Diphotons, new vacuum angles, and strong CP

The Standard Model contains a well-understood, natural, spin-0 diphoton resonance: the π⁰. Numerous studies have pointed out that the hint of a new diphoton resonance at 750 GeV could be a pion analog, identified with the pseudo-Nambu-Goldstone boson of a chiral symmetry spontaneously broken by new strong dynamics at the TeV scale. These "hypercolor" models are generically expected to violate parity through a topological angle θ̃. We discuss the physics of θ̃ and its impact on the phenomenology of the new sector. We also describe some of the theoretical implications of a nonzero θ̃. In particular, θ̃ can generate an O(1) threshold correction to the QCD vacuum angle θ near the TeV scale, sharply constraining ultraviolet solutions to the strong CP problem. Alternatively, finding that θ̃ is small may be interpreted as evidence in favor of UV solutions to strong CP, particularly those based on spontaneously broken P or CP symmetries.