February 17, 1997

Symmetry at its smallest

Or, can supersymmetry save the Standard Model of the universe from itself?

By Theobolt Leung

From a butterfly's wings or a daisy's petals to the cyclical repetition of the seasons, nature often shows a delicate, measured regularity. A cursory glance at the cramped equations scrawled all over the whiteboard in Michael Dine's office suggests not the slightest trace of such congruity. But it's there if you know where to look.

Dine, a noted theorist and professor of physics at UCSC, sketches in mathematics the interactions of elementary subatomic particles, the stuff of which everything is made. As his reference he uses the controlled subatomic collisions conducted at high-powered particle accelerators, such as Fermilab, the European Center for Nuclear Research (CERN), or the Stanford Linear Accelerator Center. He pieces together his models from clues gathered by experimental particle physicists, who sift through the fleeting rubble of the impacts that peel apart the fundamental building blocks of matter.

Yet in these minute, violent experiments, Dine sees traces of refined symmetry that rivals even butterflies. "Nature is subtle," he says, "even at its most stripped-down level."

In a talk entitled "Physics at the Shortest Distances," Dine spoke about some of the elusive patterns that may shape our universe--including what might be one of the most fundamental patterns of all, supersymmetry. The talk was part of a session on "Symmetries and Asymmetries in Science" on February 14 at a meeting of the American Association for the Advancement of Science in Seattle.

Physicists try to re-create the fiery conditions at the beginning of the universe in particle colliders. By crashing speeding protons, electrons, and their kin into each other, they produce showers of even finer grains that naturally existed in the hot primordial soup. Like fossils, the fragments hint at what forces were in play seconds after the Big Bang.

Things have cooled considerably since, and the universe has settled into what seems to be a dual personality. Theorists have forged a set of equations describing the laws of physics that rule the scale of our everyday lives, but they need a completely different set for what goes on at atomic distances. The combination of these two models--the "Standard Model" of particle physics--falls short in a number of ways. It can't explain why matter weighs as much as it does or why only certain particles show up in the colliders. Dine thinks supersymmetry may solve these mysteries.

Researchers already have spotted an unruly zoo of elementary grains. So far they mostly have seen bits of matter dubbed "fermions." These include leptons--the electron's family--and baryons, to which protons and neutrons belong. Physicists also have seen tiny packets they call "bosons," which carry the fundamental forces that govern nature. The photon, for example, is responsible for delivering the electromagnetic force in the form of light to your eye--allowing you to see. Both the forces and the particles are quite well studied and described in the Standard Model, but they are treated as unrelated species.

The theory of supersymmetry proposes that all fermions are mirrored by counterpart bosons. Scientists call the electron's hypothetical mate the "selectron," for example, and the photon's partner the "photino." In this way, the theory outlines an elegant hierarchy into which all elementary particles would fall--like the periodic table of the elements--thus bridging the gap between force and matter.

The theory also suggests the existence of the Higgs boson, which imparts matter's unaccounted-for mass. Physicists are optimistic that they will find these particles using the next generation of higher powered colliders.

Dine is trying to generalize the Standard Model to include supersymmetry. But the theory has its limitations. ("In the 1970s, lots of things were called 'super,'" say Dine, "so don't read much into it.") The theory fails to resolve the Standard Model's split personality, for example. Our best bet for a unified theory to accomplish that, says Dine, is a separate class of ideas called string theory.

So far, researchers have only the smallest wisps of experimental data that hint at the possibility of supersymmetry, and absolutely none for string theory. Scientists spotted apparent evidence of a supersymmetric particle during an experiment at Fermilab's Tevatron accelerator in April 1995. In the wake of the finding many theorists, including Dine, published a flurry of papers proposing explanations for the phenomenon. But the resolution of the data was poor, and now physicists agree that what they saw could simply have been a glitch in measurement. The Tevatron is currently being upgraded, which should allow researchers to confirm or eliminate the April anomaly. Its next run is scheduled for 1999.

In the meantime, it's back to the whiteboard for Dine and his colleagues. As he continues to probe for clues to the texture of the universe's fabric, Dine often pauses to thinks about supersymmetry, its implications, and its beauty. The theory, says Dine, is "so rich and so pretty, we sometimes forget that we haven't had any evidence that it has anything to do with anything."

Return to the Currents home page

Go to UCSC's home page