Optical transition seen in antihydrogen

Precise measurements of antimatter systems might cast light on why the Universe is dominated by matter. The observation of a transition in an antihydrogen atom heralds the next wave of high-precision antimatter studies. See Letter p.506

On page 506, Ahmadi et al.1 report a milestone in antimatter research — the first measurement of a light-induced transition in a pure antimatter atom, antihydrogen. This experimental masterpiece paves the way for investigations of unexplored regions of the antimatter world with unprecedented sensitivity.

In the standard model of particle physics, every particle has its own antimatter counterpart — particles of quasi-identical fundamental properties, the most well-known of which have opposite magnetic moments and electric charge from the matter particle. However, these anti-particles are not just 'model' particles. They can be observed in radioactive decay processes, cosmic rays and high-energy reactions such as particle collisions.

Anti-particles can even be produced using a process called pair production, an application of Einstein's famous equation of mass–energy equivalence, E = mc2. When the kinetic energy (E) of a collision reaction exceeds a certain threshold, collisions naturally produce particles of mass m. The particles are produced in matter–antimatter pairs as a consequence of several conservation laws. Electrons and positrons are examples of matter–antimatter counterparts from the lepton family of particles, whereas protons and antiprotons are from the baryon family.

These reactions can be reversed in processes called annihilations. Some annihilations produce tiny excesses of matter2, but these are almost ten orders of magnitude too small to explain why, on cosmological scales, the Universe is composed almost entirely of matter. This imbalance is one of the most intriguing puzzles and hottest topics in modern physics.

Experimentalists compare the fundamental properties of matter particles with those of their antimatter counterparts with high precision, hoping to find tiny dissimilarities that might contribute to a consistent explanation for the Universal imbalance. In the 1980s, the fundamental properties of charged leptons were compared3 with some precision, down to parts per billion. Then, in the 1990s, techniques were developed to catch and cool antiprotons in electromagnetic containers known as Penning traps. This led to a comparison4 of the charge-to-mass ratio of protons with that of antiprotons with a precision of 90 parts per trillion (p.p.t.). A later study improved the precision to 69 p.p.t. (ref. 5).

Electrically neutral matter–antimatter counterparts have also been investigated. For example, neutral K-meson masses were compared6 with fractional precisions of the order of parts per quintillion (1 quintillon is 1018). The study of another neutral system, antiprotonic helium (which consists of an antiproton and an electron orbiting a helium nucleus), has also enabled precise measurements to be made of the antiproton-to-electron mass ratio7. So far, the examined matter–antimatter counterparts seem to be exact analogues of each other at the achieved levels of experimental uncertainty.

However, the optical spectra of neutral anti-atoms made entirely from antimatter had never been compared to those of their matter counterparts. This experiment is now reported by Ahmadi and colleagues (members of the ALPHA Collaboration at CERN, Europe's particle physics laboratory near Geneva in Switzerland). They have observed the response of trapped antihydrogen atoms to optical light: the transition of the antihydrogen's positron from the 1s ground state to the excited 2s state. The wavelength of the light needed to generate the transition has been determined with a precision greater than nine significant figures, and seems to be identical to that needed to excite the equivalent transition in hydrogen. This achievement is based on the use of a sophisticated experimental machine that took almost 20 years to develop. The measurement required great experimental skill and combined key techniques of particle physics, non-neutral plasma physics, trap physics and high-resolution laser spectroscopy.

Experiments with charged anti-particles use well-established electromagnetic trapping techniques3,4,5, but investigations of antihydrogen require very different approaches. First, the electrically neutral antimatter atoms must be synthesized and trapped. This is immensely difficult, because neutral-atom traps are orders of magnitude more shallow than their ion-trap partners, and require strong magnetic gradients that are technically complicated to produce. The first report8 of antihydrogen trapping was published in 2010 after a series of innovative experimental developments, and achieved a trapping rate of only about 0.11 atoms per trial. Higher trapping rates of about 5 per trial were reported 2 years later9.

The challenge of having such a small number of trapped neutral particles to work with had to be overcome by Ahmadi and colleagues. A key ingredient in their success was their improvement of methods for synthesizing antihydrogen, which increased the trapping rate they could achieve to approximately 14 atoms per trial. This advance greatly increases the strength of the signals that can be obtained from their antihydrogen experiments.

The current result is just the starting point for a multitude of measurements in which antihydrogen is used to test matter–antimatter symmetry. Planned full scans of the 1s–2s and other optical transitions will profit from the use of advanced spectroscopy techniques10 that have yielded fractional uncertainties of the order of just 10−15 in studies of hydrogen.

The ALPHA Collaboration previously reported11 the first spectroscopic measurements of a phenomenon called 'ground-state hyperfine splitting' in antihydrogen, and further such experiments are planned by CERN's ASACUSA collaboration12. The resolution of these measurements will benefit greatly from the improved antihydrogen trapping rate reported by Ahmadi and co-workers. Unlike the 1s–2s transition, hyperfine splitting is a purely magnetic phenomenon, and its spectroscopy will thus probe very different physics from the current experiment.

How precise will future experiments need to be to persuade physicists that matter and antimatter are equivalent? In truth, no result will be precise enough — as long as ideas are available for ways to further reduce experimental uncertainties, the arguments to continue investigating are too strong. Moreover, the reason for studying matter–antimatter counterparts so precisely is to search for as yet unknown, presumably rather weak, interactions, that could manifest themselves at any level of precision. Some of these interactions might exclusively involve antimatter, which thus constitutes a crucial probe to test related aspects of physics beyond the standard model13.

One very weak interaction is gravity. Even the gravitational field of a body as large as Earth causes only fractional redshift effects (a modification in the frequency of electromagnetic radiation) of about 10−16 per metre. Several experiments that will profit from Ahmadi and colleagues' findings — especially the improved trapping rate — are planned to study the gravitational behaviour of antihydrogen. Some model-dependent limits on the gravitational behaviour of antimatter have already been derived from measurements of antiprotons4,5. Studies of falling antihydrogen are planned by the AEGIS Collaboration14 at CERN, and by ALPHA's side-project, ALPHA-g; these will provide model-independent results.

CERN will continue to strongly support antimatter physics research by setting up the Extra Low Energy Antiproton (ELENA) synchrotron, a facility that will enable much more efficient use of antiprotons. CERN's upgrade package also includes a new antihydrogen gravity experiment, GBAR (ref. 15). A key component of this experiment is the production of positively charged antihydrogen ions. If used with methods reported in 2011 for cooling ions16, GBAR will provide colder, and thus more controllable, antihydrogen atoms than are currently available, making the future of high-precision antimatter physics even brighter.Footnote 1


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Ulmer, S. Optical transition seen in antihydrogen. Nature 541, 467–468 (2017).

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