In a recent paper, the BASE collaboration presented results from a direct search for interactions of antimatter with dark matter and placed direct constraints on the interaction of ultralight axion-like particles (dark-matter candidates) with antiprotons. Their analysis constrained the value of the axion–antiproton interaction parameter in the range of  0.1 to 0.6 GeV for axion mass in the range of 2 × 10⁻²³ to 4 × 10⁻¹⁷ eV.

The authors present also astrophysical limits on the axion-antiproton interaction up to a mass of 0.01 eV by considering a hypothetical axion emission from antiprotons in supernova SN1987A, which represents the current best limit from such kinds of observations. The new laboratory result reported by BASE improves the sensitivity for ultra-light axion-like particles compared to the limits from astrophysical observations by about a factor of 100 000. The same analysis also sets limits on six combinations of previously unconstrained Lorentz- and CPT-violating terms of a model, which discusses effects of CPT-and Lorentz-violating coefficients on the standard model, recognized as the non-minimal standard model extension.

It is the first time that antimatter is used as an antenna to probe the existence of new particles like the hypothesized axion (see https://ep-news.web.cern.ch/content/rise-axions). In 1977, Robert Peccei and Helen Quinn developed a theory introducing a new field that could resolve the “strong CP problem” - the fact why CP violation is not observed in strong interactions. The proposed mechanism, in turn, led to the invention of CP-axions and, more general, axion-like particles, which are excellent dark matter candidates.

Many experiments around the world have launched extensive DM axion searches to cover the relevant parameter space. Existing theoretical underpinnings of the axion do not predict a value for the axion mass. Cosmological considerations indicate that if the axion is the source of the cold dark matter in the universe, it should have a mass in the range of about 1–25 μeV∕c² - about 10¹⁵ times smaller than the masses of weakly interacting massive particles (WIMPs). With such a small mass, in our Solar System, trillions of axions per cubic centimeter are required to account for the observed dark matter density. However, the interactions of axions with ordinary matter and photons are expected to be so feeble that their detection requires extremely sensitive techniques.

So far, intense axion searches have not given any hints for the existence of this particle while the variety of experimental approaches has allowed to exclude certain parts of the parameter space leaving though still a lot of room for future experimental searches. The lack of experimental evidence inspired the BASE collaboration to come up with the approach to search for axions within a certain mass range by probing possible axion-effects on antimatter. Using a single antiproton in a Penning trap, the BASE team searched for possible modulations in the frequency at which the spin of the antiproton precesses, as illustrated in Fig. 1. Such modulations would hypothetically be imposed by oscillating axions, which couple to the antiproton spin.

Figure 1. Illustration of the detection principle. The antiproton precesses in the magnetic field of the Penning trap magnet, a time series signal would lead to a signal as shown upper right. The axion coupling would modulate this signal to a shape as shown lower left. BASE was analyzing their magnetic moment data set for such modulations. The non-detection of such signals enabled BASE to set first constraints on axion/antiproton coupling.

To record data-sets which enable such studies, BASE stores single antiprotons in ultra-stable, high-precision Penning traps. In the strong magnetic field of the trap the antiprotons oscillate at characteristic frequencies, the spin precesses at the Larmor frequency νL, while the particle oscillates in parallel in the superconducting magnet at a complex trajectory, from which the cyclotron frequency ν_c  is obtained.  Measuring the Larmor-to-cyclotron frequency ratio ν_L/ν_c almost a thousand times over the course of about three months, BASE determined the time-averaged frequency of the antiproton’s precession ν_L of around 80 MHz with an uncertainty of 120 mHz. From this measurement, the BASE scientists were able to improve their previous best measurement of the antiproton magnetic moment by more than a factor of 350. By looking for periodic variations in the time-sequence of this extended experimental data-set, the BASE team was now able to set first direct laboratory limits on axion–antiproton interactions, as shown in Fig. 2. For the interpretation of the results BASE teamed up with Dima Budker and Yevgeny Stadnik, scientists from the PRISMA+ Cluster at the University of Mainz, which have great expertise in dark matter research.

Figure 2. Constraining axion–antiproton interactions. Recently BASE published experimental limits on the coupling between axion dark matter and antiprotons. These bounds are expressed in terms of an axion–antiproton interaction parameter and vary with the axion mass or the frequency of the axion (Credits: BASE Collaboration).

Currently BASE is gearing up the experiment to enable measurements of the antiproton magnetic moment at a precision level of at least 150 parts in a trillion. It is moreover planned to implement experiment schemes which allow for higher sampling rates and longer data-acquisition campaigns. A comparable analysis of these future results would increase the mass-bandwidth and improve the antiproton/axion interaction limits by at least a factor of 10.

The author would like to thank Christian Smorra and Stefan Ulmer for their invaluable help in preparing this article.

Further Reading: 

https://www.nature.com/articles/s41586-019-1727-9