It turns out that when a fundamental particle decays, it changes into a less massive particle and a force-carrier particle (always a W boson for fundamental particle decays). These force carriers may then re-emerge as other particles. So, a particle does not just change into another particle type; there is an intermediate force-carrier particle which mediates particle decays.

In many cases, these temporary force-carrier particles seem to violate the conservation of energy because their mass is greater than the available energy in the reaction. However, these particles exist so briefly that, because of Heisenberg's Uncertainty Principle, no rules are broken. These are called virtual particles.