A near-minimal leptoquark model for reconciling flavour anomalies and generating radiative neutrino masses (1906.01870v3)

Abstract: We introduce two scalar leptoquarks, the SU$(2)L$ isosinglet denoted $\phi\sim(\mathbf{3}, \mathbf{1}, -1/3)$ and the isotriplet $\varphi\sim(\mathbf{3}, \mathbf{3}, -1/3)$, 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 $b\to s \mu\mu$ 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.

Near-Minimal Leptoquark Model for Flavour Anomalies and Neutrino Mass Generation

Introduction and Motivations

The paper presents a strategic approach to address pressing issues in particle physics, particularly flavour anomalies and the generation of neutrino masses. Through introducing two scalar leptoquarks within an extended particle model, the authors aim to reconcile deviations observed in semileptonic $B$-meson decays and explore plausible regions in the parameter space that can accommodate such anomalies alongside radiative neutrino masses. This exploration is critical not only for understanding the fundamental properties of neutrinos beyond the Standard Model (SM) but also for addressing augmentation induced by Beyond SM (BSM) physical processes that challenge the prevailing theoretical framework.

Model Framework

The authors present a near-minimal model encapsulating two scalar leptoquarks: an SU(2)$_L$ isosinglet $\phi \sim (\mathbf{3}, \mathbf{1}, -1/3)$ and an isotriplet $\varphi \sim (\mathbf{3}, \mathbf{3}, -1/3)$. The design of these leptoquarks is instrumental in the assessment of flavour anomalies characterized by persistent tensions within decay observables $R_{D^{(*)}$, $R_{K^{(*)}$, and angular observables in $b \rightarrow s \mu\mu$ transitions. Moreover, the introduction of vector-like quark $\chi \sim (\mathbf{3}, \mathbf{2}, -5/6)$ contributes necessary lepton-number violation required for neutrino mass generation.

Theoretical Implications and Analysis

This modelling approach is theoretically significant with respect to:

1. Flavour Anomalies: The paper illustrates how these leptoquarks can harbor BSM contributions to $b \to s \mu\mu$ transitions through tree-level processes induced by $\varphi$, as opposed to $\phi$, which accommodates loop-level contributions. This structural dichotomy potentially explains discrepancies noted in experimental results, predominantly in semi-leptonic and b-quark decay processes.
2. Neutrino Mass Generation: The paper leverages these scalar leptoquarks within frameworks of radiative mass generation for neutrinos. Moving beyond tree-level mass considerations typical in SM approaches, the role of leptoquarks in formulating mass terms through loop-level mechanisms embodies an intricate response to neutrino mass disparities.

Constraints and Predictions

The manuscript entertains rigorous constraints, key among them being collider bounds pertinent for vector-like quark masses and $\mu-e$ conversion processes in nuclei. Highlighting current constraints, such as $b$-quark mixing parameters and LFU violations, provides a robust check against experimental constraints. The predicted rates of muon-electron conversion linked to BSM physics, particularly testable in upcoming studies like COMET and Mu2e, present fascinating avenues for empirical verification. Furthermore, the authors discuss the potential experimental prospects with high-pT dilepton production, offering essential insight into testing and possibly falsifying this model.

Conclusion and Future Directions

Through its synthesis of scalar leptoquarks, the paper makes an impressive commitment to linking neutrino properties with flavour anomalies, offering a condensed yet proficient model to clinicians and researchers. Though richly supported by theoretical exploration, it also underscores limitations and regional preferences in parameter space, prompting future empirical pursuits at the LHC and through specific $B$-meson decay observations. The research speculates further advancements employing collider data to project tests that respond to the theoretical provisions asserted herein, fostering the ongoing dialogue in particle physics reforms adaptive to BSM developments.

This paper stands as a pioneering guidepost in detailing how particle phenomenology can continue to accommodate novel findings from precision physics, driving global efforts to understand the intricate fabric accounting for matter and its interlinked anomalies. The authors’ contribution as a discreet yet comprehensive parameter framework for novel collider experiments is indispensable, guiding future scrutiny and refinements in theory-driven explanations across the physics community.