POSITRONIUM
The electron and it's anti-particle, the positron, can orbit each other in a matter/anti-matter unfriendly alliance called a positronium before they decay by mutual annihilation. The lifetime of such a contrivance depends upon the relative orientation of their individual spins. If the spins are paired anti-parallel with each other, with a total spin of zero, then the lifetime is about 8 nanoseconds. If, on the other hand, they are aligned parallel, with a total spin of 1, then the lifetime is about a thousand times longer at 7 microseconds. The reason being that the spin-zero state must decay to a pair of gamma ray photons, each with spin-1, whereas the spin-1 positronium cannot do this and must instead decay into a minimum of three gamma ray photons (in order to conserve momentum and angular momentum), and this decay is much less probable requiring it to generate three photons simultaneously.
