the W and Z bosons
/ double-yoo and zee BOH-zonz /
The W and Z bosons are the messenger particles of the weak force, just as the photon is the messenger of electromagnetism and gluons of the strong force. They are the heaviest force carriers in the Standard Model and the reason the weak force behaves the way it does. There are three of them: a positively charged W, its negatively charged antiparticle, and the electrically neutral Z.
Their defining feature is their enormous mass. The W weighs about 86 times as much as a proton and the Z about 97 times — heavier than an entire iron atom packed into a single point-like particle. This is wildly unusual, because the cousin carrier in electromagnetism, the photon, is exactly massless. That heaviness is what gives the weak force its tiny range and apparent feebleness. The charged W bosons can carry electric charge from one particle to another, which lets the weak force change a particle's identity; the neutral Z carries no charge and so leaves particles' identities intact while still nudging them.
Discovering the W and Z in 1983 was a triumph that confirmed the electroweak theory and earned a Nobel Prize. Today they are produced by the millions at high-energy colliders, where they show up as short-lived resonances that decay almost instantly into pairs of lighter particles. Studying exactly how often they appear and how they decay is one of the most precise tests of the Standard Model we have.
At a collider, a Z boson is often spotted by its decay into a muon and an antimuon. Add up the energies and momenta of that pair and they always reconstruct to the same mass, about 91 GeV — a sharp spike in the data that is the unmistakable signature of the Z.
A muon-antimuon pair reconstructing to 91 GeV is the calling card of a Z boson.
The W and Z are heavy not by accident but because of the Higgs mechanism, which gives them mass while leaving the photon massless — but that origin story is a separate topic.