Neutrino magnetic moment in the doublet-singlet Leptoquark model

TL;DR

This work studies the Majorana transition magnetic moment of neutrinos in a doublet-singlet scalar leptoquark model consisting of $S_1(\bar{3},1,1/3)$ and $\widetilde{R}_2(3,2,1/6)$. The authors derive a general one-loop expression for the transition magnetic moment, enhanced by Higgs-induced mixing between the leptoquarks and the $b$-quark in the loop, and they evaluate it for the $\mu^M_{\nu_{\mu\tau}}$ channel. They constrain the model by collider limits on leptoquark masses, RD(*) anomalies, muon $g-2$, and LFV decays like $\tau\to\mu\gamma$, finding that sizable Yukawa couplings can be allowed due to parameter degeneracies, while satisfying all bounds. Their numerical analysis shows that, under the neutrino mass bound $\sum m_\nu\le 0.26$ eV, the transition magnetic moment can reach $\mathcal{O}(10^{-13})\,\mu_B$ for $m_{\rm LQ}=1.5$ TeV in favorable sign configurations, which remains below current direct-detection limits but could be probed by upcoming experiments. This connects neutrino electromagnetic properties with flavor anomalies and collider constraints, suggesting a testable framework for new physics beyond the Standard Model.

Abstract

The transition magnetic moment for Majorana neutrinos is studied in a simple extension of the Standard Model. This extension incorporates two scalar Leptoquarks $S_1$ and $\widetilde{R}_2$ with quantum numbers $(\bar{3},1,1/3)$ and $(3,2,1/6)$ respectively. It is found that these Leptoquarks generate a sizable transition magnetic moment, particularly when the quark bottom is running in the loop. For our analysis of the parameter space, we include the latest measurement of the muon magnetic moment and combine it with the experimental constraint on the branching ratio Br$(τ\to μγ)$. We found that, despite the recent agreement on the $(g-2)_μ$ value, large values for Leptoquark Yukawa couplings are allowed due to a degeneracy in the parameters. Additionally, we explore how the Leptoquark model addresses the anomalies observed in the ratios of semileptonic $B$ meson decays, $R_{D^{(*)}}$. We determine that the restrictions derived from our analysis are consistent with the most recent experimental limits reported by the XENONnT and LUX-ZEPLIN collaborations. This conclusion is based on our evaluation of the transition magnetic moment from muon neutrino to tau neutrino, focusing on the allowed region for the Leptoquark Yukawa couplings.

Figures (6)

• Figure 1: One loop diagrams representing the scalar Leptoquarks $S^{1/3}$ and $R^{1/3}$ contribution to the transition neutrino magnetic moment. The arrows indicate the fermion flow and the convention for the four-momenta is depicted in the diagram (a).
• Figure 2: Leading-order parton-level Feynman diagrams that contribute to the $B$ meson decays for the SM contribution and the new physics contribution of $S^{1/3}$ and $R^{1/3}$.
• Figure 3: Allowed points with $95 \%$ C.L. in the $y^{L}_{23}y_{33}$ vs $y^L_{33}y^R_{23}$ plane consistent with the $R_{D^{(*)}}$ anomalies for two values of the LQ mass.
• Figure 4: Allowed points in the plane $y^L_{22}y^{R}_{22}$ vs $y^L_{32}y^{R}_{32}$ at $95\%$ C.L. consistent with the $(g-2)_\mu$ processes for $m_{LQ} = 1.5$ TeV (left panel) and $2$ TeV (right panel). Notice that in cases with opposite sign, a degeneracy in the parameters can lead to large values of the coupling products .
• Figure 5: Allowed areas with $95\%$ C.L. consistent with the limits on the LFV decay $\tau \to \mu\gamma$ and $(g-2)_\mu$. This also takes into account the constraints from the processes of the $B$ meson decays shown in Fig. \ref{['fig:space_RD']}, for a LQ mass of $1.5$ TeV (top panels) and $2$ TeV (bottom panels)