A beam of antihydrogen atoms has for the first time been successfully produced by an experiment at CERN’s Antiproton Decelerator (AD). The ASACUSA collaboration reports the unambiguous detection of antihydrogen atoms 2.7 m downstream from their production, where the perturbing influence of the magnetic fields used to produce the antiatoms is negligibly small. This result is a significant step towards precise hyperfine spectroscopy of antihydrogen atoms.

High-precision microwave spectroscopy of ground-state hyperfine transitions in antihydrogen atoms is a main focus of the Japanese-European ASACUSA collaboration. The research aims at investigating differences between matter and antimatter to test CPT symmetry (the combination of charge conjugation, C, parity, P, and time reversal, T) by comparing the spectra of antihydrogen with those of hydrogen, one of the most precisely investigated and best understood systems in modern physics.

One of the key challenges in studying antiatoms is to keep them away from ordinary matter. To do so, other collaborations take advantage of antihydrogen’s magnetic properties and use strong, non-uniform magnetic fields to trap the antiatoms long enough to study them. However, the strong magnetic-field gradients degrade the spectroscopic properties of the antihydrogen. To allow for clean, high-resolution spectroscopy, the ASACUSA collaboration has developed an innovative set-up to transfer antihydrogen atoms to a region where they can be studied in flight, far from the strong magnetic field regions.

In ASACUSA, the antihydrogen atoms are formed by loading antiprotons and positrons into the so-called cusp trap, which combines the magnetic field of a pair of superconducting anti-Helmholtz coils (i.e., coils with antiparallel excitation currents) with the electrostatic potential of an assembly of multi-ring electrodes (CERN Courier March 2011 p17). The magnetic-field gradient allows the flow of spin-polarized antihydrogen atoms along the axis of the cusp trap. Downstream there is a spectrometer consisting of a microwave cavity to induce spin-flips in the antiatoms, a superconducting sextupole magnet to focus the neutral beam and an antihydrogen detector. (The microwave cavity was not installed in the 2012 experiment.)

The detector, located 2.7 m from the antihydrogen-production region, consists of single-crystal bismuth germanium oxide (BGO) surrounded by five plates of plastic scintillator. Antihydrogen atoms annihilating in the crystal emit three charged pions on average, so the signal required consists of a coincidence between the crystal and at least two plastic scintillators. Simulations show that this requirement reduces the background, from antiprotons annihilating upstream and from cosmic rays, by three orders of magnitude.

The ASACUSA researchers investigate the principal quantum number, n, of the antihydrogen atoms that reach the detector, because their goal is to perform hyperfine spectroscopy on the ground state, n = 1. For these measurements, field-ionization electrodes were positioned in front of the BGO, so that only antihydrogen atoms with n < 43 or n < 29 reached the detector, depending on the average electric field. The analysis indicates that 80 antihydrogen atoms were unambiguously detected with n < 43, with a significant number having n < 29.

This analysis was based on data collected in 2012, before the accelerator complex at CERN entered its current long shutdown. Since then, the collaboration has also been preparing for the restart of the experiment at the AD in October this year. A new cusp magnet is under construction, which will provide a much stronger focusing force on the spin-polarized antihydrogen beam. A cylindrical high-resolution tracker and a new antihydrogen-beam detector are also under development. In addition, the positron accumulator will lead to an order of magnitude more positrons. The team eventually needs a beam of antihydrogen in its ground state (n = 1) so the updated experiment will employ an ionizer with higher fields to extract antihydrogen atoms that are in effect in the ground state.