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

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 $R(D^*)$.