Dark matter and the imbalance between matter and antimatter are two of the biggest mysteries of the universe. Astronomical observations tell us that dark matter makes up most of the matter in the cosmos but we do not know what it is made of. On the other hand, theories of the early universe predict that both antimatter and matter should have been produced in equal amounts, yet for some reason matter prevailed. Could there be a relation between this matter-antimatter asymmetry and dark matter?
In a paper published today in the journal Nature, the BASE collaboration reports the first laboratory search for an interaction between antimatter and a dark-matter candidate, the hypothetical axion. A possible interaction would not only establish the origin of dark matter, but would also revolutionise long-established certainties about the symmetry properties of nature. Working at CERN's antimatter factory, the BASE team obtained the first laboratory-based limits on the existence of dark-matter axions, assuming that they prefer to interact with antimatter rather than with matter.
Axions were originally introduced to explain the symmetry properties of the strong force, which binds quarks into protons and neutrons, and protons and neutrons into nuclei. Their existence is also predicted by many theories beyond the Standard Model, notably superstring theories. They would be light and interact very weakly with other particles. Being stable, axions produced during the Big Bang would still be present throughout the universe, possibly accounting for observed dark matter. The so-called wave-particle duality of quantum mechanics would cause the dark-matter axion's field to oscillate, at a frequency proportional to the axion's mass. This oscillation would vary the intensity of this field's interactions with matter and antimatter in the laboratory, inducing periodic variations in their properties.
Laboratory experiments made with ordinary matter have so far shown no evidence of these oscillations, setting stringent limits on the existence of cosmic axions. The established laws of physics predict that axions interact in the same way with protons and antiprotons (the antiparticles of protons), but it is the role of experiments to challenge such wisdom, in this particular case by directly probing the existence of dark-matter axions using antiprotons.
In their study, the BASE researchers searched for the oscillations in the rotational motion of the antiproton's magnetic moment or "spin" - think of the wobbling motion of a spinning top just before it stops spinning; it spins around its rotational axis and "precesses" around a vertical axis. An unexpectedly large axion-antiproton interaction strength would lead to variations in the frequency of this precession.
To look for the oscillations, the researchers first took antiprotons from CERN's antimatter factory, the only place in the world where antiprotons are created on a daily basis. They then confined them in a device called a Penning trap to avoid their coming into contact with ordinary matter and annihilating. Next, they fed a single antiproton into a high-precision multi-Penning trap to measure and flip its spin state. By performing these measurements almost a thousand times over the course of about three months, they were able to determine a time-averaged frequency of the antiproton's precession of around 80 megahertz with an uncertainty of 120 millihertz. By looking for regular time variations of the individual measurements over their three-month-long experimental campaign, they were able to probe any possible axion-antiproton interaction for many values of the axion mass.
The BASE researchers were not able to detect any such variations in their measurements that would reveal a possible axion-antiproton interaction. However, the lack of this signal allowed them to put lower limits on the axion-antiproton interaction strength for a range of possible axion masses. These laboratory-based limits range from 0.1 GeV to 0.6 GeV depending on the assumed axion mass. For comparison, the most precise matter-based experiments achieve much more stringent limits, between about 10 000 and 1 000 000 GeV. This shows that today's experimental sensitivity would require a major violation of established symmetry properties in order to reveal a possible signal.
If axions were not a dominant component of dark matter, they could nevertheless be directly produced during the collapse and explosion of stars as supernovae, and limits on their interaction strength with protons or antiprotons could be extracted by examining the evolution of such stellar explosions. The observation of the explosion of the famous supernova SN1987A, however, set constraints on the axion-antiproton interaction strength that are about 100 000 times weaker than those obtained by BASE.
The new measurements by the BASE collaboration, which teamed up with researchers from the Helmholtz Institute Mainz for this study, provide a novel way to probe dark matter and its possible interaction with antimatter. While relying on specific assumptions about the nature of dark matter and on the pattern of the matter-antimatter asymmetry, the experiment's results are a unique probe of unexpected new phenomena, which could unveil extraordinary modifications to our established understanding of how the universe works.
