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 one of many types of type of onium.

Like all onia, protonium is a boson with all quantum numbers (baryon number, flavour quantum numbers, etc.) and electrical charge equal to 0.

There are two known methods to generate protonium. One method involves violent particle collisions. The other method involves putting antiprotons and protons into the same magnetic cage. The latter method was first used during the experiment ATHENA (ApparaTus for High precision Experiment on Neutral Antimatter) by Evandro Lodi Rizzini's working group at the CERN laboratory in Geneva in 2002, but it was not until 2006 that scientists realized protonium was also generated during the experiment.

Reactions involving a proton and an antiproton at high energies give rise to many-particle final states. In fact, such reactions are the basis of particle colliders such as the Tevatron at Fermilab. Indirect searches for protonium at LEAR (Low Energy Antiproton Ring at CERN) have used antiprotons impinging on nuclei such as helium, with unclear results. Very low energy collisions in the range of 10 eV to 1 keV may lead to the formation of protonium.

...
Wikipedia