For every particle there is an antiparticle, which is (as far as we know) a perfect mirror image, except that it has opposite charge. Mixing antiprotons and antielectrons (called positrons) one can combine them to antihydrogen, i.e. the antimatter counterpart to ordinary hydrogen. This was done by the ATHENA experiment at CERN in 2002.

I work together with the ALPHA experiment, which is a continuation of ALPHA. The goal of this experiment is to trap antihydrogen atoms, and to study them using laser spectroscopy. In this way one may find some tiny difference between matter and antimatter. Possibly, such a difference could explain why our present universe seems to contain only ordinary matter, although at the Big Bang we would expect matter and antimatter to have been created at similar amounts.

Trapping of antimatter is difficult since it annihilates as soon as it meets ordinary matter. For charged particles, such as the antiprotons and positrons electric fields can be used to hold them in place. But the antihydrogen atom is neutral so this is not possible. Instead one has to use magnetic forces. These are very weak, so the antihydrogen has to be very cold, less than about a Kelvin, to stay in the trap. Creating antihydrogen at temperatures low enough to allow trapping is a major challenge.

My own work has centered on two areas:

• Formation of antihydrogen. Antihydrogen is formed when antiprotons are injected into a positron plasma. The reaction involves two positrons, since one extra is required to take away the energy released when the bound state is formed. The antihydrogen is initially formed in a very loosely bound and fragile state. More collisions is required to achieve a more tightly bound antiatom which can survive through trapping. But the collisions can also destroy the antiatom. The process is further complicated by the electric and magnetic fields present in the trap.
• Interaction of antihydrogen with ordinary matter. I have calculated cross sections for scattering of antihydrogen on ordinary hydrogen or Helium at low energies. These were the first quantum mechanical calculations of these systems. Possible reactions include elastic scattering (the atom and antiatom simply bounces off again), annihilation, formation of positronium (the bound state of an electron and a positron), formation of protonium (proton and antiproton), and even a rather exotic "molecule" consisting of a hydrogen atom and an antihydrogen atom.