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

Constantia Alexandrou, Jacob Finkenrath, Lena Funcke, Karl Jansen, Bartosz Kostrzewa, Ferenc Pittler, Carsten Urbach
2020 Physical Review Letters  
The infamous strong CP problem in particle physics can in principle be solved by a massless up quark. In particular, it was hypothesized that topological effects could substantially contribute to the observed nonzero up-quark mass without reintroducing CP violation. Alternatively to previous work using fits to chiral perturbation theory, in this Letter, we bound the strength of the topological mass contribution with direct lattice QCD simulations, by computing the dependence of the pion mass on
more » ... of the pion mass on the dynamical strange-quark mass. We find that the size of the topological mass contribution is inconsistent with the massless up-quark solution to the strong CP problem.
doi:10.1103/physrevlett.125.232001 pmid:33337203 fatcat:63b7i2dlc5hlbi467iisexvlna