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Massless particle
A massless particle is an elementary particle whose invariant mass is zero.
Explanation
The two known massless particles are both gauge bosons: the photon (carrier of electromagnetism) and the gluon (carrier of the strong force). However, gluons are never observed as free particles, since they are confined within hadrons.
Their energy is expressed in electronvolt eV.
Gravity
The graviton (carrier of gravitational force) is a hypothetical tensor boson. Whether a graviton would be massless if it existed is an open question. A gravitino is its symmetric partner.
Example 1
For a massless light particle naturally holds true