Standard Model tests with smeared experiment and theory
For Standard Model processes in which on-shell intermediate hadronic states contribute - including inclusive semileptonic decays and long-distance effects in rare exclusive decays such as $D\to π\ell\ell$ and $B\to K^{(\ast)}\ell\ell$ - spectral-reconstruction techniques provide a promising route to model-independent lattice QCD predictions for use in phenomenological predictions. The central ingredient is the computation of the energy-smeared spectral density. Following the continuum and infinite-volume limits, the physical amplitude is recovered as the limit of vanishing smearing width. However, achieving sufficiently small smearing for a controlled extrapolation remains a significant challenge for current lattice simulations. In this paper, we therefore propose Standard Model tests, in which both experimental results and theory predictions are smeared with finite width, similar to what has previously been done in the literature for experimental and lattice $R$-ratio data in the context of the muon $(g-2)_μ$. As concrete examples, we discuss the cases of inclusive meson decay and long-distance contributions to rare semileptonic meson decay.