Why is the mission impossible? -- Decoupling the mirror Ginsparg-Wilson fermions in the lattice models for two-dimensional abelian chiral gauge theories

TL;DR

This paper investigates the decoupling of mirror fermions in lattice chiral gauge theories, demonstrating through models and simulations that breaking mirror-fermion symmetries can lead to consistent, local fermion measures.

Contribution

It introduces a simplified four-flavor axial gauge model and provides numerical evidence that mirror fermions can be decoupled by breaking symmetries with quartic operators, addressing the 'Mission: Impossible' challenge.

Findings

Mirror fermions show regular local behavior in the simplified model.

Quartic operators effectively break mirror-fermion symmetries.

Fermion measure satisfies locality, enabling consistent chiral gauge theories.

Abstract

In the mirror fermion approach with Ginsparg-Wilson fermions, it has been argued that the mirror fermions do not decouple: in the 345 model with Dirac- and Majorana-Yukawa couplings to XY-spin field, the two-point vertex function of the (external) gauge field in the mirror sector shows a singular non-local behavior in the PMS phase. We re-examine why the attempt seems a "Mission: Impossible" in the 345 model. We point out that the effective operators to break the fermion number symmetries ('t Hooft operators plus others) in the mirror sector do not have sufficiently strong couplings even in the limit of large Majorana-Yukawa couplings. We observe also that the type of Majorana mass term considered there is singular in the large limit due to the nature of the chiral projection of the Ginsparg-Wilson fermions, but a slight modification without such singularity is allowed by virtue of the very nature. We then consider a simpler four-flavor axial gauge model, the 1$^4$(-1)$^4$ model, in which the U(1)$_A$ gauge and Spin(6)(SU(4)) global symmetries prohibit the bilinear terms, but allow the quartic terms to break all the other continuous mirror-fermion symmetries. In the strong-coupling limit of the quartic operators, the model is well-behaved and simplified. Through Monte-Carlo simulations in the weak gauge coupling limit, we show a numerical evidence that the two-point vertex function of the gauge field in the mirror sector shows a regular local behavior, and we still argue that all you need is killing the continuous mirror-fermion symmetries with would-be gauge anomalies non-matched. Finally, by gauging a U(1) subgroup of the U(1)$_A$$\times$ Spin(6)(SU(4)) of the previous model, we formulate the $2 1 (-1)^3$ chiral gauge model and argue that the induced fermion measure term satisfies the required locality property and provides a solution to the reconstruction theorem.