Application of the light-front coupled-cluster method to ${\varphi }^4$ theory in two dimensions

As a first numerical application of the light-front coupled-cluster (LFCC) method, we consider the odd-parity massive eigenstate of ${\varphi }_{1+1}^4$ theory. The eigenstate is built as a Fock-state expansion in light-front quantization, where wave functions appear as coefficients of the Fock states. A standard Fock-space truncation would then yield a finite set of linear equations for a finite number of wave functions. The LFCC method replaces Fock-space truncation with a more sophisticated truncation, one which reduces the eigenvalue problem to a finite set of nonlinear equations without any restriction on Fock space. We compare our results with those obtained with a Fock-space truncation that yields the same number of equations.