Fermionic Zero Modes and Spontaneous Symmetry Breaking on the Light Front

Spontaneous symmetry breaking is studied within a simple version of the light-front sigma model with fermions. Its vacuum structure is derived by an implementation of global symmetries in terms of unitary operators in a finite volume with periodic fermi field. Due to the dynamical fermion zero mode, the vector and axial U(1) charges do not annihilate the light-front vacuum. The latter is transformed into a continuous set of degenerate vacuum states, leading to to the spontaneous breakdown of the axial symmetry. The existence of associated massless Nambu-Goldstone boson is demonstrated.