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The infamous strong CP problem in particle physics can in principle be solvedby a massless up quark. In particular, it was hypothesized that topologicaleffects could substantially contribute to the observed nonzero up-quark masswithout reintroducing CP violation. Alternatively to previous work using fitsto chiral perturbation theory, in this Letter, we bound the strength of thetopological mass contribution with direct lattice QCD simulations, by computingthe dependence of the pion mass on the dynamical strange-quark mass. We findthat the size of the topological mass contribution is inconsistent with themassless up-quark solution to the strong CP problem.

Ruling Out the Massless Up-Quark Solution to the Strong CP Problem by Computing the Topological Mass Contribution with Lattice QCD