 Brookhaven J pair-spectrometer experiment

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.