Principal 3-Bundles with Adjusted Connections

We explore the notion of adjusted connection for principal 3-bundles. We first derive the explicit form of an adjustment datum for 3-term $L_\infty$-algebras, which allows us to give a local description of such adjusted connections and their infinitesimal symmetries. We then introduce the notion of an adjusted 2-crossed module of Lie groups and provide the explicit global description of principal 3-bundles with adjusted connections in terms of differential cocycles. These connections appear in a number of context within high-energy physics, and we list local examples arising in gauged supergravity and a global example arising in various contexts in string/M-theory. Our primary motivation, however, stems from U-duality, and we define a notion of categorified torus that forms an adjusted 2-crossed module, which we hope to be useful in lifting T-duality to M-theory.