Uncovering novel phase structures in $\Box^k$ scalar theories with the renormalization group

Abstract: We present a detailed version of our recent work on the renormalization group approach to multicritical scalar theories with higher derivative kinetic term of the form $\phi(-\Box)^k\phi$ and upper critical dimension $d_c = 2nk/(n-1)$. Depending on whether the numbers $k$ and $n$ have a common divisor two classes of theories have been distinguished which show qualitatively different features. For coprime $k$ and $n-1$ the theory admits a Wilson-Fisher type fixed point with a marginal interaction $\phi^{2n}$. We derive in this case the renormalization group equations of the potential at the functional level and compute the scaling dimensions and some OPE coefficients, mostly at leading order in $\epsilon$. While giving new results, the critical data we provide are compared, when possible, and accord with a recent alternative approach using the analytic structure of conformal blocks. Instead when $k$ and $n-1$ have a common divisor we unveil a novel interacting structure at criticality. In this case the phase diagram is more involved as other operators come into play at the scale invariant point. $\Box^2$ theories with odd $n$, which fall in this class, are analyzed in detail. Using the RG flows that are derived at quadratic level in the couplings it is shown that a derivative interaction is unavoidable at the critical point. In particular there is an infrared fixed point with a pure derivative interaction at which we compute the scaling dimensions. For the particular example of $\Box^2$ theory in $d_c=6$ we include some cubic corrections to the flow of the potential which enable us to compute some OPE coefficients as well.