Compute the residual discrete anomaly

Compute the discrete anomaly of the residual symmetry group acting on the eight light Weyl fermions in model N1, and determine whether that anomaly permits or obstructs a fully symmetric gapped light phase.

Background

The existence of a symmetric gap is linked to the ’t Hooft anomaly of the symmetry group left exact by the explicit source. The residual symmetry acts on the eight light Weyl fermions as a real representation, which excludes a perturbative anomaly, but the exact residual group also contains order-four elements with a Spin-$Z_4$-type action on the light zero modes.

The paper does not evaluate the corresponding discrete anomaly. Such a computation is needed to decide whether the symmetric gapped option is globally consistent, beyond the perturbative anomaly test.