Question leads on from others where s^1/2 = 200GeV mH = 70GeV EH = 91.5GeV and p = 59GeV, (three momentum). The reaction was e+e- -> HZ0 where H is higgs boson and Z0 is the Z0 boson with mass 91GeV It says the Z0 decays to two muon and antimuon. They are observed to be equal energy in the lab frame, what is the angle between them in the detector? They are massless. It says use the the three-momentum already worked out. I haven't a clue how to describe the particles after the decay, I didn't think you can predict where they go, isn't it random?