The J/psi meson is the ground spin-triplet S-wave vector state of charmonium, with leading valence content and quantum numbers . It has zero net charm and can decay into hadrons or charged-lepton pairs.[ref]
Review of Particle Physics by Particle Data Group (2022). J/psi(1S) listing, pp. 1–2: quantum numbers, width and decay modes
Its comparatively narrow decay width makes it a conspicuous resonance in reconstructed mass distributions. An electron–Positron mass peak in a fixed-target experiment and an enhanced event rate during an energy scan in an electron-positron collider were complementary observations in the Richter-Ting independent J/psi discovery experiments of 1974.[ref]
Experimental Observation of a Heavy Particle J by J. J. Aubert. Abstract: proton–beryllium experiment and electron–positron invariant-mass spectrum
[ref]Discovery of a Narrow Resonance in e+e- Annihilation by J.-E. Augustin. Abstract: enhanced collision cross section during the center-of-mass energy scan
A peak's measured width also includes detector resolution or beam-energy spread, so it must not automatically be identified with the intrinsic lifetime width.