The charm quark as a naturalness success

Abstract

Undeniably, the naturalness principle has had a major role in particle physics during the last decades, in particular in model building. Nowadays, one can find a wide range of different definitions. Some of them seem mutually exclusive, but traditionally, its notion has been linked to the fine-tuning problem. Understanding naturalness as the imposition that fine-tuning problems have to vanish, for instance, due to the existence of new particles, new models as those based in supersymmetry were built. In order to palliate the fine-tuning problem the Higgs sector of the standard model seems to suffer, new physics should have appeared already in the last LHC run. Thus, the persistence of fine-tuning has originated numerous works exploring both, the limits and the different conceptual definitions of naturalness. However, little work has been done re-examining precisely one of the main pillars naturalness advocates: its historical successes. Given the current period in which the critics to the naturalness principle are undergoing, we find important to explore in detail the historical examples often cited, primarily the charm quark proposal and its mass prediction, and see how can this contribute to the debate.