The Atomic Spectroscopy And Collisions Using Slow Antiprotons (ASACUSA) experiment focuses on the fundamental differences in the behaviour of matter and antimatter. Instead of directly comparing atoms with their corresponding antiatoms (as do the ATRAP and ALPHA experiments), ASACUSA’s physicists are creating hybrid atoms such as “antiprotonic helium”.

Helium has the second simplest atomic structure after hydrogen. It contains two electrons orbiting a central nucleus. The ASACUSA team make antiprotonic helium by replacing one of these electrons with an antiproton (the antimatter equivalent of a proton). This is possible because, like the electron, the antiproton has negative charge. The process of creating these hybrid atoms is easier than making antihydrogen atoms (the antimatter version of hydrogen), and they can also be kept for longer.

The ASACUSA team uses the Antiproton Decelerator at CERN to send a beam of antiprotons into cold helium gas. Most of the antiprotons quickly annihilate with ordinary matter in the surroundings, but a tiny proportion combines with the helium to form hybrid atoms that contain both matter and antimatter. Using laser beams to excite the atoms, ASACUSA can measure the mass of the antiproton to an unprecedented level of accuracy for comparison with the proton.