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Missing transverse momentum is the negative vector sum of reconstructed visible transverse momenta, with detector corrections specified. Under the ideal hypotheses of zero total incoming transverse momentum, complete visible reconstruction and exactly one invisible neutrino, momentum conservation givesThe neutrino is not directly seen. Even under these ideal hypotheses, its longitudinal momentum $p_{z\nu }$ is not measured. Beam remnants can carry substantial unobserved longitudinal momentum; the colliding quarks' longitudinal momenta are also unknown event by event. Imposing an assumed parent mass can constrain $p_{z\nu }$, but that imports a hypothesis rather than measuring it. Finite coverage, poorly reconstructed particle jets and other invisible particles can also generate or modify missing transverse momentum.

Choose the beam as the $z$ direction. A particle's transverse momentum is its momentum projected onto the perpendicular plane; $p_T=\sqrt{p_x^2+p_y^2}$. A Lorentz boost along the beam changes energy and longitudinal momentum, while leaving transverse momentum unchanged. Large transverse momentum identifies motion substantially across the beam rather than merely along it.

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]