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.