The Z Particle is almost 100 time more massive than the proton and has a lifetime of only about 10-25 (-25 should be the small number at the top right referring to scientific notation) a second (which is much below a second of time).  Z particles were first observed in 1983 by Carlo Rubbia and Simon vander Meer at CERN.  More studies have shown a natural variation in mass that is related to the particle's lifetime through the uncertainty principle.  Measurements show that when Z Particles decay to neutrino-antineutrino pairs, they produce only three types of lightweight neutrino, indicating that there are only three sets each of leptons and quarks.