When a Dirac fermion is analyzed as two coupled Weyl fermions, this quantity of the particle acts as the coupling constant between the chiralities. Unlike chirality, helicity is not Lorentz-invariant for particles with nonzero values of this quantity, as one can boost to a frame going faster than the particle to reverse its helicity. Mixing between flavor eigenstates and this quantity’s eigenstates causes a process described by the PMNS matrix. The surface in k-space where the energy-momentum relation is satisfied is a (*) “shell” named after this quantity. The oscillation of neutrinos (-5[1])implies they do not have a value of zero for this quantity, (10[2])as (10[1])once thought. A particle is relativistic when its kinetic energy exceeds the energy due to this property of the particle. (10[1])For 10 points, for a particle at rest, E equals what quantity times c-squared? ■END■ (10[1])