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Register a new userA near-minimal leptoquark model for reconciling flavour anomalies and generating radiative neutrino masses
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We introduce two scalar leptoquarks, the SU$(2)_L$ isosinglet denoted $phisim(mathbf{3}, mathbf{1}, -1/3)$ and the isotriplet $varphisim(mathbf{3}, mathbf{3}, -1/3)$, to explain observed deviations from the standard model in semi-leptonic $B$-meson decays. We explore the regions of parameter space in which this model accommodates the persistent tensions in the decay observables $R_{D^{(*)}}$, $R_{K^{(*)}}$, and angular observables in $bto s mumu$ transitions. Additionally, we exploit the role of these exotics in existing models for one-loop neutrino mass generation derived from $Delta L=2$ effective operators. Introducing the vector-like quark $chi sim (mathbf{3}, mathbf{2}, -5/6)$ necessary for lepton-number violation, we consider the contribution of both leptoquarks to the generation of radiative neutrino mass. We find that constraints permit simultaneously accommodating the flavour anomalies while also explaining the relative smallness of neutrino mass without the need for cancellation between leptoquark contributions. A characteristic prediction of our model is a rate of muon--electron conversion in nuclei fixed by the anomalies in $b to s mu mu$ and neutrino mass; the COMET experiment will thus test and potentially falsify our scenario. The model also predicts signatures that will be tested at the LHC and Belle II.
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Motivated by an explanation of the $R_{K^{(*)}}$ anomalies, we propose a Standard Model extension via two scalar SU(2)$_L$ triplet leptoquarks and three generations of triplet Majorana fermions. The gauge group is reinforced by a $Z_2$ symmetry, ensuring the stability of the lightest $Z_2$-odd particle, which is a potentially viable dark matter candidate. Neutrino mass generation occurs radiatively (at the three-loop level), and leads to important constraints on the leptoquark couplings to leptons. We consider very generic textures for the flavour structure of the $h_1$ leptoquark Yukawa couplings, identifying classes of textures which succeed in saturating the $R_{K^{(*)}}$ anomalies. We subsequently carry a comprehensive analysis of the models contributions to numerous high-intensity observables such as meson oscillations and decays, as well as charged lepton flavour violating processes, which put severe constraints on the flavour structure of these leptoquark extensions. Our findings suggest that the most constraining observables are $K^+ to pi^+ u bar u$ decays, and charged lepton flavour violating $mu -e$ conversion in nuclei (among others). Nevertheless, for several classes of flavour textures and for wide mass regimes of the new mediators (within collider reach), this Standard Model extension successfully addresses neutrino mass generation, explains the current $R_{K^{(*)}}$ tensions, and offers a viable dark matter candidate.
We explore the connection of the leptoquark solution to the recently reported $B$-meson anomalies with a mechanism of neutrino mass generation and a viable dark matter candidate. We consider a model consisting of two scalar leptoquarks and three generations of triplet fermions: neutrino masses are radiatively generated at the 3-loop level and, by imposing a discrete $Z_2$ symmetry, one can obtain a viable dark matter candidate. We discuss the constraints on the flavour structure of this model arising from numerous flavour observables. The rare decay $Kto pi^+ ubar u$ and charged lepton flavour violating $mu-e$ conversion in nuclei are found to provide the most stringent constraint on this class of models.
We propose a leptoquark model with two scalar leptoquarks $S^{}_1 left( bar{3},1,frac{1}{3} right)$ and $widetilde{R}^{}_2 left(3,2,frac{1}{6} right)$ to give a combined explanation of neutrino masses, lepton flavor mixing and the anomaly of muon $g-2$, satisfying the constraints from the radiative decays of charged leptons. The neutrino masses are generated via one-loop corrections resulting from a mixing between $S^{}_1$ and $widetilde{R}^{}_2$. With a set of specific textures for the leptoquark Yukawa coupling matrices, the neutrino mass matrix possesses an approximate $mu$-$tau$ reflection symmetry with $left( M^{}_ u right)^{}_{ee} = 0$ only in favor of the normal neutrino mass ordering. We show that this model can successfully explain the anomaly of muon $g-2$ and current experimental neutrino oscillation data under the constraints from the radiative decays of charged leptons.
We propose a one-loop induced radiative neutrino mass model with anomaly free flavour dependent gauge symmetry: $mu$ minus $tau$ symmetry $U(1)_{mu-tau}$. A neutrino mass matrix satisfying current experimental data can be obtained by introducing a weak isospin singlet scalar boson that breaks $U(1)_{mu-tau}$ symmetry, an inert doublet scalar field, and three right-handed neutrinos in addition to the fields in the standard model. We find that a characteristic structure appears in the neutrino mass matrix: two-zero texture form which predicts three non-zero neutrino masses and three non-zero CP-phases from five well measured experimental inputs of two squared mass differences and three mixing angles. Furthermore, it is clarified that only the inverted mass hierarchy is allowed in our model. In a favored parameter set from the neutrino sector, the discrepancy in the muon anomalous magnetic moment between the experimental data and the the standard model prediction can be explained by the additional neutral gauge boson loop contribution with mass of order 100 MeV and new gauge coupling of order $10^{-3}$.
We reconsider a model introducing a scalar leptoquark $phi sim (mathbf{3}, mathbf{1}, -1/3)$ to explain recent deviations from the standard model in semileptonic $B$ decays. The leptoquark can accommodate the persistent tension in the decays $bar{B}rightarrow D^{(*)}tau bar{ u}$ as long as its mass is lower than approximately $10 text{ TeV}$, and we show that a sizeable Yukawa coupling to the right-chiral tau lepton is necessary for an acceptable explanation. Agreement with the measured $bar{B}rightarrow D^{(*)}tau bar{ u}$ rates is mildly compromised for parameter choices addressing the tensions in $b to s mu mu$, where the model can significantly reduce the discrepancies in angular observables, branching ratios and the lepton-flavor-universality observables $R_K$ and $R_{K^*}$. The leptoquark can also reconcile the predicted and measured value of the anomalous magnetic moment of the muon and appears naturally in models of radiative neutrino mass derived from lepton-number violating effective operators. As a representative example, we incorporate the particle into an existing two-loop neutrino mass scenario derived from a dimension-nine operator. In this specific model, the structure of the neutrino mass matrix provides enough freedom to explain the small masses of the neutrinos in the region of parameter space dictated by agreement with the anomalies in $bar{B}rightarrow D^{(*)}tau bar{ u}$, but not the $b to s$ transition. This is achieved without excessive fine-tuning in the parameters important for neutrino mass.
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