Protonium
Protonium (symbol: Pn), also known as antiprotonic hydrogen, is a type of exotic atom in which a proton (symbol: p) and an antiproton (symbol: p) orbit each other. Since protonium is a bound system of a particle and its corresponding antiparticle, it is an example of a type of exotic atom called an onium. Protonium has a mean lifetime of approximately 1.0 μs and a binding energy of −0.75 keV. Like all onia, protonium is a boson with all quantum numbers (baryon number, flavour quantum numbers, etc.) and electrical charge equal to 0. (Wikipedia).
