Enlarging the symmetry of pure $R^2$ gravity, BRST invariance and its spontaneous breaking

TL;DR

This paper explores how the symmetry of pure R^2 gravity can be extended beyond restricted Weyl invariance by introducing a scalar field and BRST symmetry, leading to spontaneous symmetry breaking and emergence of Einstein gravity.

Contribution

It introduces a novel BRST symmetry for pure R^2 gravity involving Grassmann variables, and demonstrates spontaneous symmetry breaking to Einstein gravity.

Findings

BRST symmetry extends Weyl invariance in R^2 gravity.

Spontaneous symmetry breaking yields Einstein gravity from R^2 theory.

Conformal anomaly persists after symmetry breaking.

Abstract

Pure $R^2$ gravity was considered originally to possess only global scale symmetry. It was later shown to have the larger restricted Weyl symmetry where it is invariant under the Weyl transformation $g_{\mu\nu} \to \Omega^2(x)\, g_{\mu\nu}$ when the conformal factor $\Omega(x)$ obeys the harmonic condition $\Box \Omega(x)=0$. Restricted Weyl symmetry has an analog in gauge theory. Under a gauge transformation $A_{\mu}\to A_{\mu} + \frac{1}{e}\partial_{\mu} f(x)$, the gauge-fixing term $(\partial_{\mu}A^{\mu})^2$ has a residual gauge symmetry when $\Box f=0$. In this paper, we consider scenarios where the symmetry of pure $R^2$ gravity can be enlarged even further. In one scenario, we add a massless scalar field to the pure $R^2$ gravity action and show that the action becomes on-shell Weyl invariant when the equations of motion are obeyed. We then enlarge the symmetry to a BRST symmetry where no on-shell or restricted Weyl condition is required. The BRST transformations here are not associated with gauge transformations (such as diffeomorphisms) but with Weyl (local scale) transformations where the conformal factor consists of a product of Grassmann variables. BRST invariance in this context is a generalization of Weyl invariance that is valid in the presence of the Weyl-breaking $R^2$ term. In contrast to the BRST invariance of gauge theories like QCD, it is not preserved after quantization since renormalization introduces a scale (leading to the well-known Weyl (conformal) anomaly). We show that the spontaneous breaking of the BRST symmetry yields an Einstein action; this still has a symmetry which is also anomalous. This is in accord with previous work that shows that there is conformal anomaly matching between the unbroken and broken phases when conformal symmetry is spontaneously broken.