Higher-order structure of Hamiltonian truncation effective theory

We study the Hamiltonian truncation for the two-dimensional 𝜆⁢𝜙^4 theory within the framework of Hamiltonian truncation effective theory, where truncation artifacts are mitigated through a systematic inclusion of corrective terms organized in inverse powers of the ultraviolet energy cutoff 𝐸_{max} . Building on the leading-order matching program, we develop two complementary extensions. First, we derive compact all-order expressions for the local matching corrections to the mass and quartic coupling by resumming infinite classes of diagrams sharing fixed topologies within the local approximation. Second, we extend the nonlocal sector by computing the next-to-next-to-local corrections contributing at 𝒪⁡(𝐸^{-4}_{max}), following a continuum-first matching procedure, in which the effective corrections are computed in infinite volume and the spatial direction is subsequently recompactified to obtain a discrete basis of free-Hamiltonian eigenstates on which the truncated operator construction is implemented. Our results show that an increasingly rich operator basis is necessary to describe the theory beyond leading order.