Particle Physics

First beam of anti-hydrogen atoms at CERN

Print edition : February 21, 2014

The ASACUSA experiment at CERN. Photo: Yasunori Yamakazi

THE ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) experiment at the European Organisation for Nuclear Research’s (CERN) Antiproton Facility has for the first time succeeded in producing a beam of anti-hydrogen atoms. These are the antimatter equivalent of hydrogen atoms. They are identical to hydrogen atoms except that the electron and the proton of normal atoms are replaced by an anti-electron, called a positron, and an antiproton. The objective of the experiment is the study of anti-hydrogen and exotic atoms known as antiprotonic helium (which consists of an antiproton and an electron orbiting around the helium nucleus). The anti-hydrogen experiment aims at high-precision microwave spectroscopy of hyperfine transitions in a weak magnetic field region.

In 2011, the ALPHA experiment at CERN announced the trapping of anti-hydrogen atoms for 1,000 seconds and, in 2012, reported the observation of hyperfine transitions of trapped anti-atoms. In 2013, the ATRAP experiment announced the first direct measurement of the antiproton’s magnetic moment with a fractional precision of 4.4 parts in a million.

In a paper published recently in Nature Communications, the ASACUSA collaboration reported the unambiguous detection of 80 anti-hydrogen atoms 2.7 metres downstream of their production, where the perturbing influence of the magnetic fields used initially to produce the anti-atoms is small. This result is a significant step towards precise hyperfine spectroscopy of anti-hydrogen atoms.

Primordial antimatter has so far never been observed in the universe, and its absence remains a major scientific enigma. Nevertheless, it is possible to produce significant amounts of anti-hydrogen in experiments at CERN by mixing positrons and low-energy antiprotons produced by the Antiproton Decelerator. The spectra of hydrogen and anti-hydrogen are predicted to be identical, so any tiny difference between them would immediately open a window to new physics and could help in solving the antimatter mystery.

With its single proton accompanied by just one electron, hydrogen is the simplest existing atom, and one of the most precisely investigated and best understood systems in modern physics. Thus, comparisons of hydrogen and anti-hydrogen atoms constitute one of the best ways to perform highly precise tests of matter/antimatter symmetry.

Matter and antimatter annihilate immediately when they meet, so aside from creating anti-hydrogen, one of the key challenges for physicists is to keep anti-atoms away from ordinary matter. To do so, experiments take advantage of anti-hydrogen’s magnetic properties (which are similar to hydrogen’s) and use very strong non-uniform magnetic fields to trap anti-atoms long enough to study them. However, the strong magnetic field gradients degrade the spectroscopic properties of the (anti-)atoms. To allow for clean high-resolution spectroscopy, the ASACUSA collaboration developed an innovative set-up to transfer anti-hydrogen atoms to a region where they can be studied in flight, far from the strong magnetic field. The next step for the experiment will be to optimise the intensity and kinetic energy of anti-hydrogen beams and to better understand their quantum state.