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

John Parsons is poised to enter a new dimension.

It is a cloudy Monday morning in April, and the fifty-year-old physics professor is sitting in a swivel chair in his Pupin Hall office with his face inches away from his computer. On the screen is a colorful chart showing what happened when two protons zipped around the seventeen-mile-long circular tunnel of the world’s most powerful particle accelerator, the Large Hadron Collider (LHC), outside Geneva, and smashed together at nearly the speed of light. When the protons met, the immense energy that had been stored up in their motion was suddenly released in the form of quarks, photons, electrons, gluons, muons, pions, kaons, and other particles that materialized like tornadoes churned out by a furious weather system. The new particles spun outward in all directions, each one moving in a manner that revealed its personality: some made a beeline for the collider’s outer wall, others floated gently like streamers, and still others spiraled, as if trying to return from where they came. Their journeys, which took less than a billionth of a second and were tracked by computerized sensors, now made for a staggeringly complex and beautiful-looking puzzle.

Within the tangle, Parsons zeroes in on a single particle. He can tell it is a photon, or a light particle, by the clean angle with which it hit the detector. Photons are the fastballs of the particle world, always firing straight and hard. Parsons hopes that this one is pointing him to one of the most extraordinary scientific discoveries in a century.

“Look at where it came from,” Parsons says, zooming in on the image. “Not from the point of the original collision, but a few centimeters off.” The photon seems to have come from nowhere. This is a sign that it’s a byproduct of another particle that was produced by the collision and then quickly decayed and disappeared. What could that be? “One possibility is a neutralino,” Parsons says. “And where there’s a neutralino, there may also be a gravitino.”

The gravitino is precious quarry for experimental physicists: a new particle whose discovery would radically alter our understanding of nature. Since the 1970s, the so-called Standard Model of particle physics has been the law of the cosmos; its seventeenth and final cog, the Higgs boson, was found last year. A gravitino would be a different sort of beast entirely. This little ball of thunder would provide the first evidence for a controversial idea called supersymmetry, which holds that each of the seventeen particles already identified has a nearly identical cousin still hiding in the shadows: the photon gets a photino, the electron a selectron, each of six types of quark a squark, each of eight types of gluon a gluino, and so on. The theory goes that these supersymmetrical particles, or sparticles, are the missing puzzle pieces needed to solve mysteries that the Standard Model doesn’t even try to address, mysteries such as: What came before the Big Bang? How does gravity work? Why is our universe expanding?

“If supersymmetry is right, we’re basically talking about the key to a unified theory of everything,” Parsons says. “The thought of it is thrilling.”

“The search for supersymmetry is exciting because nobody knows if it's real or not. Finding it would be a lightning bolt.”
— John Parsons

If he is to find a gravitino, Parsons will need to be a creative sleuth. This is because a gravitino is thought to be impervious to electromagnetism and other forces. It is, in other words, invisible. Whereas most particles that pass through the LHC’s silicon, liquid-argon, and iron sensors will knock electrons loose from their atoms and leave an irradiated trail of ions in their stead, a gravitino will slip through these heavy materials as if they were cheesecloth.

How do you discover something you can’t see?

“You look for curious absences,” says Parsons, who has been conducting the analysis with Columbia graduate student Nikiforos Nikiforou and physicists at the University of Liverpool. “First, you’ll see less energy coming out of a collision than went into it, which suggests that something is sneaking past your sensors on the way out. And perhaps you’ll notice these little oddities in the direction of your photons.”

Parsons, a rosy-skinned Canadian with a calm demeanor, believes he has as good a chance as anybody of finding a sparticle. After all, he has spent the past twenty years designing, testing, and calibrating the circuit boards that act as the electronic brains in the LHC’s largest detector, known as ATLAS. He did this work with Bill Willis, a Columbia professor who died this past fall and who was among the first prominent American physicists to get involved in the European-led LHC project, in the early 1990s.

“If you spend two decades developing a piece of machinery, you really get to know it,” Parsons says. “You know its strengths, its limitations, and its quirks. And this gives you an intuitive feel for working with the data.”

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