Warped Passages - Lisa Randall [129]
Once these pioneers had worked out a four-dimensional supersymmetric theory, more physicists paid attention. However, the Wess-Zumino model of 1973 couldn’t accommodate all the Standard Model particles; no one yet knew how to add force-carrying gauge bosons to a four-dimensional supersymmetric theory. The Italian theorists Sergio Ferrara and Bruno Zumino solved this difficult problem in 1974.
On a train trip from Cambridge to London, where we had just attended the Strings 2002 conference, Sergio told me how finding the right theory would have been an impossibly difficult problem had it not been for the formalism of superspace, an abstract extension of spacetime that has additional fermionic dimensions. Superspace is an extremely complicated concept, and I shall not attempt a description of it. The important point here is that this entirely different type of dimension—which is not a dimension of space—played a crucial role in supersymmetry’s development. This purely theoretical device continues to simplify supersymmetry calculations today.
The Ferrara-Zumino theory told physicists how to include electromagnetism and the weak and strong forces in a supersymmetric theory. However, supersymmetric theories did not yet include gravity. So the remaining question for a supersymmetric theory of the world was whether it could incorporate this remaining force. In 1976, three physicists, Sergio Ferrara, Dan Freedman, and Peter van Nieuwenhuizen, solved this problem by constructing supergravity, a complicated supersymmetric theory that contains gravity and relativity.
The interesting thing is that while supergravity was being formulated, string theory was marching forward independently. In one of the key theoretical developments in string theory, Ferdinando Gliozzi, Joel Scherk, and David Olive discovered a stable string theory as an outgrowth of the fermionic string theory that Ramond, along with Neveu and Schwarz, had developed. Fermionic string theory turned out to contain a type of particle that no one had previously encountered except in supergravity theories. The new particle had identical properties to the supersymmetric partner of the graviton known as the gravitino, and this is indeed what it turned out to be.
Because of the concurrent development of supergravity, physicists seized on and pursued this common element of the two theories, and soon realized that supersymmetry was present in fermionic string theory. With that, the superstring was born.
We’ll return to string theory and the theory of the superstring in the following chapter. For now, we’ll focus on the other important application of supersymmetry: its consequences for particle physics and the hierarchy problem.
The Supersymmetric Extension of the Standard Model
Supersymmetry would be most economical and compelling if it paired known particles with each other. However, for this to be true the Standard Model would have to contain equal numbers of fermions and bosons—but it doesn’t satisfy this criterion. That tells us that if our universe is supersymmetric, it must contain many new particles. In fact, it must contain at least twice the number of particles that experimenters have so far observed. All the fermions of the Standard Model—the three generations of quarks and leptons—must be paired with new, as yet undiscovered bosonic superpartners. And the gauge bosons—the particles that communicate the forces—must have superpartners, too.
In a supersymmetric universe, the partners of quarks and leptons would be new bosons. Physicists, who enjoy whimsical (but systematic) nomenclature, call them squarks and sleptons. In general, the bosonic supersymmetric partner of a fermion has the same name as the fermion, but with an “s” at the beginning. Electrons are paired with selectrons, for example, and top quarks with stop squarks. Every fermion