The 1983 UA2 experiment Z boson paper reports eight candidate electron pairs, including radiative decays, at . Four events entered its invariant mass determination. The published result was , the second uncertainty systematic. These are that paper's reported numbers, not inputs fitted to the illustration below.[ref]
Evidence for Z0 → e+e− at the CERN proton-antiproton collider by P. Bagnaia. Physics Letters B 129 (1983), pp.130–140, abstract: eight candidates, four-event mass determination and stated uncertainties
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The UA2 experiment was a high-energy particle physics experiment conducted at the Super Proton Synchrotron (SPS) at CERN during the 1980s. The primary aim of the experiment was to investigate proton-antiproton collisions, which were produced by colliding protons with antiprotons at high energies.