How well does supersymmetry explain the physics beyond the Standard Model?
Matthew Feuer Supersymmetry was created by physicists in the early 1970s as a mathematical relationship between elementary particles. 1 It is a hypothetical extension to the Standard Model of particle physics and a framework for numerous proposed theories, which attempt to explain the properties of the universe beyond the Standard Model. 2 Although the Standard Model has had great success in predicting the outcome of experiments, it is an incomplete theory of the universe as it fails to explain all natural phenomena. 3 Theoretical physicists have proposed various different theories, many incorporating and needing supersymmetry, as candidates for the ‘theory of everything’; a theory of everything is the ultimate goal of physics, as it would explain the physics beyond the Standard Model and describe every aspect of nature, from large galaxies to sub-atomic elementary particles. 4 The Standard Model divides the elementary particles into two classes, fermions and bosons, based on the particle’s spin; fermions make up ordinary matter whereas bosons are responsible for carrying the fundamental forces. Spin is a quantum-mechanical intrinsic type of angular momentum, which is fixed at integer or half-integer values. Differences in spin between particles determine whether they are fermions or bosons as fermions all have half-integer spin and bosons all have integer spin. These differences in spin create very different properties; fermions obey the Pauli exclusion principle so cannot occupy the same quantum state however bosons don’t obey the exclusion principle so can behave collectively. 5 Supersymmetry proposes a new symmetry of nature, whereby the equations of the supersymmetric theory will remain the same when fermions are replaced by bosons and bosons replaced by fermions. 6 Supersymmetry thus remarkably implies that fermions and bosons are fundamentally identical, although to us they seem to exhibit different properties. 7 Supersymmetry predicts that every particle would have a supersymmetric partner (super-partner) with spin differing by ½; this means each fermion would be paired with a boson with the same properties (except spin). 8 Here we run into a problem, in a true symmetry the super-partner should have the same mass as its partner but different
1 (Greene, 1999) 2 (Lykken & Spiropulu, 2014) 3 (CERN, n.d.) 4 (Duff, 2011) 5 ('t Hooft, 1997, pp. 27-28)
6 (Hey & Walters, 2003) 7 ('t Hooft, 1997, p. 136) 8 (Greene, 1999)
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