Making anti-atoms

In addition to studying fundamental particles and antiparticles produced in high-energy collisions, experiments at CERN are also investigating antimatter through the simplest anti-atom: antihydrogen. Just like hydrogen, which is formed from the combination of a proton and electron, antihydrogen is formed from an antiproton and an anti-electron (positron).

The ALPHA experiment at CERN has successfully made and trapped hundreds of antihydrogen atoms.

To form antihydrogen the positrons and antiprotons are combined one at a time. The positrons are easily obtained from radioactive sources, but the antiprotons are only available at the unique Antiproton Decelerator (AD) facility at CERN.

Charged particles can be easily manipulated, as they respond in well- known ways to electric and magnetic fields. Such fields can be used to confine the particles in so-called electromagnetic bottles, away from the material walls of the surrounding container. This is particularly important for antiparticles, which would otherwise annihilate with the atoms in the walls of the chamber if they come into contact!


The ALPHA Experiment

The AD provides ALPHA with about 30 million antiprotons every two minutes. Some of these are captured in a Penning trap where they are slowed down and carefully mixed them with the awaiting positrons to form the anti-atoms. In each mixing cycle many thousands of antihydrogen atoms are produced.

A Penning trap uses a stack of electrodes to provide an axial electrical well, together with a strong magnetic field pointing along the axis of the chamber to pin the antiparticles radially and stop them drifting towards the walls. These traps allow positrons and antiprotons to be stored for long periods until scientists are ready to mix them together to form antihydrogen.


Once the electrically neutral antihydrogen atom is formed, it is no longer held in place by the Penning trap. To prevent the antihydrogen reaching a wall and annihilating, another trap is used. This is made from a special combination of magnets that creates a magnetic bowl, which has a minimum in the centre of the device. For the trap to work, and keep antihydrogen atoms away from the walls, the atoms must have kinetic energies corresponding to a temperature of less than half a degree kelvin above absolute zero (-273˚C). Typically only one antihydrogen atom is trapped each time the experiment is performed and can be held for around 15 minutes, allowing detailed studies can be made.


Once the very cold antihydrogen has been trapped, the ALPHA experiment shines a laser on it to make very precise measurements of the antihydrogen spectrum. The experiment has recently used a similar technique with microwaves to make the first observation of quantum jumps in an antiatom. ALPHA is now poised to make the most precise comparison of the properties of matter and antimatter. In doing so it may shed some light on the mystery of the fate of antimatter.