Global $SU(3)_C\times SU(2)_L\times U(1)_Y$ linear sigma model: axial-vector Ward Takahashi identities, and decoupling of certain heavy BSM particles due to the Goldstone theorem

Abstract

Dedicated to the memory of Raymond Stora (1930-2015). In the $SU(2)_L\times SU(2)_R$ Linear Sigma Model with PCAC, towers of Ward-Takahashi Identities (WTI) have long been known to give relations among 1-Scalar-Particle-Irreducible Green's functions, and among I- Scalar-Particle-Reducible T-Matrix elements, for external scalars (i.e. the Brout-Englert-Higgs scalar and 3 pseudoscalars). We extend these WTI and the resulting relations to the $SU(3)_C\times SU(2)_L\times U(1)_Y$ Linear Sigma Model including the heaviest generation of Standard Model (SM) fermions supplemented with the minimum necessary neutrino content -- right-handed neutrinos and Yukawa-coupling-induced Dirac neutrino mass. We extract powerful constraints on the effective Lagrangian: e.g. showing that they make separate tadpole renormalization unnecessary, and guarantee infra-red finiteness. Crucially, ultra-violet quadratic divergences (UVQD) and all other relevant operators contribute only to $m_\pi^2$, a Nambu-Goldstone boson (NGB) mass-squared. A WTI between T-Matrix elements (i.e. the Goldstone Theorem) then enforces $ m_\pi^2=0$ for the true NGB in the spontaneous symmetry breaking mode of the theory. All relevant operator contributions originating to all-loop-orders from virtual scalars, quarks and leptons, vanish identically! Our regularization-scheme-independent results are unchanged by the addition of certain heavy CP-conserving matter, such as originate in certain Beyond the SM models. We demonstrate this with two examples: a heavy singlet real scalar field with $Z_2$ symmetry and no VEV; and a heavy singlet right-handed Type I See-saw Majorana neutrino. Specifically, we prove that these heavy degrees of freedom decouple completely from the low-energy effective Lagrangian, contributing only irrelevant operators after quartic-coupling renormalization.