The vector boson Z⁰ of the weak force is electrically neutral. Unlike the photons and gluons, it is not massless; its mass is 91 GeV.

The largest electron-positron accelerator that has so far existed (Feb 2004) was LEP (Large Electron-Positron Collider), at CERN (the European Centre for Particle Physics). It was constructed primarily to study the Z⁰ particle. When the colliding electron and positron energies are about 45.5 GeV the 91 GeV heavy Z⁰ particle can be created. The Z⁰ particle then rapidly decays (in about 10^-24 s) to either a pair of quarks (quark-antiquark), or to a pair of leptons (lepton-antilepton)

In the chapter "Picture analysis" you will learn more about how to interpret pictures where the Z⁰ particle has decayed. In the chapter "Particle collisions" you can find suggestions of exercises that can be solved by analysing pictures of Z-decays.