The Standard Model of particle physics assumes that nature conserves CPT symmetry. In other words, it assumes that the laws of physics do not change if all the particles in an interaction are replaced by their antiparticles (C), all three directions in space are reversed (P), and time is reversed (T). It is well know that nature violates CP (charge-parity) symmetry but there is no experimental evidence that CPT symmetry is not conserved in nature.

Any violation of CPT symmetry would show up as a slight difference in the frequency of the electronic transition from the ground state to the first excited state in hydrogen and antihydrogen. This frequency has been measured with an accuracy of 1.8 parts in 1014 in laser spectroscopy experiments on cold hydrogen atoms. Creating and trapping cold antihydrogen atoms are clearly the first steps in any comparison.

Small numbers of antihydrogen atoms have been made at CERN and Fermilab before, but these antiatoms were moving too fast to be useful for precision experiments. Now the ATHENA collaboration - which includes physicists from Brazil, Denmark, Italy, Japan, Switzerland and the UK - has managed to produce large numbers of antihydrogen atoms for the first time.

All the traps in the ATHENA experiment are variations on the well established Penning trap, which uses an axial magnetic field and various electric fields to trap charged particles. Antiprotons from the antiproton decelerator (AD) at CERN are slowed down in a thin foil, trapped and then further cooled through collisions with cold electrons. The AD delivers about 20 million antiprotons in short pulses at 100 second intervals, and about 3000 of these are available for making antihydrogen after the trapping and cooling stages. Meanwhile positrons from the radioactive decay of sodium-22 are accumulated in a separate Penning trap.

To produce antihydrogen about 70 millions positrons are trapped and cooled to about 15 kelvin. Next about 10000 antiprotons (i.e., three AD shots) are launched into the positron cloud by changing the electric field, and the antiprotons and the positrons are allowed to mix. Antiatoms can only form if the excess energy and momentum in the antiproton-positron collisions are carried off by a third particle (so-called three-body recombination) or a photon (radiative recombination).

Evidence for antihydrogen production is obtained when an antiatom manages to escape from the trap and annihilates in the electrodes. The antiproton typically creates neutral or charged pions, while the positron emits back-to-back photons with a characteristic energy. The apparatus is maintained at a temperature of 15 kelvin and the antihydrogen atoms are thought to have a similar temperature. No antiatoms were detected when the temperature of the positrons was increased to several thousand kelvin.

The ATHENA team estimate that about 50,000 antihydrogen atoms were produced during the experiment but they are not sure of the absolute production rate, the dominant recombination method or the quantum state in which the antiatoms are produced. This last point is particularly important because hydrogen atoms (or antiatoms) can only be trapped if they are in their ground state.

The next step, says ATHENA team-member Mike Charlton of University of Wales Swansea, is to understand the antiproton-positron reactions in detail. Adding a laser system for spectroscopy experiments is likely to take several years, he says, and trapping antihydrogen atoms will take longer.