In the above definition d μ μ denotes a covariant vector depending on the ( d + 1 ) -dimensional momentum p ( d space + 1 time dimension), e a μ μ is the vierbein describing the curvature of the space, m the quasi-particle mass and v D the Dirac velocity. Note that since in Dirac matter the Dirac equation gives the effective theory of the quasiparticles, the energy from the mass term is m v D 2 , not the rest mass m c 2 of a massive particle. γ γ μ μ refers to a set of Dirac matrices, where the defining for the construction is given by the anticommutation relation,