Antihydrogen

Antihydrogen is the antimatter counterpart of ordinary hydrogen. Whereas a hydrogen atom consists of a proton orbited by an electron, antihydrogen comprises an antiproton and a positron. As the simplest known anti-atom, it provides a unique platform for testing fundamental symmetries in physics, especially those concerning matter–antimatter equivalence. Scientific interest centres on questions such as the origin of the observed matter dominance in the universe, known as the baryon asymmetry problem, and on whether antimatter exhibits identical structural and gravitational behaviour to matter as predicted by the CPT theorem.

Experimental history

Antihydrogen occurs nowhere naturally and must be produced artificially in particle accelerator environments. Hot antihydrogen was first detected in the 1990s when high-energy antiprotons interacted with target materials to produce positron–antiproton pairs. Significant progress occurred in the early 2000s with the ATHENA and ATRAP collaborations at CERN. In 2002, ATHENA produced cold antihydrogen by combining antiprotons and positrons within electromagnetic Penning traps. ATRAP followed soon after, employing similar techniques to generate large numbers of cold atoms suitable for further study.
A major breakthrough came in 2010 when the ALPHA collaboration at CERN successfully trapped antihydrogen for the first time. Using a magnetic minimum trap created by mirror and multipole fields, the team confined neutral antihydrogen atoms for over a hundred milliseconds, marking a fundamental step toward precision spectroscopy. This success built on the capabilities of CERN’s Antiproton Decelerator, designed to provide low-energy antiprotons for antimatter research.
Subsequent developments allowed the ALPHA and GBAR experiments to cool and manipulate anti-atoms with increasing precision. In 2016, ALPHA reported the first measurement of the 1s–2s transition in antihydrogen, a key atomic spectral line. The transition frequency closely matched that of ordinary hydrogen within the sensitivity limits of the experiment, strongly supporting matter–antimatter symmetry.

Spectroscopy and fundamental tests

The 1s–2s transition in hydrogen is among the most precisely measured quantities in physics. A central aim of antihydrogen spectroscopy is to compare this value with extreme precision to test the CPT theorem, which states that matter and antimatter must have identical mass, charge-to-mass ratios and atomic transition frequencies. In the experiment:

• A laser tuned to half the expected transition frequency illuminated trapped antihydrogen.
• Two-photon excitation elevated positrons in the anti-atoms from the 1s to the 2s state.
• Excited atoms subsequently decayed, ionised or underwent spin-flip transitions, allowing annihilation signatures to be detected.

ALPHA found significantly enhanced annihilation during resonance conditions compared with off-resonance or no-laser controls, demonstrating successful excitation of antihydrogen. The results indicated equality with hydrogen to within parts per trillion, providing one of the most stringent CPT tests ever performed.

Characteristics

According to the CPT theorem, antihydrogen should share all intrinsic properties of hydrogen except for reversed charge signs. Thus antimatter atoms should:

• possess the same mass,
• have identical magnetic moments,
• exhibit equal spectroscopic transitions, and
• follow the same internal quantum structure.

Excited antihydrogen should therefore emit light at the same wavelengths as ordinary hydrogen. A key open question concerns the gravitational behaviour of antimatter. Standard theory predicts that antimatter experiences the same gravitational attraction as matter. However, alternative speculative models have proposed the possibility of negative gravitational mass or repulsive gravitational interactions. Ongoing experiments aim to measure the free-fall behaviour of antihydrogen with high precision to settle these questions empirically.
When antihydrogen encounters ordinary matter, its constituents annihilate. The positron annihilates with an electron to produce gamma rays, while the antiproton annihilates with nucleons, generating a cascade of high-energy pions that quickly decay into lighter particles such as muons, neutrinos and electrons.

Production techniques

The earliest production of antihydrogen was achieved in 1995 by a CERN team led by Walter Oelert. Antiprotons accelerated in the Low Energy Antiproton Ring were fired into xenon clusters, generating electron–positron pairs and enabling antiproton–positron capture. However, this method had a very low yield and produced extremely energetic anti-atoms unsuitable for trapping.
Greater efficiency arrived with the construction of the Antiproton Decelerator, which delivers low-energy antiprotons for precision experiments. In cold-antihydrogen synthesis:

• Positrons are collected and cooled in dedicated traps.
• Antiprotons are similarly cooled and confined.
• Both species are merged under controlled electromagnetic conditions to form antihydrogen atoms at low kinetic energies.

By the mid-2000s, millions of antihydrogen atoms had been produced, though most annihilated on the apparatus walls due to their high temperature. Advances in trap design and laser-cooling methods are gradually improving confinement times and enabling more detailed investigations.
A separate technique using Rydberg positronium—a highly excited bound state of an electron and positron—has enabled pulsed production of antihydrogen suitable for beam-based measurements, including proposed gravitational interferometry using Moiré deflectometry.

Larger antimatter atoms

The synthesis of heavier anti-atoms presents even greater challenges. Antideuterium, antitritium and antihelium nuclei have been produced in high-energy collision environments, but their large kinetic energies and short survival times make neutral anti-atom formation technically difficult. Nevertheless, identifying and studying such species remains a long-term goal in antimatter physics.