Charged Lepton Flavor Violating Processes and Scalar Leptoquark Decay Branching Ratios in the Colored Zee-Babu Model

TL;DR

This paper explores a scalar leptoquark model that links neutrino mass generation to charged lepton flavor violation, predicting measurable effects and decay patterns that could guide future experiments and collider searches.

Contribution

It introduces a specific scalar leptoquark and diquark model generating neutrino masses at two loops, establishing connections with lepton flavor violation and predicting decay branching ratios.

Findings

Predicted lower bounds for $Z$ and $l ightarrow l' ext{gamma}$ decay branching ratios.

Certain parameter space regions are accessible to upcoming lepton flavor violation experiments.

Leptoquark decay branching ratios are predicted, aiding collider search strategies.

Abstract

We consider a neutrino mass generating model which employs a scalar leptoquark, $\Delta$, and a scalar diquark, $S$. The new scalars $\Delta$ and $S$ carry the standard model $SU(3)_c\times SU(2)_L\times U(1)_Y$ quantum numbers $(3,1,-1/3)$ and $(6,1,-2/3)$, respectively. The neutrino masses are generated at the two-loop level, as in the Zee-Babu model\cite{Zee-Babu}, and $\Delta/S$ plays the role of the doubly/singly charged scalar in the Zee-Babu model. With a moderate working assumption that the magnitudes of the six Yukawa couplings between $S$ and the down-type quarks are of the same order, strong connections are found between the neutrino masses and the charged lepton flavor violating processes. In particular, we study $Z\rightarrow \overline{l} l'$, and $l\rightarrow l' \gamma$ and find that some portions of the parameter space of this model are within the reach of the planned charged lepton flavor violating experiments. Interesting lower bounds are predicted that $B(Z\rightarrow \overline{l} l')\gtrsim 10^{-16}-10^{-14}(10^{-14})\times(1\mbox{TeV}\cdot m_S/7 m_\Delta^2)^2$ and $B(l\rightarrow l' \gamma)\gtrsim 10^{-17}-10^{-16}(10^{-18}-10^{-16})\times(1 \mbox{TeV}\cdot m_S/7 m_\Delta^2)^2$ for neutrino masses being the normal(inverted) hierarchical pattern. The type of neutrino mass hierarchy could also be determined by measuring the charged lepton flavor violating double ratios. Moreover, definite leptoquark decay branching ratios are predicted when there is no Yukawa interaction between the right-handed fermions and $\Delta$ ( the branching fraction of $\Delta$ to a charged lepton and a quark is 50\%), which could help refine the collider search limit on the scalar leptoquark mass.