On November 10, 1974, SLAC's Burton Richter and colleagues found evidence for a particle they called the Ψ (the Greek letter Psi). Meanwhile on the east coast of the United States, Samuel Ting and his colleagues found comparable evidence for a particle they called the J. Both were the same particle and papers from both groups were published in Physical Review Letters on 2 December, 1974, as the first evidence for what is now known as the J/Ψ. Richter and Ting were jointly awarded the Nobel Prize in Physics in 1976 for the discovery, a mere two years after the work was done.
As part of PRL's 50-year anniversary, the American Physical Society has now made those papers freely accessible to everybody. You can read both the Richter and colleagues, and Ting and colleagues papers via this introductory page. Back in September 2005, we published a reproduction plot of Richter's logbook with the original data for the Ψ particle in symmetry.