At the heart of the antimatter mystery

An extremely accurate measurement of the proton’s magnetic moment could help to explain the surplus of matter in the Universe

Fractions of a second after the Big Bang, matter and antimatter formed in equal measure – only to annihilate each other again. But a small surplus of matter survived and formed the Universe we know today. What caused this small surplus has remained one of the great mysteries of physics. A precise comparison of the properties of matter and antimatter could contribute to finding a solution. One of these properties is the magnetic moment of the proton, which a scientific cooperation, including researchers from the Max Planck Institute for Nuclear Physics in Heidelberg, has now determined with a precision never achieved before. In the next step, the scientists want to measure the magnetic moment of the antiproton.

Precision instrument for difficult measurements: A single proton is stored in the gold-plated Penning trap. It consists of annular electrodes which are separated by insulating rings of synthetic sapphire. The magnetic moment of the proton can be determined from its oscillations in the trap. © C. Rodegheri, MPI for Nuclear Physics Precision instrument for difficult measurements: A single proton is stored in the gold-plated Penning trap. It consists of annular electrodes which are separated by insulating rings of synthetic sapphire. The magnetic moment of the proton can be determined from its oscillations in the trap. © C. Rodegheri, MPI for Nuclear Physics

We owe our existence to a very tiny crack in the cosmic mirror. Matter and its mirror image, antimatter, were created in almost equal proportions during the hot formation phase of the Universe. Since everything was very close together in the hot infant universe, the antagonistic, fraternal matter and antimatter met and annihilated each other in the process. The echo of this huge explosion still reverberates today in the cosmic background radiation. If the annihilation of matter had occurred with perfect symmetry back then, our universe would have transformed into a bubble of pure radiation which expands and cools without any further interesting events. The fact that there are galaxies, stars, planets and ourselves is thanks only to a slight error in the cosmic accounting. A tiny deviation from the perfect mirror symmetry between matter and antimatter could have ensured the survival of a slight surplus of matter.

The question of what caused this tiny crack in the cosmic mirror is one of the major, as yet unsolved questions of physics. For decades, various physics disciplines have been using different strategies in the search for a solution. A promising approach consists in comparing fundamental properties of the building blocks of matter with their antimatter mirror images. Attractive candidates for such a programme of comparisons are the proton and the antiproton. The former is one of the building blocks of atomic nuclei, together with the neutron. It also teams up with an electron to form hydrogen, the simplest and most abundant element in the universe.

A new precision measurement 42 years after the most accurate measurement until then

The proton not only carries an electric charge, it is 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, i.e. the strength of its magnetism quasi, with a precision never achieved before. “The most precise measurement until then was 42 years old and what’s more, it was only an indirect measurement,” says Klaus Blaum. “Its interpretation required many additional assumptions, which represents a limitation.”

The oscillating proton (red) generates a tiny current that is detected by highly sensitive electronics. The magnetic moment of the proton is indicated by a red arrow, the green lines trace out the magnetic field in the trap. The proton can align itself in two opposing directions in the magnetic field so that it oscillates in the trap with different speeds. The strength of the magnetic moment can be determined from the oscillation frequencies. © G. Schneider, University of Mainz The oscillating proton (red) generates a tiny current that is detected by highly sensitive electronics. The magnetic moment of the proton is indicated by a red arrow, the green lines trace out the magnetic field in the trap. The proton can align itself in two opposing directions in the magnetic field so that it oscillates in the trap with different speeds. The strength of the magnetic moment can be determined from the oscillation frequencies. © G. Schneider, University of Mainz

The Director at the Max Planck Institute for Nuclear Physics in Heidelberg is a member of a team participating in the cooperation. He attempts to make the huge technical challenge comprehensible to lay persons. “If one thinks of the proton as a small bar magnet, then its magnetic moment is 24 orders of magnitude, i.e. one millionth of a billionth of a billion, weaker than a typical compass needle,” explains the physicist: “And this comparison also applies to the ratio of the moment of this compass needle to the magnetic field of Earth as a whole.” Many years of development work were necessary simply to trap a single proton and store it. The experiment, which requires an almost perfect vacuum, among other things, is located at the Johannes Gutenberg University of Mainz. “We can meanwhile store a proton in our trap for a year,” says Blaum, “this is how good the vacuum is.”

The apparatus is based on the principle of the so-called Penning trap. Andreas Mooser, who has worked for five years on setting up the experiment, first as an undergraduate and then as a doctoral student, explains: “We trap the single proton in free space with skilfully chosen electric and magnetic fields.” But how does he know whether the tiny particle is stored in the trap at all? A stored proton oscillates to and fro in the trap, almost like the pendulum of a clock. With its charge, it thus generates an extremely weak current, which the highly sensitive apparatus can detect as the proton’s signal. “It involves tiny currents on the femto-ampere scale,” is how Andreas Mooser emphasises the challenge. As a comparison: A standard AA battery can supply up to ten amperes of current for a short time; one femto-ampere is ten million billion times weaker.

The magnetic moment of the proton is derived from its oscillations

The principle of the method used in the Mainz-based experiment is to determine the spatial alignment of the proton as a tiny bar magnet. To this end, the scientists use the strange rules of the quantum world, which state that the proton, as a small compass needle, may point only in one of two opposing directions in an external magnetic field. The proton oscillates faster or slower in the trap, depending on its alignment. The physics Nobel laureate Hans Georg Dehmelt developed this method back in the 1980s to measure the magnetic moment of the electron. “Since the magnetic moment of the proton is almost 700 times smaller, this represents a special challenge,” says Klaus Blaum. Therefore, another thirty years were needed before this method was successfully transferred to the proton.

The team has employed this method to determine the magnetic moment of the proton with very high precision and only a tiny error. This error is of the order of one billionth of the measured value. The precision here is so unbelievable that the cooperation group wants to measure the magnetic moment of the antiproton using the same method. To this end, a team headed by Stefan Ulmer from the Japanese RIKEN Institute is setting up an identical experiment at an antiproton source at the CERN European research laboratory in Geneva. Should the team even discover a different value for the antiproton, this would be an important step in solving the antimatter mystery. “This would be a clue to a new physics outside the Standard Model of today’s particle physics,” says Blaum. He and Andreas Mooser, who will be a postdoc when he goes to Geneva, are duly excited about the antiproton experiment.

RW