particle physics

In LHC’s Shadow, America’s Collider Awakens

The Relativistic Heavy Ion Collider recently fired up for its 15th run to take a deeper look at the building blocks of atoms.

America’s last major particle collider lies coiled beneath the pine barrens and sparse outbuildings of Brookhaven National Laboratory on Long Island, N.Y. The Relativistic Heavy Ion Collider (RHIC), as it’s called, recently came out of hibernation equipped with new gear for spilling the secrets of atoms.

RHIC pales next to Europe’s Large Hadron Collider when it comes to the energy with which its particles collide — energy that determines whether collisions will give rise to new, exotic particles. But the machine clings to relevance (and Department of Energy funding) by forgoing the new in favor of a closer look at the mysterious familiar: the quarks and gluons that comprise the cores of atoms, and thus 99 percent of all visible matter, about which several things are not known.

In a typical run, gold nuclei fly in opposite directions through RHIC’s central artery at 99.995 percent the speed of light before slamming together with the energy of colliding mosquitoes — objects 10 trillion times their weight — inside the two main detectors, STAR and PHENIX. The crash momentarily produces a several-trillion-degree droplet of “quark-gluon plasma” — matter in which quarks and gluons shed their individuality and form a single, flowing entity. It was here at RHIC in the early 2000s that experiments first definitively recreated this strange liquid, which researchers believe filled the universe in its infancy.

At RHIC, the plasma droplet survives for about a hundred-thousandth of a billionth of a billionth of a second before cooling and condensing into individual particles. Measurements over the years, along with calculations that exploit the plasma’s peculiar mathematical relationship to black holes, have revealed that it is an almost “perfect” liquid, possessing the lowest viscosity (or internal friction) allowed by quantum physics.

With gold nuclei and their nearly 200 constituent protons and neutrons, scientists take a shotgun approach to the problem of initiating contact between quantum-scale targets. A collision can produce thousands of particles. “It’s like trying to reconstruct a firecracker from the debris,” said Gene Van Buren, co-leader of STAR’s computing group.

The strong force, which binds quarks together into protons and neutrons and those objects into atomic nuclei, is conveyed through the exchange of gluons. But the equations that describe the strong force are so difficult to solve that physicists do not have a complete understanding of quark-gluon dynamics. For example, quark-gluon plasma droplets form much faster than expected during collisions. For RHIC’s 15th run, which began Feb. 10, scientists have upgraded the PHENIX detector with a new tungsten-silicon hybrid tracking device to help detect radiation from gluons deep inside the colliding particles. “One idea is that we have the wrong picture of gluon distribution,” explained PHENIX scientist Barbara Jacak, who is a professor of physics at the University of California, Berkeley. A super-dense gluon “field,” rather than discrete gluons, might permeate the protons, she said.

As the world’s only polarized proton collider, RHIC also aims to address what’s known as the “spin crisis,” an unresolved question concerning a property of particles called “spin.” A proton’s three quarks only account for one-fifth of its spin, suggesting the lion’s share comes from the spins of gluons and from quarks and gluons orbiting one another. By colliding protons as they spin in a range of directions, scientists hope to identify the spins and orbits of their component parts.

The future of RHIC, which employs 850 people and costs the Department of Energy about $160 million annually, is uncertain. In a 2012 white paper making the case for continued operations, scientists argued that “RHIC is in its prime” with new upgrades poised to answer key questions in nuclear physics. Yet a panel of scientists recommended shuttering the collider in the stead of two other nuclear physics facilities vying for the same funding. So far, all three laboratories have made the cut, yet every run of RHIC could be its last.

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