  We propose an evolution of the Mu2e experiment, called Mu2e-II, that would
leverage advances in detector technology and utilize the increased proton
intensity provided by the Fermilab PIP-II upgrade to improve the sensitivity
for neutrinoless muon-to-electron conversion by one order of magnitude beyond
the Mu2e experiment, providing the deepest probe of charged lepton flavor
violation in the foreseeable future. Mu2e-II will use as much of the Mu2e
infrastructure as possible, providing, where required, improvements to the Mu2e
apparatus to accommodate the increased beam intensity and cope with the
accompanying increase in backgrounds.
