The Relativistic Heavy Ion Collider’s end marks a new beginning for U.S. particle physics

When the universe first burst into being, all of space was a cosmic cauldron filled with a roiling, fiery liquid of fundamental particles heated to trillions of degrees. But this seething primordial soup—the stuff of future galaxies, stars, planets and people—only lasted a few microseconds. Matter’s more ordinary building blocks, protons and neutrons, settled out of it as the universe expanded and cooled, and the strange stuff vanished, never to be seen again.

Until, that is, it showed up 13.8 billion years later in, of all places, Long Island—specifically at Brookhaven National Laboratory (BNL) around the turn of the millennium, summoned by a newly built experiment called the Relativistic Heavy Ion Collider (RHIC). RHIC was designed to recreate the universe’s earliest moments by smashing together proton-and-neutron-packed atomic nuclei at close to the speed of light, rekindling the long-lost fire of creation in subatomic explosions that endured for less than a trillionth of a billionth of a second.

And for the past quarter-century it’s done just that, again and again, making this revolutionary replication of the early universe seem almost routine. During its record-breaking 25-year run, RHIC illuminated nature’s thorniest force and its most fundamental constituents. It created the heaviest, most elaborate assemblages of antimatter ever seen. It nearly put to rest a decades-long crisis over the proton’s spin. And, of course, it brought physicists closer to the big bang than ever before.

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But much like the short-lived soup itself, RHIC’s days were numbered and are now at an end. Today at BNL, a control room full of scientists, administrators and members of the press gathered to witness the experiment’s final collisions. The vibe had been wistful, but the crowd broke into applause as Darío Gil, the Under Secretary for Science at the U.S. Department of Energy, pressed a red button to end the collider’s quarter-century saga.

“It’ll be good to sleep well for a while,” says Travis Shrey of BNL, who coordinated the final run—the experiment’s longest. “I’m excited to reach the finish line.”

Others had more mixed emotions—such as Angelika Drees, a BNL accelerator physicist. “I wish I could go sit in a corner and cry, to be honest,” she says. “I’m really sad—it was such a beautiful experiment and my research home for 27 years. But we’re going to put something even better there.”

That “something” will be a far more powerful electron-ion collider to further push the frontiers of physics, extend RHIC’s legacy and maintain the lab’s position as a center of discovery. This successor will be built in part from RHIC’s bones, especially from one of its two giant, subterranean storage rings that once held the retiring collider’s supply of circulating, near-light speed nuclei.

Seeing inside the proton

RHIC’s purpose was to shed light on the strong force, the most obscure and counterintuitive of the four fundamental ways we know of that nature tugs on things.

The strong force operates between quarks, the particles that physicists realized must exist when they discovered in the 1960s that protons and neutrons can be split like atoms. Three quarks come together to form protons and neutrons alike, which in turn form the nuclei of atoms.

That would suggest the stuff we see all around us is, by mass, mostly quarks. But counterintuitively, the three quarks that make up a proton only sum to about 1 percent of its mass. The rest comes from the “glue” that binds them together—particles called gluons that are constantly interchanged between quarks and, stranger still, are themselves entirely massless. How could it be, physicists wondered, that a few light quarks and a sea of massless gluons add up to the mass of a bulky, giga-electron-volt proton?

Where the proton gets its spin is an even gnarlier puzzle. Like almost every other particle, protons have “spin,” a quantum property akin to a twirling top. The proton’s quantum spin should come from its constituent quarks, but in 1987 physicists found that it didn’t. To find the missing source of the spin, they realized they’d need a way to shatter protons and study their innards.

Even to particle physicists, quarks are slippery, almost whimsical things—the six specimens have names such as “strange” and “charm,” and they carry an arcane analogue of electric charge called “color.” All these eccentric titles befit their elusive nature. Unlike the three other forces, the confusingly named strong force between quarks actually gets weaker, not stronger, as the particles get closer together. Quarks crammed in tight can roam about freely, but try to separate them and the glue kicks in with a vengeance.

This explains why quarks and gluons behave so very differently now than they did in the first split seconds of cosmic time. In today’s relatively cold and diffuse universe, quarks have settled down to sedate lives within their protonic and neutronic homes. But in the inconceivably hot and dense conditions immediately following the big bang, quarks and gluons alike were so squeezed together that they briefly behaved as one omnipresent fluid—that is, the fiery primordial soup. Physicists named this distinct phase of weird matter the quark-gluon plasma.

The strong force’s paradoxes make its interactions incredibly difficult to predict. The behavior of even a few quarks and gluons is incalculable without the world’s most advanced supercomputers. In a sense, the quark-gluon plasma seems impossible. And yet it’s the origin of everything.

In the early 1980s physicists began planning for what would eventually become RHIC—a way to recreate that plasma and then hopefully settle the proton crises and pin down the most elusive force of nature. The trick was to concoct the plasma from precise, head-on crashes between two nuclei of a heavy element such as gold, each moving fast enough (99.995 percent the speed of light) to spit out ample quark fuel. (The technical term for such nuclei, which have been stripped of their electrons, is “ions,” which accounts for RHIC’s full name.) The facility would also, however, be able to separately send two protons colliding with precisely aligned spins—something that, even today, no other experiment has yet matched. Both operating modes would rely on a pair of 2.4-mile-wide particle-storage rings—which, even now, remain the largest in the U.S.

Discoveries in the rearview—and ahead

When RHIC at last began full operations in 2000, its initial heavy-ion collisions almost immediately pumped out quark-gluon plasma. But demonstrating this beyond a shadow of a doubt proved in some respects more challenging than actually creating the elusive plasma itself, with the case for success strengthening as RHIC’s numbers of collisions soared.

By 2010 RHIC’s scientists were confident enough to declare that the hot soup they’d been studying for a decade was hot and soupy enough to convincingly constitute a quark-gluon plasma. And it was even weirder than they thought. Instead of the gas of quarks and gluons theorists expected, the plasma acted like a swirling liquid unprecedented in nature. It was nearly “perfect,” with zero friction, and set a new record for twistiness, or “vorticity.”

For Paul Mantica, a division director for the Facilities and Project Management Division in the DOE’s Office of Nuclear Physics, this was the highlight of RHIC’s storied existence. “It was paradigm-changing,” he says.

But the collider had much more to offer. In 2023, based on RHIC’s trillions of spin-aligned proton collisions, BNL physicists announced they were a huge step closer to solving the proton spin puzzle. They accounted precisely for the spin of both the quarks and the gluons. But a hefty slice remains unexplained, arising mysteriously from the two constituents’ combined motion.

RHIC’s last smash isn’t really the end; even when its collisions stop, its science will live on.

“Most of our scientific productivity sits ahead of us,” says David Morrison of the sPHENIX collaboration, which used an eponymous detector that began collecting data at BHL just three years ago to squeeze a final set of answers out of RHIC before its closure. sPHENIX’s focus was on how particularly energetic particles burst through the muck of quarks and gluons, and it proved so prolific that it generated most of the hundreds of petabytes of data gathered during RHIC’s last run—more than all of RHIC’s previous campaigns combined.

“I’m elated,” says Linda Horton, interim director of the Office of Science at the DOE, which owns and operates BNL. “The collider’s gone, but RHIC will live on through the data.”

In fact, data from the final run (which began nearly a year ago) has already produced yet another discovery: the first-ever direct evidence of “virtual particles” in RHIC’s subatomic puffs of quark-gluon plasma, constituting an unprecedented probe of the quantum vacuum.

RHIC’s end is meant to mark the beginning of something even greater. Its successor, the Electron-Ion Collider (EIC), is slated for construction over the next decade. That project will utilize much of RHIC’s infrastructure, replacing one of its ion rings with a new ring for cycling electrons. The EIC will use those tiny, fast-flying electrons as tiny knives for slicing open the much larger gold ions. Physicists will get an unrivaled look into the workings of quarks and gluons and yet another chance to grapple with nature’s strongest force.

“We knew for the EIC to happen, RHIC needed to end,” says Wolfram Fischer, who chairs BNL’s collider-accelerator department. “It’s bittersweet.”

EIC will be the first new collider built in the US since RHIC. To some, it signifies the country’s reentry into a particle physics landscape it has largely ceded to Europe and Asia over the past two decades. “For at least 10 or 15 years,” says Abhay Deshpande, BNL’s associate laboratory director for nuclear and particle physics, “this will be the number one place in the world for [young physicists] to come.”