SUSY:
We Know It's There ... Or Do We?

Reviewed by Steve Martin

The triumph of discovering the top quark at Fermilab was inversely proportional to its surprise value.

Theory told us it was there, and how it would behave. We knew we would find it--knew we HAD to find it--and many of us delighted in having an explanation ready when curious, non-physicist friends asked the question they thought would stump us before the discovery.

"So, let's see if I've got this straight," their challenge might begin. "People at Fermilab are smashing protons and antiprotons together to try to make top quarks. Nobody has ever seen one of these top quarks, and yet you say you are absolutely sure they exist. Why?"

The best answer lies in the symmetries of the equations of the Standard Model of elementary particles and forces, the foundation of modern particle physics. One particular symmetry, called "weak isospin," requires that some equations don't change if the symbol used to represent each particle is exchanged with one belonging to a partner particle. The up and down quarks are partners of each other; so are the strange and charm quarks. Every quark must have its own partner, or the mathematics of the Standard Model would fall apart. The Standard Model was so well-tested in many other ways that this was inconceivable. Thus, the undiscovered partner of the bottom quark, the top quark, simply HAD to exist.

This was hardly the first time that a symmetry had insisted on predicting a new particle. In one of the earliest examples, Dirac used the relativistic symmetries of space and time to make the stunning prediction that the electron must have a partner. The positron was soon discovered.

In his new book "Supersymmetry," University of Michigan physics professor Gordon Kane uses these examples and other arguments to show how we might anticipate the next great discoveries. In clear language aimed at a wide audience, he explains what supersymmetry is, why many particle physicists believe in it, how to look for it, and what a successful hunt might bring home.

Supersymmetry is a particularly ambitious cousin of the known symmetries. Like weak isospin, it predicts that every fundamental particle has a new partner, called a superpartner. The revolutionary, or "super," part is that the superpartners must carry different amounts of spin. No other symmetry dares to do such a thing, because particles with different spins don't seem to behave much like each other.

Circumstantial evidence yields the only clues to superpartners, and not all particle physicists are convinced they exist, as they were about the top quark. Still, as Kane explains, "Supersymmetry is now a sufficiently mature area, and sufficiently close to confirmation... that a wider understanding of its content and implications is both possible and worthwhile." And he illustrates why many particle theorists have fallen in love with supersymmetry as the preferred pathway beyond the Standard Model.

Kane reviews the Standard Model with special attention to puzzles that point towards supersymmetryóincluding the Higgs boson, postulated as the source of mass, the next great quest for particle physicists. He explains that the tiny length scales at which a Higgs boson would be important are actually huge compared to the length scales where gravity produces strong quantum effects. Without supersymmetry, this appears to be very odd; these two important length scales are hard to separate in the equations. With supersymmetry, pairs of superpartners can easily conspire to allow the two length scales to be as different as they indeed are.

All the superpartner particles ("sparticles") are introduced next, with such whimsical-sounding names as squark, slepton, photino, and Wino ("Wee-no"). Using easy-to-understand diagrams, Kane offers a guide to some of their habits and personalities. There is a specific, but not technical, discussion of how we expect supersymmetric particles to be produced in colliders, and how the results of their decays are detected. Kane clearly explains how to tell the resulting events apart from those produced by known particles. Not surprisingly, Fermilab plays a central role here, and Kane strongly makes the point that this is a great place to discover superpartners, if they exist. Kane even extends an invitation to visit Fermilab, complete with photographs. Kane also argues eloquently for the value of future projects and the necessity of funding them.

What will we learn if supersymmetry is found? Among other possibilities, Kane proposes supersymmetry as a solution to the dark matter problem. He is not afraid to point out that supersymmetry provokes some challenges of its own. He argues, however, that discovering sparticles may help us to get at what he calls the "primary theory." This is an insightful (and sensible) term for what others have called the "theory of everything" (slightly grandiose, considering it doesn't address the behavior of complex systems) or the "final theory" (unnecessarily bleak). While tests of a primary theory will be possible, it isn't so clear that anything like decisive tests will be forthcoming, so I found this part of the book to be especially provocative.

For nearly two decades, Kane has been an enthusiastic advocate for taking sparticles seriously in experimental physics, not always a popular position. For the first ten years or so after its proposal in the early 1970's, supersymmetry was widely regarded as a theoretical plaything. Kane recalls those days when theory research funds were granted (to others) with a strict condition that they NOT be spent on investigating supersymmetry.

Whether or not supersymmetry wins final vindication within the decade to come, Kane's book will remain a readable and fascinating account of how physicists make educated guesses and test them.