Gauge symmetry and the light-front vacuum structure

A residual gauge symmetry, exhibited by light-front gauge theories in finite-volume formulation, is studied at the quantum level. Its implications for the structure of the vacuum are clarified first within the massive Schwinger model for simplicity. Unitary operators, which implement the symmetry, transform the trivial Fock vacuum into an infinite set of degenerate coherent-state vacua. A fermionic component of the vacuum emerges naturally without the need to introduce a Dirac sea. Gauge invariant θ vacuum is constructed as a superposition of degenerate vacuum states. Finally, a generalization of the above ideas to the vacuum problem of more complicated gauge field theories is discussed.