W boson

The W bosons $W^{+}$ and $W^{-}$ are massive charged electroweak gauge bosons mediating weak charged currents. Before mass diagonalization, the charged combinations are $W^{\pm }=(W^1\mp i{W}^2)/\sqrt{2}$; electroweak symmetry breaking gives their tree-level mass $gv/2$.

A W-mediated interaction changes the electric charge of its matter partners. For example, neutron beta decay contains a down-to-up quark transition and an electron–antineutrino pair; at low energy the exchanged W is virtual, not a real emitted particle. A real W produced at high energy can decay to a charged lepton and neutrino or an allowed quark pair. The large W mass explains why low-energy weak processes can be approximated by a short-range four-fermion interaction.