In nuclear reactions, catalysts can potentially affect the reaction rates by providing alternative pathways or by stabilizing intermediate states. For example, in muon-catalyzed fusion, muons (heavier analogs of electrons) replace electrons in hydrogen isotopes, bringing the nuclei closer together and facilitating fusion at lower temperatures. This effect is due to the muon's ability to reduce the Coulomb barrier, which is the energy barrier due to electrostatic repulsion between positively charged nuclei.
