Phase Diagram of the Two-Flavor Schwinger Model at Zero Temperature

TL;DR

This paper investigates the phase structure of the two-flavor Schwinger model at zero temperature, revealing a BKT-type transition at nd a non-perturbative mass gap, using Hamiltonian lattice gauge methods.

Contribution

It provides a detailed phase diagram of the two-flavor Schwinger model at nd introduces numerical lattice techniques that incorporate discrete chiral symmetry effects.

Findings

Logarithmic RG flow of BKT type at or small masses

Non-perturbative mass gap  e^{-A g^2/m^2} in certain regimes

Identification of phase boundaries where charge conjugation symmetry is broken

Abstract

We examine the phase structure of the two-flavor Schwinger model as a function of the $\theta$-angle and the two masses, $m_1$ and $m_2$. In particular, we find interesting effects at $\theta=\pi$: along the $SU(2)$-invariant line $m_1 = m_2 = m$, in the regime where $m$ is much smaller than the charge $g$, the theory undergoes logarithmic RG flow of the Berezinskii-Kosterlitz-Thouless type. As a result, in this regime there is a non-perturbatively small mass gap $\sim e^{- A g^2/m^2}$. The $SU(2)$-invariant line lies within a region of the phase diagram where the charge conjugation symmetry is spontaneously broken and whose boundaries we determine numerically. Our numerical results are obtained using the Hamiltonian lattice gauge formulation that includes the mass shift $m_\text{lat} = m- g^2 a/4$ dictated by the discrete chiral symmetry.