years after the most accurate determination to date

The proton is not only electrically charged, but also magnetic. This magnetism is a promising item on the scientific matter-antimatter checklist. An international cooperation has now succeeded in measuring the magnetic moment of the proton, in other words the strength of its magnetism, with unprecedented precision. “The most accurate measurement to date was 42 years old and, moreover, only indirect,” says Klaus Blaum. "Your interpretation required many additional assumptions, which is a limitation."

The director at the Max Planck Institute for Nuclear Physics in Heidelberg is involved in the cooperation with a team. He tries to make the enormous technical challenge tangible for laypeople. "If you look at the proton as a small bar magnet, then its magnetic moment is around 24 orders of magnitude, that is, a millionth of a billionth of a billionth, weaker than a typical compass needle," explains the physicist: "The moment of this compass needle is in turn related to the magnetic field in exactly the same way of the entire earth. ”Capturing and storing a single proton required years of development work. The experiment, for which you need an almost complete vacuum, is at the Johannes Gutenberg University in Mainz. "We can now store such a proton in our trap for a year," says Blaum, "that's how good the vacuum is."

The apparatus is based on the principle of the so-called Penning trap. Andreas Mooser, who helped set up the experiment as a graduate student and then as a doctoral student, explains it: “We hold the individual proton in free space with cleverly chosen electric and magnetic fields.” But how do you tell if it's tiny Particle is stored in the trap at all? A stored proton swings back and forth in the trap, almost like a clock pendulum. With its charge, it produces an extremely weak current that the highly sensitive apparatus can detect as a signal from the proton. "It's all about tiny femto-ampere currents," says Andreas Mooser, emphasizing the challenge. For comparison: a standard AA cell can briefly deliver up to ten amperes of current, a femtoampere is ten billion billion times weaker.

Its magnetic moment results from the oscillation of the proton

At its core, the method used in the Mainz experiment is about determining the spatial orientation of the proton as a tiny bar magnet. To do this, the scientists use the strange rules of the quantum world, according to which the proton is only allowed to point in two opposite directions as a small compass needle in an externally applied magnetic field. Depending on the orientation, the proton swings faster or slower in the trap. Nobel laureate in physics, Hans Georg Dehmelt, used this method back in the 1980s to measure the magnetic moment of the electron. “But since the magnetic moment of the proton is almost 700 times smaller, this poses a special challenge,” says Klaus Blaum. It took another thirty years before this method could be transferred to the proton.

With this method, the team determined the magnetic moment of the proton with the exception of a tiny error. This error is on the order of a billionth of the reading. This is so incredibly precise that the cooperation aims to measure the magnetic moment of the antiproton using the same method. To this end, a team led by Stefan Ulmer from the Japanese RIKEN Institute is setting up an identical experiment on an antiproton source at the European research laboratory CERN in Geneva. Should the team even discover a different value for the antiproton, that would be an important step towards solving the antimatter riddle. “That would then be an indication of new physics outside of the standard model of today's particle physics,” says Blaum. Accordingly, he and Andreas Mooser, who will be a postdoctoral fellow in Geneva, are excited about the antiproton experiment.

RW