Grand Unification

A puzzling feature of the standard model is the large disparity in strength between electroweak interactions and the strong interaction of QCD. Yet these interactions have very similar structures, both modeled on the gauge invariance of electromagnetism. Soon after the emergence of the standard model, it was realized and confirmed experimentally that the strengths of these interactions change with distance: As the distance gets smaller, the strong interaction grows weaker and the weak interaction grows stronger. They ultimately come together at distances roughly a million billion times smaller than those now being probed. At this tiny distance the strong and electroweak theories may combine into a single grand unified theory, incorporating symmetries beyond those of the strong and electroweak theories.

An especially dramatic prediction of grand unified theories is that the proton, a basic building block of matter, decays into lighter particles with a very small probability. In fact, a large class of grand unified theories predict that protons have a mean lifetime in excess of a trillion trillion times the age of our universe. This explains why we still have plenty of protons. Despite 20 years of careful searches with massive detectors located far underground to avoid spurious signals, no proton decay has been seen, ruling out the simplest grand unified theories. However, some of the most appealing versions of these theories incorporating supersymmetry predict that current detectors are very close to having the required sensitivity.

Neutrinos

A startling discovery made recently may be a different sign of grand unification. Nuclear and particle experimentalists using underground detectors similar to those involved in the search for proton decay detect neutrinos produced by the Sun and by cosmic-ray interactions in Earth's atmosphere (see sidebar “Massive Neutrinos and Neutrino Astrophysics”). The results indicate that a neutrino belonging to one family can spontaneously transform into a neutrino from another, a process known as neutrino oscillation. This can happen only if neutrinos have a nonzero mass, a possibility outside the standard model but easily accommodated in grand unified theories. In fact, the mass determined by the Super-Kamiokande atmospheric neutrino experiment in Japan may be a sign of new interactions at the grand unified scale. Housed deep within a mine in the Japanese alps, Super-Kamiokande contains 50,000 tons of ultrapure water, the inner portion of which is viewed by an array of 13,000 photomultiplier tubes.