Interface Yang-Mills, Supersymmetry, and Janus

We consider theories consisting of a planar interface with $\N=4$ super-Yang-Mills on either side and varying gauge coupling across the interface. The interface does not carry any independent degrees of freedom, but is allowed to support local gauge invariant operators, included with independent interface couplings. In general, both conformal symmetry and supersymmetry will be broken, but for special arrangements of the interface couplings, these symmetries may be restored. We provide a systematic classification of all allowed interface supersymmetries. We find new theories preserving eight and four Poincar\'e supersymmetries, which get extended to sixteen and eight supersymmetries in the conformal limit, respectively with $SU(2) \times SU(2)$, $SO(2) \times SU(2)$ internal symmetry. The Lagrangians for these theories are explicitly constructe d. We also recover the theory with two Poincar\'e supersymmetries and SU(3) internal symmetry proposed earlier as a candidate CFT dual to super Janus. Since our new interface theories have only operators from the supergravity multiplet turned on, dual supergravity solutions are expected to exist. We speculate on the possible relation between the interface theory with maximal supersymmetry and the near-horizon limit of the D3-D5 system.