A particle can be defined to be on-shell if an equation involving rest mass and this quantity, the mass-shell condition, is satisfied. (15[1])The dependence of a propagator on this quantity is determined by its values for incoming and outgoing particles, between which it may vary (15[1])for virtual particles. (-5[1])The Pauli–Lubański pseudovector (-5[2])generates the maximal subgroup of the Poincaré group that leaves the eigenvalues of this quantity invariant, which is its (*) little group. Mandelstam variables are functions (10[1])of this quantity for two scattering particles. (-5[1])This quantity for tachyons would be a space-like vector. This quantity’s operator equals positive (-5[1])i h-bar times a certain gradient, in contrast to a (10[1])non-relativistic quantity whose operator equals i h-bar times the gradient. For 10 points, name this generalization of the product of mass and velocity to Minkowski space. (10[1])■END■ (10[4])