where the field-strength dual is defined as Fμνa = 12e μναβF a αβ. In this dual counterpart E and B are flipped into B and E. Thus, F a μνF μνa ∝ E · B, while F a μνF μνa ∝ E − B. We know that E and B are the vector and the axial vector fields, respectively, which means that the parity transformation, P, changes E and B into −E and B. Thus, we can understand that F a μνF is P-even and F a μνF μνa is P-odd. Also, the time reversal transformation, T , makes no difference in the charge distribution but it changes the current direction, leading to E → E and B → −B. Again, F a μνF is T -even, while F a μνF is T -odd. In condensed matter physics T is under experimental control but it is not so in the relativistic heavy-ion collision experiment, and we usually refer to CP instead of T using the fact that CPT is never broken in the Standard Model. The question is how large or small θ in Eq. (1.1) could be. The most typical observable for P and CP violation is the electric dipole moment (EDM) of nucleons. As illustrated in Fig. 1 it is obvious that the EDM changes its direction if P is applied. One may think that the EDM is intact under CP because the replacement of + and − by P is precisely canceled by the charge conjugation C. Since the EDM should be considered relative to the spin direction, however, a finite EDM indicates