Linked-cluster Tamm–Dancoff field theory

To solve the relativistic bound-state problem one needs to systematically and simultaneously decouple the high-energy from the low-energy modes and the many-body from the few-particle states using a consistent renormalization scheme. In a recent Letter we have shown that one such approach can be a combination of the coupled cluster method as used in many-body theory and the Wilsonian exact renormalization group. Even though the method is intrinsically non-perturbative, one can easily implement a loop expansion within it. In this Letter we provide further support for this aspect of our formalism by obtaining results for the two-loop renormalized φ4 theory. We show that the non-unitary representation inherent in our method leads to an economic computation and does not produce any non-Hermiticity in the relevant terms.