COVER STORY

Heady Collisions

Move over, Higgs boson. Columbia scientists at the Large Hadron Collider are searching for the key to a unified theory of everything.

by David J. Craig Published Summer 2013
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Illustration by Keith Negley

Over the past few years, Parsons and Willis used their intimate knowledge of the ATLAS detector to develop new analytic techniques that enabled scientists for the first time to identify photons that are second-generation byproducts of collisions. The method involves not only tracing the photons’ paths but also recording how much time they take to reach the detector’s sensors: even the slightest delay will signal that they sprouted from other, slower particles that traveled the first few centimeters from the collision.

Parsons and Willis’s techniques were instrumental in finding the Higgs boson, which had been a Holy Grail for physicists since being theorized in the 1960s. The Higgs is not invisible, although it might as well be: it appears only fleetingly in high-energy circumstances before transmuting into more ordinary particles like photons, the way a hurricane downgrades into a tropical storm.

“One of the Higgs’s most distinctive decay patterns is turning into two photons,” Parsons says. “So analyzing the journeys of these light particles was helpful in pulling off its mask.”

The importance of the Higgs boson’s discovery, which was announced last summer, is difficult to overstate. The Higgs is a physical manifestation of an energy field that permeates space and acts like cosmic molasses, slowing down particles as they move. It thus explains how particles acquire mass, which was the last remaining gap in the Standard Model.

To Parsons and to many other scientists working at the Large Hadron Collider, however, the real drama is just beginning. As important as the discovery of the Higgs boson was, it hardly came as a surprise. The existence of the Higgs had been theorized for so long, and physicists had accumulated so much indirect evidence for it, that the work had come to feel like a long, slow march toward the inevitable.

“There is an entire generation of physicists, of which I’m a part, who have spent their careers with the Higgs looming over the horizon,” says Parsons. “It is satisfying to have finished the job. But frankly, we’re also happy to move on to other things. The search for supersymmetry is exciting because nobody knows if it’s real or not. Finding it would be a lightning bolt.”

INTO THE WEIRD

The concept of supersymmetry grew out of physicists’ attempts in the 1970s and ’80s to resolve a paradox they had observed in nature: that tiny objects and larger ones are controlled by different forces. In the quantum world, activity between particles is governed by electromagnetism and the so-called weak and strong forces that bind atoms together. Gravity is imperceptibly weak in this realm — so weak, in fact, that particle physicists don’t even figure it into their equations. Yet the interactions of more massive bodies — from dust particles, say, all the way up to planets, stars, and galaxies — are dominated by gravity.

“It turns out that our best descriptions of the quantum world and the macroscale world just don’t mix,” says Brian Greene, a Columbia theoretical physicist and mathematician. “And if you’re in the business of trying to identify truly universal laws to describe nature, that’s a problem.”

Supersymmetry’s solution is to propose that the differences between the forces are not fundamental or irreconcilable at all but merely seem this way because of the limitations of our perspective — namely, that we are stuck observing a universe that has been cooling and expanding for fourteen billion years. If only we could glimpse the universe in the first trillionth of a second after the Big Bang, say proponents of supersymmetry, we would see that the forces initially acted on equal terms. Scientists are doubtful they will ever build a particle accelerator powerful enough to recreate the high-energy conditions of the universe’s birth. But experiments at existing colliders have recreated phenomena from within the first billionth of a second after the Big Bang, and they have shown that the forces do act more similarly, although not identically, at these energies. Many theorists take this as a sign that the forces were once part of a single entity called the “superforce.”

“The arrows seem to point back to a unity that may have existed among all the disparate parts of the universe,” says Greene. “It seems that in the first moments after the Big Bang there was an elegant simplicity, a grand synthesis that shattered and eventually crystallized out into the messy world that we see around us.”

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