It doesn't. The ground state orbital of the antiproton is in principle stable, just like the electron orbitals in a regular atom. The difference is that antipro

It doesn't. The ground state orbital of the antiproton is in principle stable, just like the electron orbitals in a regular atom. The difference is that antiprotons can annihilate with protons from the helium nucleus when they get too close. This is why the experiments observe orbitals with high quantum numbers where the overlap between the antiproton wavefunction and the nucleus is small.