= Brookhaven J pair-spectrometer experiment
{c}
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>.<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]{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.
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