Merging the theory of relativity and the microscopic world of quantum physics into a single framework has yielded some profound insights into the nature of the Universe. Among them is the concept of antiparticles, the elusive twins of ordinary particles such as the electron, the proton and the neutron. But determining the properties of these particle twins is a difficult task, because they don’t mix well — to the extent that, on contact, a particle–antiparticle pair annihilates, leaving only a flash of intense light. However, the theories of relativity and quantum physics also imply that a mirror-like symmetry exists, called CPT invariance, that ensures that a particle and its antiparticle have the same mass, but opposite charge. Writing in Nature, Borchert et al.1 report the most stringent test of this aspect of mirror symmetry performed so far, by studying the motion of single negatively charged hydrogen ions and antiprotons in an apparatus known as a Penning trap.
The challenges posed by antimatter experiments are often circumvented by establishing physical laws using matter and then extrapolating them to antimatter by assuming CPT invariance. This approach has been extraordinarily successful, and will indisputably continue to hold sway in fundamental-physics research. This fact alone highlights the necessity for improved CPT tests.
But there is also a more intriguing side to mirror symmetry. Certain observations are difficult, although not impossible, to reconcile with our current CPT-invariant laws. One such observation is the imbalance of matter and antimatter in the Universe, despite the prevailing view that the Big Bang created equal quantities of both. There are also conceptual issues associated with our existing laws that could be resolved by allowing minuscule departures from mirror symmetry. These exciting developments have imparted particular urgency to the need for improved CPT tests2. Observations of departures from mirror symmetry, however small, would have a huge impact, requiring serious revisions to relativity, quantum physics or both.
In preparation for the authors’ measurement of the charge-to-mass ratio of antiprotons, these antiparticles were created at CERN, the European particle-physics laboratory near Geneva, Switzerland. Borchert et al. stored the antiparticles in a Penning trap, a device designed to suspend charged particles or antiparticles inside a vacuum with carefully chosen magnetic and electric fields, ready to be deployed for experiments. Such a delicate storage method prevents the antiprotons from making contact with ordinary matter, which would lead to their immediate disintegration.
The key idea behind the measurement is based on the circular trajectory of a charged particle when it moves in a constant magnetic field, such as that in a device known as a cyclotron: the rotational frequency of this motion is proportional to the antiproton’s charge-to-mass ratio. For the actual measurement, Borchert et al. extracted a single antiproton from storage and shuttled it to a region of the trap that had judiciously tuned electromagnetic fields. This region was equipped with ultrasensitive detectors capable of picking up the motion of the single antiproton by detecting its electromagnetic signature in a circuit. The authors obtained the cyclotron frequency from this motion.
In principle, comparing this antiproton measurement with the corresponding measurement for the proton provides a test of mirror symmetry. However, the proton’s opposite electric charge requires an inversion of the electric trapping field, an adjustment that introduces large systematic errors. So, instead, Borchert et al. used a negatively charged hydrogen ion. The effects of introducing two electrons to a proton to produce this ion are well understood, so the authors were able to correct for these effects in their data analysis. The final result was the ratio of the cyclotron frequencies of the antiproton and the hydrogen ion, minimizing uncertainties due to possible irregularities in the magnetic field. Borchert and colleagues measured the magnitude of this ratio to be one, with a fractional uncertainty of 16 parts per trillion. This value is precise enough to be considered confirmation of mirror symmetry, and it improves the precision of the previous best test3 by a factor of 4.3.
This measurement also places severe constraints on deviations from Albert Einstein’s special theory of relativity, one of the pillars on which mirror symmetry rests, when interpreted within a framework known as the standard-model extension. This framework predicts the observable effects of hypothetical departures from Einstein’s ideas in both general relativity, the theory that governs gravity, and the standard model of particle physics, the theory that describes how fundamental particles interact with each other. The standard-model extension contains coefficients describing physics that is dependent on the orientation of the laboratory, but this dependence is incompatible with the principle of relativity. The fact that Borchert et al. measured a constant cyclotron-frequency ratio as the orientation of the Penning-trap apparatus changed with Earth’s rotation implies that the measurement was in agreement with relativity — an observation that provides four times the precision of previous limits for ten of the coefficients of the standard-model extension.
The authors also used the Penning trap to study the question of whether antimatter obeys the weak equivalence principle. This principle states that all bodies in the same gravitational field experience the same acceleration, regardless of their properties. Its origin is often attributed to Galileo Galilei, who is said to have dropped unequal masses from the top of the Leaning Tower of Pisa in Italy. To test whether the antiproton has an anomalous coupling to the gravitational field, in violation of the weak equivalence principle, the team used cyclotron-frequency data collected over extended periods spanning months. On such timescales, the distance between Earth and the Sun varies enough to induce changes in the Sun’s gravitational potential at the location of the Penning trap (Fig. 1). This made it possible to estimate constraints on how much the measured cyclotron-frequency ratio varied with the gravitational potential — the world’s first such analysis.
The next step will be to design a portable trap for antiprotons, which would allow cyclotron-frequency measurements to be performed in a low-noise laboratory without the disturbances present at the antiproton facility at CERN4. Other developments might take inspiration from quantum computing. A closely related mirror-symmetry test exists that involves the anomaly frequency — the difference between the cyclotron frequency and the precession frequency of the particle’s spin angular momentum in a magnetic field. A comparison of the anomaly frequencies of the proton and antiproton in a Penning trap could benefit from laser-based techniques that were originally developed for manipulating ions in quantum-computing experiments5. Finally, theoretical insights into mirror-symmetry violations in the context of gravity, reported last year6, have the potential to provide much more sophisticated interpretations of tests of CPT invariance. With these developments on the horizon, the future for mirror-symmetry studies involving antiprotons looks bright indeed.