Exchange particles

Every single force must be exchanged by particles called exchange particles. Virtual photons ($\lambda$) are an example of these as they exchange the electromagnetic force, but cannot be detected directly.

Weak interaction

The weak nuclear force is responsible for:

• Changing a proton to a neutron in $\beta^+$ decay
• Changing a neutron to a proton in $\beta^-$ decay. It does not affect stable nuclei, and its exchange particle is the $W^+$ and the $W^-$.