The Strong CP Problem and the Visibility of Invisible Axions
One of the remarkable successes of quantum chromodynamics (QCD), the theory of hadrons, is the solution of the U(1) problem . In a quark model with three flavours one expects as a consequence of spontaneous chiral symmetry breaking nine pseudo-Goldstone bosons. Eight of them, which are related to the breaking of the nonabelian axial SU(3)A symmetry, can be identified as pions, kaons and η. The candidate for the ninth boson, however, the isoscalar meson η’, is too heavy to be the pseudo-Goldstone boson of the broken axial U(1)A symmetry.
KeywordsSymmetry Breaking Experimental Bound Current Quark Masse Symmetry Breaking Scale Neutron Electric Dipole Moment
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