@MISC{Lavoura08softcp, author = {L. Lavoura}, title = {Soft CP breaking and the strong-CP problem}, year = {2008} }

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Abstract

I put forward a model with vectorlike isosinglet quarks in which soft breaking of CP leads to strong CP violation only arising at two-loop level. Non-perturbative effects in Quantum Chromodynamics (QCD) may lead to P and CP violation, characterized by a parameter θ, in strong interactions. The experimental upper bound on the electric dipole moment of the neutron requires θ< ∼ 2 × 10 −10 [1]. (Electric dipole moments of elementary particles are both P-and T-violating; in normal field theory, T violation is equivalent to CP violation.) The presence of this unnaturally small number in QCD is known as the strong-CP problem. θ is the sum of two terms, θQCD and θQFD. θQCD is the original value of the angle θ characterizing the QCD vacuum. θQFD originates in the chiral rotation of the quark fields needed to render the quark masses real and positive. If Mp and Mn are the mass matrices of the up-type (charge 2/3) and down-type (charge