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]
Experimental Observation of a Heavy Particle J by J. J. Aubert. Experimental Observation of a Heavy Particle J, pp. 1404–1405, Fig. 1 and checks 1, 2 and 7: target, spectrometer, calibration and stability tests
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.
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