Missing momentum (${\mathbf{p}}_{\mathrm{miss}}$)

The initial momentum minus the vector sum of reconstructed final visible momenta. At an electron-positron collider the full initial four-momentum is approximately known, unlike the hard partonic longitudinal momentum at a hadron collider. Nonzero missing momentum may indicate neutrinos, other invisible particles, or a detector loss; it is not alone proof of an invisible decay.