As u know,CP symmetry is broken in certain weak decays but it is fulfilled in #QuantumChromodynamics. A problem when electroweak description was tried to QCD that was solved in 1977 by pushing a term in #Lagrangian
Associated field would be a solution of non-homogeneous #KleinGordon equation -therefore scalar-
Since fields are quantized,from there what was called #Axion was born-name of a detergent and it's a #physicists' joke-
Now they hold on to #Axion as a possible candidate for #DarkMatter