The "hierarchy problem" and the question of "naturalness" are seen as shortcomings of the otherwise eminently successful Standard Model of electroweak interactions. Although usually regarded as "aesthetic", such criteria serve as main guidelines in the practice of theoretical model-building, and regarding them purely as expressions of abstract ideals of beauty or simplicity does not do justice to their multiform heuristic functions. In my talk I shall discuss how these notions may be explored using an integrated historical-philosophical approach in which their origins are reconstructed with special attention to the motivation for introducing them in the first place and the transformation they went through in the last three decades.
Today, the terms "hierarchy problem" and the "naturalness" are seen as closely connected with each other and linked to the fine tuning of the Higgs mass corrections and to the difference between the electroweak and the Planck scale. However, their origins in the 1970s are thoroughly distinct and did not concern the Higgs mass or the Planck scale. Notions of "naturalness" started circulating among theoretical physicists in the early 1970's and being used as guidelines for model-building. More often than not, they were linked to the newly famous notion of spontaneous symmetry breaking, which theorists at the time hoped might help circumvent many problems of renormalization. Steven Weinberg (1972) was among the first to search for "natural" theories in this sense, but the notion was discussed with particular care in 1974 by Howard Georgi and Abraham Pais, who proposed their own idea of "naturalness" as a guideline to construct new models of particle interactions. The link between naturalness and the fine-tuning of the Higgs mass was introduced only in 1979 by Leonard Susskind - on a suggestions by Kenneth Wilson - as he formulated the first technicolor model. Later on, Gerhard 't Hooft (1980) and Martin Veltmann (1981) offered their own views on the subject which complemented, but in part also contradicted, Susskind's. Meanwhile, the original "hierarchy problem" had emerged in context of early grand unified theories (1976) and had come to be occasionally associated also to the Planck scale.
At the beginning of the 1980s, all these various threads were woven together to benefit the rise of supersymmetry, which allegedly solved the problem of naturalness as well as the hierarchy problem of grand unified theories. Yet in the late 1990's, things took a new twist, and the search for solutions to the "big" hierarchy problem concerning the Planck scale led to the emergence of a whole new class of models, i.e. those with one or more extra dimensions.

Transparencies (2.0 MB)

Transparencies

Poster (304KB)

Physikseminar_borrelli18october.pdf