spin, and we would have known for a long time if such partners existed. 9 However, this was not the end of supersymmetry; physicists discovered it could be explained by spontaneous symmetry breaking. During the high-energy early universe, the super-partners were identical massless
particles 10 but as the universe cooled and its energy decreased, the regular particles gained a small mass (via the Higgs mechanism) whereas the super-partners acquired larger masses than we are currently able to detect. 11 This means the symmetry would be broken and so supersymmetry is hidden from our low-energy view. 12
Figure 1 - The supersymmetric particles ( (TJ, 2013))
The first reason that physicists have for supersymmetry being a true property of the universe is due to aesthetics. There are a number of symmetries in the universe, including translation in space and time or rotation; in 1967, the Coleman-Mandula theorem stated that no other symmetries in space, time or motion were possible other than those already known. However, when physicists closely investigated the theorem they found it had a loophole allowing exactly one more symmetry: symmetry in spin
Figure 2 - Force unification ( (New Scientist, 2011))
(supersymmetry). It is mathematically possible to have a universe without symmetry in spin; however, many physicists were sceptical that the universe would have every possible symmetry except one and believed a universe lacking supersymmetry compromises the beauty of nature. 13 Supersymmetry is also believed to explain the physics beyond the Standard Model. The first major problem with the current Standard Model is the ‘hierarchy problem’, 14 closely related to the problems of ‘fine-tuning’ and ‘naturalness’. The hierarchy problem concerns the mass of the Higgs boson, which is the boson of the field responsible for giving elementary particles mass. It was discovered in 2012 at the Large Hadron Collider (LHC) with a mass of 125 GeV; however, due to quantum interactions with virtual particles, the theoretical mass enormously increases to 10 19 GeV. This mass is on the Planck scale; at this point gravity becomes as strong as the other forces, causing space-time to warp and fill with black holes. 15 The large mass of this theoretical Higgs does not agree with the observations at the LHC; physicists could solve this problem within the Standard Model but only by manually fine-tuning some of the 20 parameters within the Standard Model. This fine-tuning required the parameters to be altered with tremendous precision; Brian Greene wrote, ‘Such precision is on par with adjusting the launch angle of a bullet fired from an enormously powerful rifle, so that
9 (Strassler, Supersymmetry — What Is It?, 2011) 10 (Ananthaswamy, In SUSY we trust: What the LHC is really looking for, 2009) 11 (Greene, 1999) 12 (Strassler, Supersymmetry — What Is It?, 2011) 13 (Greene, 1999) 14 (Ananthaswamy, In SUSY we trust: What the LHC is really looking for, 2009) 15 (Chalmers, 2012)
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