From the Atkins diet to the Metabolism diet and the Russian Air Force diet, there’s no shortage of ideas on the correct way to understand the origins of human mass. When it comes to the deeper question of the masses of the elementary particles that make up the ordinary mass of the universe, things aren’t much different.
Most physicists agree that the stunning successes of the standard model of particle physics imply that particle masses must be due to their interactions with a new field known as the Higgs field. At currently accessible energies, it is thought that the Higgs field is a scalar field, and that elementary particles acquire mass because the universe is filled with a condensate of the Higgs field, through which all standard model particles are forced to wade.
One of the hints that the standard model may not be complete comes from the properties of the quantum theory of this Higgs field. For the Higgs field to perform properly, its mass must be around the weak scale (except for in an extremely narrow region of parameter space). However, the masses of scalar fields receive large corrections from quantum effects (so-called quadratic divergences) and, unless one fine-tunes the theory a truly ridiculous amount, the expected mass of the Higgs boson may be as high as the Planck scale – sixteen orders of magnitude higher than the weak scale. This tension, between the required low mass scale of the Higgs field and the ultra-high Planck scale to which it might be driven, is known as the hierarchy problem of the standard model.
Much of the motivation for the next generation of particle colliders – the Large Hadron Collider (LHC) at CERN, and the proposed International Linear Collider (ILC) – comes from the desire to identify the mechanism responsible for the generation of mass (finding the Higgs particle) and to understand the structure through which the hierarchy mentioned above is rendered stable. Other motivations come from the connections between colliders and cosmology.
In the most recent issue of Scientific American, Gordy Kane, a renowned particle theorist from the University of Michigan, discusses these issues in a nicely written article titled The Mysteries of Mass. One of the leading candidates to explain the hierarchy problem is supersymmetry, which tames the quadratic divergences by introducing a new set of particles into the model, related to the standard model particles by, well, supersymmetry. Gordy has been a major player in research into supersymmetry, and understandably this constitutes one focus of the article. It is worth pointing out, however, that there are a number of other ideas floating around (such as extra dimensions, on which JoAnne is an expert), not to mention the perennial favorite – none of the above. Whether you find the idea of supersymmetry compelling or not, the origin of mass is certainly one of the deepest questions facing physics today, and I think Gordy’s article provides an interesting summary of the main questions and challenges.
As we approach the turn on of the LHC, probably in 2007, serious attempts to figure out how to interpret its data to distinguish between the various ideas that theorists have generated have ramped up. To this end, in a few days, CERN will host the LHC Olympics – a sort of blind test of how well such a distinction might be made. They even have a pdf primer. When the results of all this are in, hopefully in a few months, I (or JoAnne if I can persuade her) will report on what happened.