Global $U(1)_Y$xBRST symmetry and the LSS theorem: Ward-Takahashi identities governing Green's functions, on-shell T-Matrix elements, and the effective potential, in the spontaneously broken extended Abelian Higgs model

Abstract

The weak-scale U(1) Abelian Higgs Model (AHM) is the simplest spontaneous symmetry breaking (SSB) gauge theory. The extended AHM (E-AHM) adds certain heavy scalars $\Phi$ and fermions $\psi$. In Lorenz gauge, these theories have a global U(1) conserved physical current, but no conserved charge. As shown by Kibble, the Goldstone theorem applies, there is a massless derivatively coupled Nambu-Goldstone boson (NGB). Proof of all-loop-orders renormalizability and unitarity is tricky because the BRST-invariant Lagrangian is not U(1) symmetric. Nevertheless, Slavnov-Taylor identities guarantee that on-shell T-matrix elements of physical states are independent of anomaly-free gauge transformations. We observe that they are therefore also independent of the usual anomaly-free U(1) global transformations. It follows that the associated global current, is exactly conserved for amplitudes of physical states. We identify corresponding Ward-Takahashi identities (WTI). In Lorenz gauge, two towers of "1-soft-pion" global WTI govern the scalar-sector, and represent a new global U(1)xBRST symmetry not of the Lagrangian but of the physics. The first gives relations among off-shell Green's functions, the second governs on-shell T-matrix elements, replacing the Adler self-consistency conditions. These WTI constrain the all-loop-orders scalar-sector low-energy effective Lagrangian. Consequently, certain heavy CP-conserving heavy matter representations decouple completely in the $M_{Heavy}^2/m_{Weak}^2 \to \infty$ limit. SSB (E-)AHM physics therefore has more symmetry than does its BRST-invariant Lagrantian. The NGB decouples from the observable particle spectrum in the usual way, when the observable vector absorbs it, as if it were a gauge transformation, hiding both towers of WTI from observable particle physics.