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Brookhaven's proton beam struck stationary beryllium. A two-arm magnetic pair spectrometer measured electron–Positron momenta and directions. Cherenkov detectors, tracking chambers and shower counters rejected the much larger hadron background. Calibration used an electron beam; timing and track matching rejected accidental coincidences. The pair invariant mass distribution showed a narrow excess. Reducing magnet currents left its reconstructed position unchanged; changing target thickness and reversing spectrometer polarity supplied further checks. The unresolved intrinsic width did not mean a zero decay width.^[ref]

The logic of the magnet-current check is especially useful. A real state should retain its reconstructed invariant mass when a calibrated change of magnetic field makes its daughters follow different accepted paths. A feature tied to one detector location or one setting need not do so. This checks a possible instrumental origin; it does not by itself determine the state's quark composition.

Coauthored the observation of the J/psi meson through a narrow peak near $3.1\mathrm{GeV}$ in the invariant mass of electron-Positron pairs. The experiment used a precise pair spectrometer to analyze products of protons striking a beryllium target.^[ref]

The J/psi meson is the ground spin-triplet S-wave vector state of charmonium, with leading valence content $c\bar{c}$ and quantum numbers $J^{PC}=1^{--}$. It has zero net charm and can decay into hadrons or charged-lepton pairs.^[ref]

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][ref] 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.

In 1974, Samuel Chao Chung Ting's Brookhaven team and Burton Richter's SPEAR team independently found the narrow state now called the J/psi meson. Each discovery paper notes learning of the other result during manuscript preparation.^[ref][ref]

The common question was whether the observed excess represented a new particle resonance, rather than a peculiarity of particle identification, acceptance or calibration. The important difference was the horizontal coordinate of the search: Brookhaven reconstructed a final-state invariant mass; SPEAR scanned an initial-state center-of-mass energy. The invariant-mass selection of bubble-chamber events uses the first kind of reasoning, despite a very different recording medium.

Samuel C. C. Ting coauthored the 1974 observation of the narrow particle signal called J. A precise pair spectrometer measured the invariant mass of electron-Positron pairs produced when protons struck beryllium. A sharp feature near 3.1 GeV stood above the smooth background, indicating a new state rather than an arbitrary individual collision. This illustrates how a short-lived particle can be identified through reconstructed decay products.^[ref]

A physicist.^[ref]