Gauged $\tau$-lepton chiral currents and $B \to K^{(*)} E_{\rm miss}$

We consider a class of theories with a $U(1)_X$ gauge symmetry associated with leptonic chiral currents. The low-energy effective field theory includes a light spin-$1$ boson coupled to the electroweak gauge sector via a Wess-Zumino term, which ensures anomaly cancellation in the infrared. As a concrete application, we show that a light vector boson with mass $m_X \simeq 2.1\,\text{GeV}$, coupled to a $\tau$-lepton chiral current, can readily account for the recent $3\sigma$ excess observed in $B \to K^{(*)} E_{\rm miss}$ at Belle II, while remaining consistent with existing constraints from $Z \to \gamma E_{\rm miss}$ and direct searches for anomalon fields responsible for anomaly cancellation in the ultraviolet. After classifying phenomenologically viable models, we explore in greater detail two concrete realizations which give rise to distinctive phenomenological signatures, potentially accessible at future experiments at the high-energy and intensity frontiers.