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 implies they do not have a value of zero for this quantity, as once (10[1])thought. (10[1])A particle is relativistic when its kinetic energy exceeds the energy due to this property of the particle. For 10 points, (10[1])for a particle at rest, E equals what quantity times c-squared? ■END■ (10[1])