Dark matter in a world of supersymmetric sparticles

Supersymmetry is not just a book about supersymmetry theories, but about why and how people research into particle physics. The book's subtitle, Unveiling the Ultimate Laws of Nature is a better description of its excellent contents than its title. Gordon Kane describes accurately the interaction between theoretical and experimental physics. His description of how science is actually carried out makes the book interesting to philosophers and historians of science as well as readers interested in particle physics.

Kane is keen to point out the differences between areas of science that are conceptually and experimentally well established and those that are still developing. He calls the developing sciences "research in progress". He also draws a distinction between effective theories that answer the questions as to how things work at different distance scales, and what he calls the primary theory that would answer questions about why things work.

Before launching into the heady world of supersymmetry, Kane gives a brief description of the Standard Model that reflects our current understanding of particle physics. Kane tackles the sometimes difficult concepts head on and explains the fundamental features of the Standard Model very carefully and clearly, avoiding oversimplification and inappropriate analogies, without getting bogged down in detail. The weaknesses of the Standard Model and the need for a deeper level of description are also well described.

It is not until nearly half way through the book that we turn to supersymmetry. Within the theory, all the particles of the Standard Model have supersymmetric partners, referred to as sparticles. As yet, none of these "sparticles" has been observed.

Although there is some indirect evidence for supersymmetry, it is still "research in progress". Supersymmetry is still only an effective theory, and not a candidate for the primary theory. As an effective theory, Kane describes how the world's being supersymmetric could help us to understand some of the big questions in particle physics and cosmology. In particular, he focuses on three questions. First, that of dark matter, ie matter that we cannot see, but believe to exist because we can observe its gravitational effects. The lightest supersymmetric partner is a prime candidate to make up this matter. Second, that of Higgs physics, the mechanism that allows particles to have mass in the Standard Model. Supersymmetry could provide an elegant explanation of how Higgs physics occurs. Third, why do we have a universe that is dominated by matter, if matter and antimatter were created in equal measure at the time of the big bang? Some of the problems with supersymmetry theories are also discussed, if briefly, including the decay of the proton.

The book tries to describe how we might actually find more direct and compelling evidence to support supersymmetry. The most direct way is through the "observation" of the supersymmetric partners in particle-physics colliders. The description is clear, and Kane has a good understanding of the experimental side of particle physics as well as the theoretical.

However, this section is already dated despite having been written only a year or so ago. The author suggests two places where we might have seen some evidence for the supersymmetric partners by now. One is the Large Electron Positron Collider at Cern; the other is the Tevatron at Fermilab. The first closed down at the end of last year without seeing evidence of the supersymmetric partners, and a newly upgraded Tevatron was only restarted in March of this year. However, it is difficult to see how the author could have avoided this reference from becoming dated.

Finally, Kane discusses whether it will ever be possible for us to gain an understanding of the primary theory. Perhaps it is simply beyond our ability to comprehend. He believes we can gain such an understanding, and that it would mean the death of particle physics. Although he accepts that some find this arrogant and that such predictions have proved to be wrong in the past, Kane does give a coherent set of arguments as to why things could be different this time around.

David Colling is a research physicist, Imperial College, London.