Truly neutral particle

In particle physics, a truly neutral particle is a subatomic particle with all its charges equal to zero. This not only requires particles to be electrically neutral, but also requires that all of their other charges (like the colour charge) are neutral. Such a particle will be its own antiparticle.

Mathematically, charge conjugation replaces all the constituent particles of a particle with their corresponding antiparticles. If a particle remains the same after charge conjugation, then it is its own antiparticle, and is truly neutral.

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-1/2 particle such as the neutralino, being a truly neutral particle implies being a Majorana fermion.

Composite particles can also be truly neutral. The best known example is onium, a system composed of a particle forming a bound state with its own antiparticle.

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