"The yield of muon pairs decreased rapidly from 1 GeV to the kinematic limit of nearly 6 GeV with the exception of a curious shoulder near 3GeV. The measurement of muons was by range as determined by liquid and plastic scintillation counters interspersed with steel shielding. Each angular bin (there were 18) had four range bins, and for two muons this made a total of only 5000 mass bins into which to sort the data. Multiple scattering in the minimum of 10 feet of steel
made finer binning useless. Thus we could only note that "Indeed, in the mass region near 3.5 GeV, the observed spectrum may be reproduced by a composite of a resonance and a steeper continuum." This 1968-1969 experiment was repeated in 1974 by Aubert et al. (1974), with a
magnetic spectrometer based upon multiwire proportional chambers. The shoulder was refined by the superior resolution into a towering peak [see Figure] called the "J" particle."

Leon Lederman