If you were to mix antiprotons with ordinary matter, the particles would annihilate in less than a picosecond.But there is an exception. Mixed with helium gas, about 3% of the antiprotons will combine with helium atoms, forming antiprotinic helium.Antiprotonic helium consists of a antiproton and an electron orbiting a helium nucleus (two protons and two neutrons). They can exist for up to tens of microseconds, before decaying.http://en.wikipedia.org/wiki/Antiprotonic_helium