Truly neutral particle

In particle physics, a truly neutral particle is a subatomic particle that has no charge quantum number; they are their own antiparticle.^[1]: 131 In other words, it remains itself under the charge conjugation, which replaces particles with their corresponding antiparticles.^[1]: 135 All charges of a truly neutral particle must be equal to zero. This requires particles to not only be electrically neutral, but also requires that all of their other charges (such as the colour charge) be neutral.

Examples

Known examples of such elementary particles include photons, Z bosons, and Higgs bosons, along with the hypothetical neutralinos, sterile neutrinos, and gravitons. For a spin-½ particle such as the neutralino, being truly neutral implies being a Majorana fermion.

By way of contrast, neutrinos are not truly neutral since they have a weak isospin of ⁠±+1/2⁠, or equivalently, a non-zero weak hypercharge, both of which are charge-like quantum numbers.