1. The problem statement, all variables and given/known data Four particles having the following momenta were found: ___ p4 ----- p3 ----- p2 ----- p1 px: 6.5696, 3.4861, 3.4861, 4.0211 py: 2.5853, 2.4498, 0.1379, 2.5853 pz: 8.6726, 1.8936, 1.8936, 8.6726 All numbers are in GeV. p1 & p2 are muons, p3 & p4 are antimuons. Is it possible that two of these particles came from the decay of one? If so, what particle was it? 2. Relevant equations Conservation laws: 4-momentum & various charges (electric, leptonic, baryonic, etc.) 3. The attempt at a solution As we don't know of any particles with charge ±2, I found the center-of-mass energy of all possible muon-antimuon pairs. From this I saw that either both the 1-4 pair and 2-3 came from two 2215 MeV particles, or that the 1-3 pair came from the decay of a 4340 MeV particle and the 2-4 from a 3080 MeV one. The problem now is finding particles with suitable masses... The TA sent me to http://www-spires.dur.ac.uk/HEPDATA/PART/ where there are mountains of data. I couldn't find my hands and legs, let alone a neutral 2215 MeV particle that decays to muons. I saw in various lists there particles such as eta(2225), rho(3)(2250), etc. I understand the number in brackets is the mass, but what's the different between those and the ordinary eta and rho particles? This is the first homework assignment in the introductory course, so I suspect things are simpler than what I imagine. Can anyone help? Thanks in advance.