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A top priority for the entire field of particle physics is the search for particles predicted by supersymmetry—a theory that aspires to explain much of the physics that cannot be understood within the standard model. Now, after decades of planning and work by thousands of scientists around the world, a report appearing in Physical Review Letters from the ATLAS collaboration [1] and a similar paper from the CMS collaboration [2] are presenting the results of the first supersymmetry searches at the 7 tera-electron-volt (TeV) Large Hadron Collider (LHC) at CERN. While these early searches did not turn up the long-sought-after particles, there is good reason to believe that supersymmetry is there to be discovered at the LHC (Fig. 1).

The standard model (SM) of particle physics has been incredibly successful, but few physicists believe it is the final story. The model doesn’t explain why particles have mass, the presence of dark matter and dark energy in the universe, or the excess of matter over antimatter. Nor does the standard model incorporate a quantum theory of gravity. Among the many theories that go beyond the standard model, there are few that are sufficiently compelling that they warrant a comprehensive and systematic set of searches to see if they are realized in nature. Supersymmetry (SUSY) is one such theory [3]. Essentially, SUSY is a theory that predicts an as-yet-unobserved symmetry between fermions and bosons. For example, each of the quarks and leptons have bosonic counterparts, often called squarks and sleptons, and these and other “sparticles” can be searched for in the high-energy collisions at the LHC.

At the cost of predicting a whole set of new particles, SUSY provides a fix for a number of the standard model’s problems. For example, the standard model predicts a divergent value for corrections to the Higgs boson’s mass, but SUSY offers a way around this problem, provided the sparticles aren’t too heavy [4]. Another exciting possibility is that SUSY provides a way to unify the different forces coupling constants at very high energy. There is no a priori requirement that this must happen, but the potential unification of the electroweak and strong forces has an elegance that is tantalizing [5]. Many versions of SUSY have an extra conservation law that would prohibit the decay of the lightest SUSY particle. Not only would this particle become a dark matter candidate, but in this context SUSY can be used to provide both a full calculation of early universe physics and the dark matter relic density, a central problem in modern cosmology [6]. Finally, SUSY allows a possible connection to quantum gravity through superstring theory.

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Category: Science