One of the greatest mysteries in modern physics is why the Universe seems to contain mostly matter and almost no antimatter. This observation could be explained if a property of nature called charge–parity–time (CPT) symmetry is violated. Under CPT symmetry, the physics of particles and their antiparticles is identical. A tiny violation of CPT symmetry during the Big Bang could, in principle, be responsible for the lack of antiparticles in the Universe. In a paper in Nature, the ALPHA Collaboration1 reports high-precision spectroscopic measurements of antihydrogen — an atom comprising an antiproton and a positron (the antiparticle of an electron). The authors find that the gaps between energy levels in antihydrogen are in excellent agreement with those measured previously in ordinary hydrogen2–4, placing strong constraints on potential CPT violation.

Read the paper: Investigation of the fine structure of antihydrogen

Tests of CPT symmetry using individual particles — such as neutral kaons5, positrons6 and antiprotons7,8 — have shown no sign of CPT violation. However, studies of antihydrogen might probe the influence of factors that were not explored in previous tests.

Hydrogen is the simplest atom, and its properties can be calculated with impressive precision. For more than a century, the study of this atom has been the driving force behind groundbreaking ideas about the structure of matter. The optical spectrum of hydrogen was measured with great accuracy in the 1880s, before being quantitatively explained in the 1910s. The structure of the atom was then at the heart of the formulation of quantum mechanics and in the generalization of this theory to relativistic (fast-moving) particles in the 1920s. And it was the unexpected discovery9 of an energy gap between the 2S and 2P 1/2 excited states of hydrogen by the physicist Willis Lamb in 1947 that motivated the development of quantum electrodynamics — the theory that describes the interactions between particles and light.

This energy gap, known as the Lamb shift, exists in both hydrogen and antihydrogen. It originates mostly from quantum fluctuations, whereby particle–antiparticle pairs spontaneously emerge in empty space and then instantly annihilate each other. However, its magnitude is subtly affected by, for example, the charge radius (the spatial extent of the charge distribution) of the proton or antiproton, the weak nuclear force and, potentially, currently unknown phenomena that could be the source of the matter–antimatter asymmetry in the Universe.

Antimatter may matter

The current work was carried out using the ALPHA experiment at CERN, Europe’s particle-physics laboratory near Geneva, Switzerland. A facility called the Antiproton Decelerator delivers antiprotons to this experiment, with a source of radioactive sodium providing positrons. Every few minutes, 90,000 cold trapped antiprotons and 3 million positrons are mixed in a sophisticated charged-particle trap. This process yields about 20 cold antihydrogen atoms that are then confined to a neutral-atom trap made from superconducting magnets. These antihydrogen atoms can be stored10 for at least 60 hours, and production cycles can be repeated to obtain hundreds of such atoms.

The aim of the present study was to measure the energy differences between the 1S ground state and the 2P 1/2 and 2P 3/2 excited states of antihydrogen (Fig. 1). The ALPHA Collaboration used an approach called laser spectroscopy, which involved injecting pulses of laser light into the antihydrogen trap. This injection caused atoms to transition from the 1S state to the 2P 1/2 or 2P 3/2 state and to subsequently decay back to the 1S state. Atoms that ended up in a different magnetic substate of the 1S state from the one in which they started were expelled from the magnetic neutral-atom trap. These antihydrogen atoms then annihilated on contact with regular atoms in the walls of the ALPHA apparatus to produce particles called charged pions.

Figure 1 | Lowest-energy states of antihydrogen. The ALPHA Collaboration1 carried out high-precision spectroscopic measurements of antihydrogen — the antimatter counterpart of hydrogen. Specifically, the team determined the energy differences between the 1S ground state and the 2P 1/2 and 2P 3/2 excited states of antihydrogen. They used these results to estimate the fine-structure splitting (the 2P 1/2 –2P 3/2 energy gap). They also combined their previous determination11 of the energy gap between the 1S and 2S states with their current measurement of the 1S–2P 1/2 energy difference to infer the Lamb shift (the 2S–2P 1/2 energy gap). The authors found that all of these results are in agreement with the corresponding ones for ordinary hydrogen. (Drawing not to scale.)

The ALPHA Collaboration plotted the number of observed charged pions as a function of the frequency of the laser light. They then used the positions of the two peaks in these plots to infer the 1S–2P 1/2 and 1S–2P 3/2 energy differences in antihydrogen. These differences agree with the ones measured in ordinary hydrogen at the level of 16 parts per billion. The authors used their results to estimate the fine-structure splitting (the 2P 1/2 –2P 3/2 energy difference) in antihydrogen, with an uncertainty of 0.5%. This value is again in good agreement with the one for ordinary hydrogen.

In 2018, the ALPHA Collaboration measured the energy gap between the 1S and 2S states in antihydrogen11 to one part in 1012. In the current work, the authors combined this result with their measurement of the 1S–2P 1/2 energy difference to provide an estimate of the Lamb shift in antihydrogen. This value has an uncertainty of 11% (or 3.3%, when the fine-structure splitting in ordinary hydrogen is used in the analysis).

Over the past few years, high-precision laser spectroscopy of antihydrogen has become possible, and the ALPHA Collaboration has achieved spectacular progress. An examination of several transitions in antihydrogen would enable targeted tests of CPT symmetry, quantum electrodynamics and the standard model of particle physics. For example, a measurement of the Lamb shift with an uncertainty of less than one part in 104 would allow the antiproton charge radius to be determined12. Moreover, improved measurements of the energy gap between magnetic substates in antihydrogen would provide detailed information about the magnetic structure of the antiproton13.