# New light mediators for the $R_K$ and $R_{K^*}$ puzzles

## TL;DR

This paper investigates light scalar and vector mediators around 25 MeV as potential explanations for anomalies in lepton universality measurements $R_K$ and $R_{K^*}$, proposing models that fit the data better than previous explanations.

## Contribution

It introduces light scalar and vector mediator models with specific couplings that can simultaneously explain the $R_K$ and $R_{K^*}$ anomalies and other measurements, addressing the low $q^2$ challenge.

## Key findings

- A 25 MeV $Z'$ with $q^2$-dependent $b-s$ coupling and electron coupling explains all anomalies.
- A 25 MeV scalar provides a good fit except for low $q^2$ $R_{K^*}$.
- A 25 MeV $Z'$ with muon coupling fits $R_K$ and $R_{K^*}$ data but not low $q^2$ $R_{K^*}$.

## Abstract

The measurements of $R_{K}$ and $R_K^{*}$ provide hints for the violation of lepton universality. However, it is generally difficult to explain the $R_{K^*}$ measurement in the low $q^2$ range, $0.045 \le q^2 \le 1.1$ GeV$^2$. Light mediators offer a solution by making the Wilson coefficients $q^2$ dependent. We check if new lepton nonuniversal interactions mediated by a scalar ($S$) or vector particle ($Z^\prime$) of mass between $10-200$ MeV can reproduce the data. We find that a 25 MeV $Z^\prime$ with a $q^2$-dependent $b-s$ coupling and that couples to the electron but not the muon can explain all three anomalies in conjunction with other measurements. A similar 25 MeV $S$ provides a good fit to all relevant data except $R_{K^*}$ in the low $q^2$ bin. A 25 MeV $Z^\prime$ with a $q^2$-dependent $b-s$ coupling and that couples to the muon but not the electron provides a good fit to the combination of the $R_K$ and $R_{K^*}$ data, but does not fit $R_{K^*}$ in the low $q^2$ bin well.