The antiproton is a mirror twin of the proton: same mass, but negative charge. When it approaches an atomic nucleus, the particles can annihilate, turning into radiation. In the PUMA experiment at CERN, researchers study how antiprotons are captured by nuclei, forming exotic atoms similar to hydrogen but with an antiproton instead of an electron.
In the new work, physicists calculated how an antiproton interacts with tritium (one proton and two neutrons) and helium-3 (two protons and one neutron) nuclei. They solved complex four-body equations to find energy-level shifts and widths of these states. It turned out that the result strongly depends on the model of nucleon–antinucleon interaction: that is, on how exactly protons and neutrons are attracted to the antiproton.
Then the scientists computed the annihilation density — where in the nucleus the antiproton most often collides with a nucleon. Comparison with the nuclear matter distribution showed that annihilation occurs mostly at the nuclear periphery, not in the dense center. This is like a person walking through a crowd: they are more likely to bump into people at the edges, where it's less crowded, than in the middle of the crush.
This conclusion helps better understand how antimatter interacts with ordinary matter and could be useful for future experiments with antihydrogen.
In an atom, an antiproton replaces the electron, but because of its large mass it sits much closer to the nucleus and annihilates faster.