Zero modes in the light-front coupled-cluster method

• Extends the light-front coupled-cluster method to include zero modes.
• Illustrates with an analysis of vacuum structure for phi-3, phi-4, and Wick–Cutkosky model field theories.
• Demonstrates the applicability of the LFCC method to theories with spontaneous symmetry breaking.

The light-front coupled-cluster (LFCC) method is a technique for solving Hamiltonian eigenvalue problems in light-front-quantized field theories. Its primary purpose is to provide a systematic sequence of solvable approximations to the original eigenvalue problem without the truncation of Fock space. Here we discuss the incorporation of zero modes, modes of zero longitudinal momentum, into the formalism of the method. Without zero modes, the light-front vacuum is trivial, and the vacuum expectation value of the field is always zero. The LFCC method with zero modes provides for vacuum structure, in the form of a generalized coherent state of zero modes, as is illustrated here in two-dimensional model field theories.