Dalitz decay of $K^*(892) \rightarrow K \ell^+\ell^-$: A New Probe for Hadronic Structure and Dark Photon Searches

TL;DR

This paper predicts the branching fraction and dilepton spectrum of the rare $K^*(892)$ Dalitz decay, proposing it as a new way to study hadronic structure and search for dark photons at BESIII.

Contribution

It provides the first calculation of the branching fraction for $K^*(892) ightarrow K \, ext{lepton pairs}$ and explores its potential for dark photon searches.

Findings

Predicted the branching fraction for the decay channel.

Analyzed the dilepton mass spectrum for form factor insights.

Discussed experimental sensitivity for dark photon detection.

Abstract

We present the first comprehensive study of the rare Dalitz decay $K^*(892) \rightarrow K \ell^+ \ell^- (\ell = e, \mu)$, providing a prediction for the branching fraction and the dilepton mass spectrum. This decay involves the emission of a virtual photon which converts into a lepton pair, offering a probe of the transition form factor $F_{K^*K}(q^2)$ and underlying meson structure. Using a single pole approximation for the form factor, we present the calculation of the branching fraction for this rare decay channel for the first time. Furthermore, we also investigate the potential to search for a light $A^\prime$ boson (dark photon) appearing as a narrow resonance in the dilepton spectrum, and discuss the experimental sensitivity and new physics opportunities at the dedicated BESIII experiment. Our results establish $K^*(892) \rightarrow K \ell^+ \ell^- (l = e, \mu)$ as a new laboratory for hadronic structure and dark-sector searches.