Almost no one doubts that the Standard Model is only an effective theory that has to be incorporated into a larger framework. What this framework will ultimately look like, we do not know. Empirical facts that we cannot account for in the Standard Model, such as neutrino masses, dark matter, and dark energy, provide some guidance. Aesthetic considerations such as the desire for unification of interactions and for an understanding of the patterns of matter fermion masses and mixing angles also guide our thinking. Although this seems rather removed from particle physics today, we also hope that one day we will have a framework that consistently incorporates gravity.

It was, however, efforts to resolve the fine-tuning problem of the Standard Model that led us to arrive at the exciting conclusion that there must be new physics at the TeV scale that can be probed at high energy colliders such as the LHC or a TeV electron–positron linear collider. Weak scale supersymmetry provides an attractive resolution of this problem, and continues to hold promise also for several other reasons, detailed at the end of Chapter 2. Indeed, many of these positive aspects of supersymmetric models have become evident only in the last 10–15 years – many years after the discovery of supersymmetry, and well after the effort to explore its phenomenological implications had begun in earnest. We believe that the motivations for seriously examining supersymmetry remain as strong as ever.

These promising features notwithstanding, SUSY is not a panacea.