Abstract: Yang–Mills theory is the (non-Abelian) quantum field theory underlying the Standard Model of particle physics; mathbb{R}^4 is Euclidean 4-space; the mass gap Δ is the mass of the least massive particle predicted by the theory. Therefore, the winner must first prove that Yang–Mills theory exists and that it satisfies the standard of rigor that characterizes contemporary mathematical physics, in particular constructive quantum field theory, which is referenced in the papers 45 and 35 cited in the official problem description by Jaffe and Witten.