According to the standard model, neutrinos are massless particles.

We then recover the relationship between energy and momentum for massless particles:

By contrast, massless particles, such as photons, are not considered matter, because they have neither rest mass nor volume.

To justify giving mass to a would-be massless particle, scientists were forced to do something out of the ordinary.

It is the conversion of massless particles into one or more massive particles.

These results will be applied to both massive and massless particles.

In the same limit, the propagator for a massless particle is singular.

These techniques, however, cannot be applied to massless particles and have unsolved issues with bound states.

In 3 + 1 dimensions, the little group for a massless particle is the double cover of SE(2).

The same is true for massless particles in such system, which add invariant mass and also rest mass to systems, according to their energy.