Truly neutral particle has part(s) of the class neutralino
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.
Composite particles can also be truly neutral. A system composed of a particle forming a bound state with its antiparticle, such as the neutral pion (π0), is truly neutral. Such a state is called an "onium", another example of which is positronium, the bound state of an electron and a positron (e− e+).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. (The example presumes on evidence to date, which gives no indication that neutrinos are Majorana particles.)
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