Probing CP-violation in the fermion transition dipole moment

by Shyam Balaji (University of Sydney (AU))

Thursday 8 Oct 2020, 16:00 → 17:00 Australia/Sydney

The CP violation in the fermion transition electromagnetic dipole moment is discussed in the context of the Standard Model with an arbitrary number of right-handed singlet neutrinos as well as for the heaviest SM particle, the top quark. A full one-loop calculation of the fermion electromagnetic form factors is performed in the Feynman gauge. A non-zero CP asymmetry is generated by a required threshold condition for the initial fermion masses along with non-vanishing CP violating phases in the flavour mixing matrices. This formalism is then applied to a minimal seesaw model with two heavy right-handed neutrinos denoted $N_1$ and $N_2$. We observe that the CP asymmetries for decays into light neutrinos are extremely suppressed, maximally around $10^{−17}$. However the CP asymmetry for $N_2 → N_1\gamma$ can reach of order unity. Even if the Dirac CP phase $δ$ is the only source of CP violation, a large CP asymmetry around $10^{−5} -10{−3}$ is comfortably achieved. For the top quark, we provide an explicit analytical study of the branching ratios and CP asymmetries resulting from the from the loop level flavour violating processes $t\rightarrow c\gamma$ and $t\rightarrow cg$. These rare processes are suppressed in the SM by the Glashow-Iliopoulos-Maiani (GIM) mechanism. However, the results presented here can easily be exported for use in minimal extensions of the SM including vector-like quarks or in Two-Higgs-Doublet models where radiative fermionic decay processes can be enhanced relative to the SM by several orders of magnitude.