Flavor anomalies in leptoquark model with gauged $U(1)_{L_\mu-L_\tau}$

TL;DR

This paper explores how a scalar leptoquark with gauged $U(1)_{L__}$ symmetry can explain flavor anomalies, especially in rare meson decays and $B$-meson ratios, by analyzing its impact on specific processes.

Contribution

It introduces a novel $U(1)_{L__}$-gauged scalar leptoquark model that naturally suppresses certain couplings and explains flavor anomalies in meson decays.

Findings

Enhancement of $BR(B\to K(K^*) \nu\bar\nu)$ and $BR(K_L \to \pi^0 \nu\bar\nu)$ by up to a factor of 2.

Model can fit the experimental values of $R(D)$ and $R(D^*)$.

$BR(K^+ \to \pi^+ \nu\bar\nu)$ can reach the upper $1\sigma$ experimental limit.

Abstract

Leptoquarks (LQs) have been extensively studied in the context of $B$ anomalies. When $U(1)_{L_\mu-L_\tau}$ is introduced to a scalar LQ model with the LQ $S_1$ charged under the new symmetry, $S_1$ primarily couples to the third-generation leptons while its couplings to first and second-generation leptons are naturally suppressed. Furthermore, only $S_1$ in the scalar LQ models has the feature that down-type quarks merely couple to neutrinos but not the charged leptons, avoiding strict restrictions from $b\to s \mu^+ \mu^-$. With this distinctive characteristic of $S_1$, we investigate its impact on rare processes involving the $d_i \to d_j \nu \bar\nu$ transitions. Under the dominant constraints from $\Delta F=2$ processes, we find that the $S_1$ contributions to the branching ratios (BRs) of $B\to K(K^*) \nu \bar\nu$ and $K_L \to \pi^0 \nu \bar\nu$ can be factorized into the same multiplicative factor multiplying the standard model predictions. Enhancement in the BRs can possibly exceed a factor of 2. In particular, ${\cal B}(K^+\to \pi^+ \nu \bar\nu)$ can reach the upper $1\sigma$ error of the experimental value, i.e., $\simeq 15.4 \times 10^{-11}$. We also show that the model can fit the new world averages of $R(D)$ and $R(D^*)$.