Hi, I am an undergrad doing research with CERN and in examining data from ATLAS. I was looking for instances of Z bosons decaying into muons by taking systems with two energetic, oppositely charged muons, adding their four vectors, and getting the invariant mass of the result. There was an expected local peak at ~90 GeV (the z boson mass), but there was an even higher peak at about 3 GeV. There were also peaks at ~0.75, ~1.1, ~3.75, and 9 GeV. My professor said it was probably caused by Psi and Upsilon Resonances. Could someone explain what exactly these are to me? I haven't found any direct explanations by searching, mostly just very technical explanations and specific papers not about these phenomenon in general.