Phenomenology of scalar leptoquarks

We study the simplest renormalizable scalar leptoquark models where the standard model is augmented only by one additional scalar representation of $SU(3)\times SU(2)\times U(1)$. The requirement that there be no proton decay from renormalizable interactions singles out two such models, one of which exhibits an unusual top mass enhancement of the $\mu \to e\gamma$ decay rate. We analyze the phenomenology of the model with the unusual top mass enhancement of loop level chirality changing charged lepton processes in the light of existing and upcoming experiments. Both of the models that do not allow proton decay from renormalizable interactions have dimension-5 operators that, even if suppressed by the Planck scale, can give rise to an unacceptably high level of baryon number violation. We discuss symmetries that can forbid these dimension-5 operators.

Figure 1

Feynman diagrams contributing to the process $\mu \to e\gamma$.

Figure 2

The combination of couplings $\lambda$ from Eq. (14) as a function of the scalar leptoquark mass for two values of the $\mu \to e\gamma$ branching ratio relevant for the MEG experiment. The shaded region consists of points which do not satisfy Eq. (7).

Figure 3

The combination of couplings $\lambda$ from Eq. (14) as a function of the scalar leptoquark mass for two values of the $\mathrm{Br}(\mu \to e\mathrm{\text{conversion in Al}})$ relevant for the Mu2e experiment. The thin solid line, corresponding to $\mathrm{Br}(\mu \to e\gamma )={10}^{-14}$, is included for reference. The shaded region consists of points which do not satisfy Eq. (7).

Figure 4

The combination of couplings $\lambda$ from Eq. (14) as a function of the scalar leptoquark mass for a branching ratio $\mathrm{Br}(\mu \to e\mathrm{\text{conversion in Al}})={10}^{-16}$ and four different positive values of the ratio of the couplings $r$ from Eq. (21). The shaded region consists of points which do not satisfy Eq. (7).

Figure 5

Same as Fig. 4 but for negative values of $r$.

Figure 6

Same as Fig. 4 but for a branching ratio $\mathrm{Br}(\mu \to e\mathrm{\text{conversion in Al}})={10}^{-17}$.

Figure 7

Same as Fig. 5 but for a branching ratio $\mathrm{Br}(\mu \to e\mathrm{\text{conversion in Al}})={10}^{-17}$.

Figure 8

The combination of couplings $|\mathrm{Im}[{\tilde{\lambda }}_e^{13}{\tilde{\lambda }}_u^{31}]|$ as a function of the scalar leptoquark mass for three different values of the electron EDM. The shaded region consists of points which do not satisfy Eq. (7).

Figure 9

Feynman diagram representing proton decay in model I.