Super Symmetry

MILESTONES DOI: 10.1038/nphys868 M iles Tone 1 3 Super symmetry The way that spin is woven into the in 1015. However, a form of symmetry very fabric of the Universe is writ between fermions and bosons called large in the standard model of supersymmetry offers a much more particle physics. In this model, which elegant solution because the took shape in the 1970s and can quantum fluctuations caused by explain the results of all particle- bosons are naturally cancelled physics experiments to date, matter out by those caused by fermions and (and antimatter) is made of three vice versa. families of quarks and leptons, which Symmetry plays a central role in are all fermions, whereas the physics. The fact that the laws of electromagnetic, strong physics are, for instance, symmetric in and weak forces that act on these time (that is, they do not change with particles are carried by other time) leads to the conservation of particles, such as photons and gluons, energy. These laws are also symmetric The ATLAS experiment under construction at the which are all bosons. with respect to space, rotation and Large Hadron Collider. Image courtesy of CERN. Despite its success, the standard relative motion. Initially explored in model is unsatisfactory for a number the early 1970s, supersymmetry is a of reasons. First, although the less obvious kind of symmetry, which, graviton. Searching for electromagnetic and weak forces if it exists in nature, would mean that supersymmetric particles will be a have been unified into a single force, the laws of physics do not change priority when the Large Hadron a ‘grand unified theory’ that brings when bosons are replaced by Collider comes into operation at the strong interaction into the fold fermions, and fermions are replaced CERN, the European particle-physics remains elusive. Second, the origins by bosons. laboratory near Geneva, in 2008. of mass are not fully understood. Although it is difficult to explain Peter Rodgers, Chief Editor, Third, gravity is not included. supersymmetry through analogies to Nature Nanotechnology Moreover, there are other, less classical physics, its consequences obvious problems with the standard are dramatic — it predicts that every ORIGINAL RESEARCH PAPERS Golfand, Y. A. & Likhtman, E. P. Extension of the algebra of model. The two natural mass scales in fundamental particle has a Poincaré group generators and violation of nature are zero and the Planck mass, superpartner with half a unit of spin P invariance. JETP Lett. 13, 323–326 (1971) | ~ 1019 GeV c−2. Neither photons nor less. The electron, for instance, has a Neveu, A. & Schwarz, J. H. Factorizable dual model of pions. Nucl. Phys. B 31, 86–112 (1971) | gluons (which carry the spin of a half, so its superpartner Ramond, P. Dual theory for free fermions. Phys. electromagnetic and strong forces, (which is known as a selectron) has Rev. D 3, 2415–2418 (1971) | Wess, J. & Zumino, B. Supergauge transformations in four dimen- respectively) have mass, but the W zero spin. This means that the sions. Nucl. Phys. B 70, 39–50 (1974) | Wess, J. & and Z bosons that are responsible for superpartner of a boson is always a Zumino, B. A Lagrangian model invariant under the weak force have masses of about fermion and vice versa. supergauge transformations. Phys. Lett. B 49, 52–54 (1974) −2 90 GeV c . Where does this mass Supersymmetry also plays a central FUrTher reading Dimopoulos, S., Raby, S. scale come from? role in theories that attempt to unify & Wilczek, F. Supersymmetry and the scale of This ‘hierarchy problem’ can be the forces in the standard model with unification. Phys. Rev. D 24, 1681–1683 (1981) | Almadi, U., de Boar, W. & Furstenau, H. Comparison solved by fine-tuning the model so gravity by treating fundamental of grand unified theories with electroweak and that various quantum fluctuations particles as vibrating strings or strong coupling constants measured at LEP. Phys. Lett. B 260, 447–455 (1991) | Greene, B. cancel out, although many physicists membranes in 10-dimensional or The Elegant Universe (Vintage, London, 2000) | are uncomfortable with this solution 11-dimensional spacetimes. In these Kane, G. & Shifman, M. (eds) The Supersymmetric because some parameters must theories the gravitational force is World: The Beginnings of the Theory (World Scientific, Singapore, 2000) be fine-tuned to better than 1 part carried by a spin-two boson called the NATURE MILESTONES | SPIN mARCH 2008 © 2008 Nature Publishing Group .

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