2002/12/06 by Daniel R. Nelson, Nelson, Daniel R.
Physics and Astronomy · #FOS: Physical sciences #High Energy Physics - Lattice (hep-lat) #hep-lat
paper · pdf · doi:10.48550/arxiv.hep-lat/0212009
318 pages, 91 figures, Ph.D. thesis
arxiv created 2002/12/06 · arxiv updated 2009/11/30
The nontrivial topological structure of the QCD gauge vacuum generates a CP breaking term in the QCD Lagrangian. However, measurements of the neutron electric dipole moment have demonstrated that the term's coefficient is unnaturally small, a dilemma known as the strong CP problem. A massless up quark is one potential solution, as the term could then be absorbed through chiral rotations. Through the light-quark-mass ratio mu / md, leading order Chiral Perturbation Theory appears to rule this scenario out. However, the Kaplan-Manohar ambiguity demonstrates that certain strong next-to-leading order corrections are indistinguishable from an up quark mass. Only a direct calculation of the Gasser-Leutwyler coefficient combination 2 L8 - L5 can resolve the issue. We carry out such a calculation, using partially quenched Nf = 3 staggered fermions and hypercubic blocking, and make a quantitative assessment of our systematic error. We find 2 L8 - L5 = (0.22 +/- 0.14) x 10-3, which corresponds to a light-quark-mass ratio of mu / md = 0.408 +/- 0.035. Thus, the massless-up-quark solution to the strong CP problem is ruled out. This is the first calculation of its type to use a physical number of light quarks, Nf = 3, and the first determination of 2 L8 - L5 to include a comprehensive study of statistical error.