iCHSTM 2013 Programme • Version 5.3.6, 27 July 2013 • ONLINE (includes late changes)
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The problem of ‘naturalness’ in high-energy physics from the 1980s to the discovery of the Higgs boson in summer 2012
Arianna Borrelli | University of Wuppertal, Germany

Since the late 1970s the „Standard Model“ has been regarded as an eminently successfull theory of particle phenomena, yet theoretical speculations on and experimental searches for physics beyond it have been underway for the last three decades, and one of the main motivations and guidelines for them has been the fact that the Stanndard Model, despite its empirical successes, displayes features which the high-energy-physics community regards as „unnatural“. There were and still are many definitions of „naturalness“ in high-energy physics, but the general idea behind all of them is that a theory describing the inner workings of nature should not simply deliver consistent, correct predictions of phenomena, but should also do so without need of „fine-tuned“ computations (e.g. two very large numerical contributions almost cancelling each other to give a very small results) or large hierarchies of input parameters (e.g. the widely diverging masses of quarks). This „naturalness problem“ is neither an empirical anomaly nor a theoretical inconsistence, and physicists are ready to characterize it as an „aesthetic“ or „philosophical“ issue. Nonetheless, they reagard it as a most serious flaw of present particle theory and a reason to believe in the existence on „new physics“ beyond it. As the renouned theorist Mikhail Shifman recently put it: „The criterion of naturalness is aesthetic, or, if you wish, philosophic. If you do not like it you can ignore it. Most people like it“ (Shifman, ArXiv:hep-ph:1211.0004, 2012).

In my contribution I will sketch how the „naturalness problem“ emerged around 1980 to rapidly become a motor of theoretical and experimental research, from its first formulation by Leonard Susskind in 1979 to the doubts finally cast upon its significance by the discovey of the Higgs boson at the Large Hadron Collider announced in July 2012.