Emergent CPT violation from the splitting of Fermi points

Abstract

In a fermionic quantum vacuum, the parameters k_\mu of a CPT-violating Chern-Simons-like action term induced by CPT-violating parameters of the fermionic sector depend on the universality class of the system. As a concrete example, we consider the Dirac Hamiltonian of a massive fermionic quasiparticle and add a particular term with purely-spacelike CPT-violating parameters b_\mu=(0,{\bf b}). A quantum phase transition separates two phases, one with a fully-gapped fermion spectrum and the other with topologically-protected Fermi points (gap nodes). The emergent Chern-Simons ``vector'' k_\mu=(0,{\bf k}) now consists of two parts. The regular part, {\bf k}^{reg}, is an analytic function of |{\bf b}| across the quantum phase transition and may be nonzero due to explicit CPT violation at the fundamental level. The anomalous (nonanalytic) part, {\bf k}^{anom}, comes solely from the Fermi points and is proportional to their splitting. In the context of condensed-matter physics, the quantum phase transition may occur in the region of the BEC-BCS crossover for Cooper pairing in the p-wave channel. For elementary particle physics, the splitting of Fermi points may lead to neutrino oscillations, even if the total electromagnetic Chern-Simons-like term cancels out.