Static weak dipole moments of the $τ$ lepton via renormalizable scalar leptoquark interactions

Abstract

The weak dipole moments of elementary fermions are calculated at the one-loop level in the framework of a renormalizable scalar leptoquark model that forbids baryon number violating processes and so is free from the strong constraints from experimental data. In this model there are two scalar leptoquarks accommodated in an $SU_L(2)\times U_Y(1)$ doublet: one of such leptoquarks is non-chiral and has electric charge of $5/3e$, whereas the other one is chiral and has electric charge $2/3e$. In particular, a non-chiral leptoquark contributes to the weak properties of an up fermion via a chirality flipping-term proportional to the mass of the virtual fermion and can also induce a non-zero weak electric dipole moment provided that the leptoquark couplings are complex. The numerical analysis is focused on the weak properties of the $τ$ lepton since they offer good prospects for their experimental study. The constraints on leptoquark couplings are briefly discussed for a non-chiral leptoquark with non diagonal couplings to the second and third fermion generations, a third-generation non-chiral leptoquark, and a third-generation chiral leptoquark. It is found that although the chirality-flipping term can enhance the weak properties of the $τ$ lepton via the top quark contribution, such an enhancement would be offset by the strong constraints on the leptoquark couplings. So, the contribution of scalar leptoquarks to the weak magnetic dipole moment of the $τ$ lepton are smaller than the standard model (SM) contributions but can be of similar size than those arising in some SM extensions. A non-chiral leptoquark can also give contributions to the weak electric dipole moment larger than the SM one but well below the experimental limit. We also discuss the case of the off-shell weak dipole moments and for completeness analyze the behavior of the $τ$ electromagnetic properties.