Is SUSY natural?

Abstract

Space-time supersymmetry (SUSY) is widely believed to play an important role in most fundamental theories of physics, and is usually invoked in order to address problems of naturalness. In this paper, we examine the question of whether SUSY itself is ‘natural’ (i.e. likely to exist as a fundamental component of nature at high-energy scales). Our approach to answering this question is based on a statistical examination of the heterotic string landscape, and our conclusion is that SUSY is an exceedingly rare phenomenon. We also find that the likelihood of SUSY appearing at the string scale is dependent on the gauge symmetries present at the string scale, with certain gauge groups strongly favoring the appearance of SUSY and others not. This paper summarizes several recent papers, yet also contains some new results. In particular, one new result is that the heterotic landscape appears to favor either the non-supersymmetric Standard Model or an SUSY GUT gauge group at the string scale; by contrast, the opposite outcomes (namely the minimal supersymmetric Standard Model (MSSM) or a non-supersymmetric GUT) are significantly disfavored.