There are three weak gauge bosons : W, W, and Z ; these mediate the weak interaction. The W bosons are known for their mediation in nuclear decay: The W converts a neutron into a proton then decays into an electron and electron-antineutrino pair. The Z does not convert particle flavor or charges, but rather changes momentum; it is the only mechanism for elastically scattering neutrinos. The weak gauge bosons were discovered due to momentum change in electrons from neutrino-Z exchange. The massless photon mediates the electromagnetic interaction. These four gauge bosons form the electroweak interaction among elementary particles.