Lattice study of two-dimensional SU(2) gauge theories with a single massless Majorana fermion

Massless overlap fermions in the real representation of two-dimensional <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>SU</a:mi><a:mo stretchy="false">(</a:mo><a:msub><a:mi>N</a:mi><a:mi>c</a:mi></a:msub><a:mo stretchy="false">)</a:mo></a:math> gauge theories exhibit a mod(2) index due to the rigidity of its spectrum when viewed as a function of the background gauge field—lattice gauge fields on a periodic torus come under two classes: ones that have one set of chirally paired zero modes and ones that do not. Focusing on SU(2) and a single Majorana fermion in an integer representation <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mi>J</e:mi></e:math>, we present numerical evidence that shows only one of these classes survives the continuum limit and this depends on the boundary conditions of the fermion and the gauge field. As such, two of the four possible partition functions are zero in the continuum limit. By defining modified partition functions which do not include the zero modes of the overlap fermions in the fermion determinant, we are able to define an expectation value for a fermion bilinear as ratios of two mixed partition functions. This observable is referred to as the topological condensate and has a nonzero expectation value on any finite physical torus and also has a nonzero limit as the size of the torus is taken to infinity. We study the spectral density of fermions and the scaling of the lowest eigenvalue with the size of the torus to show the absence of any spontaneous symmetry breaking but the emergence of zero modes in the infinite volume limit where it is prohibited in finite volume. These results remain the same for <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>J</g:mi><g:mo>=</g:mo><g:mn>1</g:mn></g:math>, 2, 3, and 4. These results motivate us to propose an independent plaquette model which reproduces the correct physics in the infinite volume limit using a single partition function.