Two of the most intriguing mysteries in modern cosmology are the apparent preponderance of ordinary matter over antimatter and the nature of dark matter, which accounts for about 85% of the mass in the Universe1. Dark matter has made its presence known only through its gravitational effects on astrophysical objects. Therefore, whatever type of particle it is made of must have feeble interactions with other matter. One leading candidate is the axion — a light neutral particle that was originally postulated to explain why the neutron lacks a measurable electric dipole moment2. Until now, researchers have looked for evidence of couplings between axion dark matter and only ordinary particles such as photons, electrons and nuclei3,4. Writing in Nature, Smorra et al.5 report a search for a coupling between axion dark matter and antimatter (specifically, antiprotons).
Every known particle can be classified as either a boson or a fermion. Bosons have integer spin (intrinsic angular momentum), and include the (spin-1) photon and the (spin-0) Higgs boson. By contrast, fermions have half-integer spin, and include the (spin-1/2) electron. The axion is expected to be a spin-0 boson that has odd parity, which means that its wavefunction changes sign if spatial coordinates are flipped.
Unlike fermionic dark matter (such as dark-matter candidates called weakly interacting massive particles, WIMPs), there is no limit to the number of axions that can exist in a certain volume of space. As a result, axion dark matter has an extremely wide range of potential masses. Astrophysical measurements place an upper limit6 on the mass of about 10–2 electronvolts (eV). This value is expressed in units of energy, in which the electron mass is 511 kiloelectronvolts and the proton mass is 938 megaelectronvolts (see go.nature.com/2bwkrqz). And a lower limit7 of about 10–22 eV comes from the fact that, when these particles are described as waves in quantum mechanics, their wavelengths cannot be larger than the size of a dwarf galaxy — otherwise, such galaxies would show deviations from their observed structure.
The particles associated with axion dark matter can be thought of as classical waves that have an oscillation frequency directly proportional to the axion mass. There are several techniques that can be used to look for such waves, and the most appropriate one depends mainly on the frequency range that is being considered. For axions that have masses below 10–17 eV (corresponding to a frequency of tens of millihertz), the waves oscillate extremely slowly. If antiprotons are held in the strong magnetic field of a device known as a Penning trap, these waves will produce changes in the frequency at which the spins of the antiprotons precess.
The Baryon Antibaryon Symmetry Experiment8 (BASE) at the European particle-physics laboratory CERN near Geneva, Switzerland, uses this technique. The BASE collaboration relies on ultrasensitive Penning traps, which use specialized configurations of magnetic and electric fields to trap antiprotons in a high-vacuum environment. This set-up allows the antiprotons to be measured continuously for long periods of time, and to be shuttled back and forth between different measurement chambers without running into ordinary matter and being annihilated. One of the main goals of the collaboration is to determine the intrinsic magnetic moment of the antiproton. This quantity can be calculated to extremely high precision using the standard model of particle physics — the current explanation of the Universe’s particles and forces.
In 2017, Smorra et al. made an ultraprecise measurement of the antiproton’s magnetic moment (to one part in a billion)9, constraining many theories of physics beyond the standard model. The key to their method was the simultaneous measurement of the spin precession and a quantity called the cyclotron frequency, which describes the cyclical motion of an antiproton in a trap. This task was challenging, because it required meticulous control of a device known as a magnetic bottle to non-destructively determine the spin state of the antiproton. The group’s measurement required hundreds of experiments, each of which lasted for almost an hour, taking place over several months.
In the current paper, Smorra and colleagues, who include members of the BASE collaboration, analysed the data from these experiments. They proposed that waves corresponding to axion dark matter that oscillated at frequencies between 10–8 and 10–2 hertz would shift the spin-precession frequency in a small but measurable way if the axion coupling to antiprotons was sufficiently strong. Although no axion signal was detected, Smorra et al. constrained the parameter that quantifies axion–antiproton interactions to values greater than 0.1–0.6 gigaelectronvolts in the axion mass range from 2 × 10−23 eV to 4 × 10−17 eV (Fig. 1). These limits are as much as 105 times stronger than astrophysical constraints (as estimated by the authors), which consider how axions might have been produced by antiprotons in the supernova 1987A.
Future work should aim to further constrain the axion–antiproton coupling and to look for evidence of interactions between axion dark matter and other forms of antimatter, such as positrons (the antiparticles of electrons). One key finding from these studies could be the observation that dark matter couples to antimatter in different ways from its couplings to ordinary matter — a result that might help to explain why there is a predominance of matter over antimatter in the Universe.
Smorra and colleagues have highlighted a growing trend in high-energy physics, whereby exquisitely precise measurements are used to nail down fundamental particle parameters and to look for evidence of physics beyond that of the standard model. Axion dark matter, which has a vast potential mass range and extraordinarily weak predicted couplings, has gone through a renaissance in terms of innovative detection techniques. The search for a preferred coupling of axion dark matter to antimatter (as opposed to ordinary matter) is an exciting prospect, and could prove to be the key to unlocking several mysteries in cosmology as technology improves.
Nature 575, 293-294 (2019)