Thrust is the longitudinal momentum along a unit vector . The maximum must be found over all possible . Planarity Here, and are defined to be axes of a Cartesian coordinate system whose third axis is . The variable indicates how well a reaction satisfies the assumption of being in a plane. The maximum is found with respect to the plane orientation.
Maximization gives the same result if is maximized.
The scattered system may have different from zero, but will be in a plane with the spectator jets and back to back in the transverse projection. Jets thus found are often subsequently contracted, viz. For higher jet multiplicities, one often uses variables reducing to three jets.