Desperately seeking supersymmetry (SUSY)

Abstract

The discovery of x-rays and radioactivity in the waning years of the 19th century led to one of the most awe-inspiring scientific eras in human history. The 20th century witnessed a level of scientific discovery never before seen or imagined. At the dawn of the 20th century only two forces of nature were known—gravity and electromagnetism. The atom was believed by chemists to be the elemental, indestructible unit of matter, coming in many unexplainably different forms. Yet J J Thomson, soon after the discovery of x-rays, had measured the charge to mass ratio of the electron, demonstrating that this carrier of electric current was ubiquitous and fundamental. All electrons could be identified by their unique charge to mass ratio.In the 20th century the mystery of the atom was unravelled, the atomic nucleus was smashed, and two new forces of nature were revealed—the weak force (responsible for radioactive β decay and the nuclear fusion reaction powering the stars) and the nuclear force binding the nucleus. Quantum mechanics enabled the understanding of the inner structure of the atom, its nucleus and further inward to quarks and gluons (the building blocks of the nucleus) and thence outward to an understanding of large biological molecules and the unity of chemistry and microbiology.Finally the myriad of new fundamental particles, including electrons, quarks, photons, neutrinos, etc and the three fundamental forces—electromagnetism and the weak and the strong nuclear forces—found a unity of description in terms of relativistic quantum field theory. These three forces of nature can be shown to be a consequence of symmetry rotations in internal spaces, and the particular interactions of each particle are solely determined by their symmetry charge. This unifying structure, describing all the present experimental observations, is known as the standard model (SM). Moreover, Einstein's theory of gravity can be shown to be a consequence of the symmetry of local translations and Lorentz transformations.As early as the 1970s, it became apparent that two new symmetries, a grand unified theory of the strong, weak and electromagnetic interactions in conjunction with supersymmetry (SUSY), might unify all the known forces and particles into one unique structure. Now 30 years later, at the dawn of a new century, experiments are on the verge of discovering (or ruling out) these possible new symmetries of nature. In this paper we try to clarify why SUSY and supersymmetric grand unified theories are the new SM of particle physics, i.e. the standard against which all other theories and experiments are measured.