W and Z discovery experiments

A W boson decay can leave one visible charged lepton and a large missing transverse momentum; a Z boson decay can instead supply two visible leptons whose four-momenta determine an invariant mass. The two signatures answer the same existence question with different missing information. The electroweak unification history recognized by the 1979 Nobel Prize in Physics predates these 1983 observations: the W boson and Z boson were predictions to test, rather than particles first proposed in 1983.

The Super Proton-Antiproton Synchrotron stored counterrotating proton and antiproton beams; stochastic cooling made accumulation of the scarce antiprotons practical. Carlo Rubbia's 1984 lecture describes track reconstruction in a large drift chamber inside a magnetic field, surrounded by segmented particle calorimeters. Their nearly complete angular coverage allowed an imbalance test for unobserved neutrinos.^[ref]

The 1983 UA1 experiment paper used proton–antiproton collisions at $\sqrt{s}=540\mathrm{GeV}$. A high-transverse momentum electron candidate required a track matched to an electromagnetic particle shower, with little energy behind it in the hadronic particle calorimeter. Test beams checked electron and pion responses; ordinary events and particle jet samples checked the imbalance resolution. Two searches, beginning with isolated electrons and with missing transverse energy, converged on the same candidates. Conversion pairs, hadrons and heavy-flavor decays were examined as backgrounds.^[ref]

The 1983 UA2 experiment Z boson paper reports eight candidate electron pairs, including radiative decays, at $\sqrt{s}=550\mathrm{GeV}$. Four events entered its invariant mass determination. The published result was $91.9\pm 1.3\pm 1.4\mathrm{GeV}/c^2$, the second uncertainty systematic. These are that paper's reported numbers, not inputs fitted to the illustration below.^[ref]

For two visible, approximately massless leptons, both measured four-momenta enterThis is a reconstructed invariant mass, including longitudinal information, rather than the lower bound obtained from transverse mass. As an original example, two opposite-azimuth leptons with $p_{T+}=p_{T-}=45\mathrm{GeV}$ and equal rapidities have $m_{\ell \ell }=90\mathrm{GeV}$. Changing their rapidity difference to one raises it to about $101.486\mathrm{GeV}$, even though their transverse momenta are unchanged.

The ideal transverse mass bound concerns the actual event's parent invariant mass, not a perfectly sharp universal number. A finite decay width spreads parent invariant masses; particle jet recoil changes daughter directions; invariant-mass resolution, momentum resolution and imperfect reconstruction can smear an observed endpoint. Recoil alone does not invalidate the ideal inequality when both daughter transverse momenta are correct. Additional invisible particles, radiated photons and a massless-daughter approximation can invalidate the simple two-body reconstruction. Likewise, a Z boson pair-mass peak requires control of identification, detection efficiency, instrumental response and backgrounds. A peak position and an endpoint are different summaries of selected event populations.