Krylov spaces for truncated spectrum methodologies

We propose herein an extension of truncated spectrum methodologies, a nonperturbative numerical approach able to elucidate the low energy properties of quantum field theories. TSMs, in their various flavors, involve a division of a computational Hilbert space, $H$, into two parts, one part, $H_1$ that is "kept" for the numerical computations, and one part, $H_2$, that is discarded or "truncated." Even though $H_2$ is discarded, truncated spectrum methodologies will often try to incorporate the effects of $H_2$ in some effective way. In these terms, we propose to keep the dimension of $H_1$ small. We pair this choice of $H_1$ with a Krylov subspace iterative approach able to take into account the effects of $H_2$. This iterative approach can be taken to arbitrarily high order and so offers the ability to compute quantities to arbitrary precision. In many cases it also offers the advantage of not needing an explicit UV cutoff. To compute the matrix elements that arise in the Krylov iterations, we employ a Feynman diagrammatic representation that is then evaluated with Monte Carlo techniques. Each order of the Krylov iteration is variational and is guaranteed to improve upon the previous iteration. The first Krylov iteration is akin to the next-to-leading order approach of Elias-Miró et al. [NLO renormalization in the Hamiltonian truncation, Phys. Rev. D 96, 065024 (2017)]. To demonstrate this approach, we focus on the ($1+1d$)-dimensional ${\varphi }^4$ model and compute the bulk energy and mass gaps in both the $Z_2$-broken and unbroken sectors. We estimate the critical ${\varphi }^4$ coupling in the broken phase to be $g_c=0.2645\pm 0.002$.