Which Particle Has the Least Mass?

In particle physics, leptons are the family of particles with the least mass. Among these, the neutrino is the smallest known particle. Although electrons are point particles with no discernible internal structure, they interact more strongly with gravitational fields than neutrinos.

Electrons also have charge, which gives them an extra dimension and ability to interact with electromagnetic forces, gravity and weak nuclear forces. Neutrinos do not have charge and only interact with weak nuclear and gravitational forces. These minute interactions enable them to traverse the universe at close to the speed of light virtually unmolested. The neutrino's tiny mass helps it play a crucial part in astrophysical processes of all kinds.