  Hadronic spectral densities play a pivotal role in particle physics, a primer
example being the R-ratio defined from electron-positron scattering into
hadrons. To predict them from first principles using Lattice QCD, we face a
numerically ill-posed inverse problem, due to the Euclidean signature adopted
in practical simulations. Here we present a recent numerical analysis of the
vector isovector spectral density extracted using the multi-level algorithm
(recently extended also to the case of dynamical fermions) and discuss its
implications.

High Energy Physics - Lattice