The Atomic Spectroscopy And Collisions Using Slow Antiprotons (ASACUSA) experiment studies the fundamental symmetries between matter and antimatter by precision spectroscopy of atoms containing an antiproton (the antimatter equivalent of the proton). The experiments focused in particular on hybrid atoms (antiprotonic helium), as well as pure antiatoms (antihydrogen). It also studies the interactions that occur during collisions between matter and antimatter.

ASACUSA aims to precisely measure a property of antihydrogen called the “hyperfine structure” and compare it to the well-known value for hydrogen. Since this quantity is very sensitive to magnetic fields, ASACUSA does not aim to trap antiatoms but rather to create a beam of antihydrogen atoms that can be transported to a region where no disturbing fields are present. To do this, a unique magnetic field configuration called “CUSP” is used to create a polarized beam of antihydrogen, which is then studied in flight using microwave radiation.

Helium has the second simplest atomic structure after hydrogen. It contains two electrons orbiting a central nucleus. The ASACUSA team makes antiprotonic helium by replacing one of these electrons with an antiproton. This is possible because, like the electron, the antiproton has negative charge. These hybrid atoms are formed by injecting antiprotons into a helium gas cell. 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 determine the mass of the antiproton to an unprecedented level of accuracy for comparison with the proton.

ASACUSA also studies the interactions that occur between matter and antimatter, by colliding beams of antiprotons on various kinds of normal atoms and molecules. These phenomena include the so-called “ionization” process where the fast antiproton rips away the electrons that circle the atoms. Another interesting process occurs when the antiprotons strike and annihilate with the atomic nuclei.

The ASACUSA team uses the Radio Frequency Decelerator downstream of Antiproton Decelerator at CERN to decelerate a 5.3 MeV antiprotons down to 100 keV.  In this way, the ASACUSA team uses antiprotons 10-100 times more efficiently than other collaborations.