Complete lattice QCD calculation of $K^{-}\to \ell^{-}\bar{\nu}_{\ell}\ell^{'+}\ell^{'-}$ form factors

TL;DR

This paper reports the first complete lattice QCD calculation of form factors for rare charged kaon decays, providing precise Standard Model predictions for multiple decay channels.

Contribution

It introduces a comprehensive lattice QCD approach to determine all relevant form factors for $K^- o ext{leptons}$ decays with controlled uncertainties and novel spectral function reconstruction.

Findings

Form factors determined across experimental kinematic regions.

Decay rates and observables computed for four decay channels.

Results enable direct comparison with experimental measurements.

Abstract

We present the first complete lattice QCD calculation of the four structure-dependent form factors governing the rare charged kaon decay $K^- \to \ell^- \bar{\nu}_\ell \ell'^+ \ell'^-$, with fully controlled statistical and systematic uncertainties. Our calculation is based on gauge ensembles generated by the Extended Twisted Mass Collaboration (ETMC) with $N_f = 2+1+1$ flavors of Wilson-clover twisted-mass fermions. Simulations are performed directly at the physical values of the light and strange quark masses, and include an estimate of the quark-disconnected contributions in which the virtual photon couples to sea quarks. All four form factors are determined across the kinematical region probed by experiments. The Spectral Function Reconstruction (SFR) method of Ref. [1] is employed to overcome the analytic continuation problem for dilepton invariant masses above the two-pion threshold. Finite-volume effects are investigated using ensembles with spatial extents $L\simeq [3.8,7.6]~\mathrm{fm}$, while the continuum limit is obtained from three lattice spacings in the range $a\in[0.057, 0.08]~\mathrm{fm}$. Our results for the form factors enable the evaluation of decay rates and differential observables for all four channels, $K^- \to e^- \bar{\nu}_e e^+ e^-$, $K^- \to e^- \bar{\nu}_e \mu^+ \mu^-$, $K^- \to \mu^- \bar{\nu}_\mu e^+ e^-$, and $K^- \to \mu^- \bar{\nu}_\mu \mu^+ \mu^-$, thereby providing first-principles Standard Model predictions against which existing and upcoming measurements can be directly compared. A detailed phenomenological analysis of the decay rates and associated observables is presented in a companion paper [2].