Supersymmetry

The first extension of the standard model that attempts to address some of its shortcomings and incorporate gravity is called supersymmetry, or SUSY for short. SUSY proposes that every fermion has a boson partner that is a totally new particle (not one of the known exchange bosons). Not only that, but all the known bosons have new fermion partners, too.

Since our goal was to simplify things, you may be wondering why we’d want to double the number of fundamental particles. Well, the reason is that the mathematics of supersymmetry actually allows for the exact unification of the strong force with the electroweak force at high energy. SUSY is also a necessary ingredient for string theory, which we discuss in the next section. With such a high degree of symmetry, more interactions can be calculated exactly. SUSY has plenty of room for a so-far unobserved exchange boson, called the graviton, to mediate the gravitational interaction and allow for a quantum field theory of gravity.

Unfortunately, none of the predicted supersymmetric partners to the well-known quarks and leptons has ever been observed. The reason could be that all of these new particles have much larger masses than their standard partners, which means that much larger and more energetic accelerators are needed to see them. But why might that be? It seems a little silly to invent another level of symmetry only to have it broken so drastically. On the other hand, one or more of the new particles could compose the mysterious Dark Matter.

By doubling the number of fundamental particles, supersymmetry has a lot of potential for explaining the huge difference in the strengths of the fundamental interactions. At this stage, however, it requires even more free parameters to do it, so it is not clear that it is a real improvement over the present standard model.