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

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arxiv 2002.07802 v3 pith:ZSAHWFUA submitted 2020-02-18 hep-lat hep-phhep-th

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

classification hep-lat hep-phhep-th
keywords masstopologicalcontributionmasslessproblemstrongup-quarkcomputing
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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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 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.

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