On the Classical Integrability of Root-$T \overline{T}$ Flows

The Root-$T \overline{T}$ flow was recently introduced as a universal and classically marginal deformation of any two-dimensional translation-invariant field theory. The flow commutes with the (irrelevant) $T \overline{T}$ flow and it can be integrated explicitly for a large class of actions, leading to non-analytic Lagrangians reminiscent of the four-dimensional Modified-Maxwell theory (ModMax). It is not a priori obvious whether the Root-$T \overline{T}$ flow preserves integrability, like it is the case for the $T \overline{T}$ flow. In this paper we demonstrate that this is the case for a large class of classical models by explicitly constructing a deformed Lax connection. We discuss the principal chiral model and the non-linear sigma models on symmetric and semi-symmetric spaces, without or with Wess-Zumino term. We also construct Lax connections for the two-parameter families of theories deformed by both Root-$T \overline{T}$ and $T \overline{T}$ for all of these models.