Measuring the Breaking of Lepton Flavour Universality in $B\to K^*\ell^+\ell^-$

TL;DR

This paper proposes sensitive measurements of angular observables in rare B meson decays to detect potential violations of lepton flavor universality by comparing electron and muon channels, with suppressed hadronic uncertainties.

Contribution

It introduces new observables in B decay analysis that are highly sensitive to differences in Wilson coefficients for electrons and muons, reducing hadronic uncertainties.

Findings

Proposed observables are highly sensitive to LFU breaking in $C_9$.

Hadronic contributions are suppressed to below 7% in the specified $q^2$ range.

Suppression of hadronic effects is stronger at high $q^2$ regions.

Abstract

We propose measurements of weighted differences of the angular observables in the rare decays $B \to K^*\ell^+\ell^-$. The proposed observables are very sensitive to the difference between the Wilson coefficients $\mathcal{C}_9^{(e)}$ and $\mathcal{C}_9^{(\mu)}$ for decays into electrons and muons, respectively. At the same time, the charm-induced hadronic contributions are kinematically suppressed to $\lesssim 7\% (4\%)$ in the region $1\,$GeV$^2 \leq q^2 \leq 6\,$GeV$^2$, as long as LFU breaking occurs only in $\mathcal{C}^{(\ell)}_{9}$. This suppression becomes stronger for the region of low hadronic recoil, $q^2 \geq 15\,$GeV$^2$.