Boson half-integer spin particles cannot be bosons and integer spin particles cannot be fermions. In large systems, the difference between bosonic and fermionic statistics is only apparent at large densities — when their wave functions overlap. At low densities, both types of statistics are well approximated by Maxwell–Boltzmann statistics, which is described by classical mechanics. All observed elementary particles are either fermions or bosons. The observed elementary bosons are all gauge bosons: photons, W and Z bosons, gluons, except the Higgs boson which is a scalar boson. Finally, many approaches to quantum gravity postulate a force carrier for gravity, the graviton,
W′ and Z′ bosons W′ and Z′ bosons In particle physics, W′ and Z′ bosons (or W-prime and Z-prime bosons) refer to hypothetical gauge bosons that arise from extensions of the electroweak symmetry of the Standard Model. They are named in analogy with the Standard Model W and Z bosons. W′ bosons often arise in models with an extra SU(2) gauge group. is spontaneously broken to the diagonal subgroup SU(2) which corresponds to the electroweak SU(2). More generally, we might have "n" copies of SU(2), which are then broken down to a diagonal SU(2). This gives rise to "n"−1 W′, W′ and Z′ bosons.