Annihilations are of course not decays, but they too occur via virtual particles. In an annihilation a matter and an antimatter particle completely annihilate into energy.

That is, they interact with each other, converting the energy of their previous existence into a very energetic force carrier particle (a gluon, W/Z, or photon). These force carriers, in turn, are transformed into other particles.

Quite often, physicists will annihilate two particles at tremendous energies in order to create new, massive particles.