The end of the Relativistic Heavy Ion Collider marks a new beginning for American particle physics.

When the universe first came into existence, all of space was a cosmic cauldron filled with a boiling, fiery liquid of fundamental particles heated to trillions of degrees. But this boiling primordial soup—the stuff of future galaxies, stars, planets, and people—lasted only a few microseconds. As the universe expanded and cooled, the more common building blocks of matter, protons and neutrons, were squeezed out of it, and the strange things disappeared, never to be seen again.

Until, that is, it appeared 13.8 billion years later in, of all places, Long Island – specifically at Brookhaven National Laboratory (BNL) around the turn of the millennium, hosted 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, re-igniting the long-lost fires of creation in subatomic explosions that lasted less than a billionth of a second.

And it has done so repeatedly over the past quarter-century, making this revolutionary replication of the early universe seem almost routine. During its record-breaking 25-year tenure, RHIC shed light on nature’s thorniest forces and its most fundamental components. This created the heaviest, most elaborate combination of antimatter ever seen. This almost ended the decades-long crisis over the rotation of the proton. And, of course, it brought physicists closer to the big bang than ever before.

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But like the short-lived soup, RHIC’s days were numbered and are now coming to an end. Today at BNL, a control room filled with scientists, administrators and members of the press gathered to watch the experiment’s final collision. The atmosphere was somber, but when Dario Gil, Under Secretary for Science at the U.S. Department of Energy, pressed the red button to end the collider’s quarter-century saga, the crowd erupted in applause.

“It would be nice to get some good sleep for a while,” says BNL’s Travis Schrey, who coordinated the last round of the experiment, which ran the longest. “I’m excited to get to the finish line.”

Others had more mixed feelings – such as Angelica Dries, a BNL accelerator physicist. “Honestly, I wish I could sit in a corner and cry,” she says. “I’m really sad – it was a very beautiful experiment and was my research home for 27 years. But we’re going to do something even better there.”

That “something” will be a far more powerful electron-ion collider to push the boundaries of physics, extend RHIC’s legacy, and maintain the laboratory’s position as a center of discovery. This successor will be built partly from the bones of RHIC, specifically from one of its two vast, underground storage rings that once held the retiring collider’s supply of circulating, near-light speed nuclei.

looking inside the proton

RHIC was intended to shed light on the strong force, the most obscure and counter-intuitive of the four fundamental ways nature pulls things together.

The strong force works between quarks, particles that must have existed when physicists discovered in the 1960s that protons and neutrons could be split like atoms. Three quarks combine to form protons and neutrons, which in turn form the nuclei of atoms.

This suggests that the things we see around us are mostly quarks by mass. But conversely, the sum of the three quarks that make up a proton is only about 1 percent of its mass. The rest comes from the “glue” that binds them together – particles called gluons that constantly exchange between quarks and, strangely, are themselves completely 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 heavy, giga-electron-volt proton?

Where the proton rotates is an even more complex puzzle. Like almost every other particle, the proton has “spin”, a quantum property similar to a spinning top. The proton’s quantum spin should come from its constituent quarks, but in 1987 physicists discovered that it did not. To find the missing source of spin, they realized they needed a way to break apart protons and study their interiors.

Even for particle physicists, quarks are slippery, almost crazy things — six samples have names like “strange” and “charm,” and they carry a mysterious analog of electrical charge called “color.” All these fantastic titles are in accordance with his elusive nature. Unlike the three other forces, the confusingly named strong force is actually found between quarks weakNot strong, because the particles come close to each other. Tightly packed quarks can move around freely, but try to pull them apart and the glue starts up with a vengeance.

This explains why quarks and gluons behave very differently now than they did in the first split second of cosmic time. In today’s relatively cold and diffuse universe, quarks have settled into cool lives within their protonic and neutronic homes. But in the unimaginably hot and dense conditions immediately after the Big Bang, quarks and gluons alike got so squeezed together that they briefly began to behave as an omnipresent fluid – that is, the fiery primordial soup. Physicists have named this specific phase of strange matter quark-gluon plasma.

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

In the early 1980s physicists began planning what would eventually become RHIC – a way to recreate that plasma and then hopefully solve the proton crisis and harness nature’s most elusive force. The trick was to create plasma by a precise, head-on collision between two nuclei of a heavy element like gold, each moving fast enough (99.995 percent the speed of light) to spew out a substantial amount of quark fuel. (The technical term for such nuclei that have been stripped of their electrons is “ions”, which is the full name of RHIC.) However, the facility will also be able to send two protons colliding with precisely aligned spins apart – something that, even today, no other experiment has yet matched. Both operating modes will rely on a pair of 2.4-mile-wide particle-storage rings—which are still, by far, the largest in the US.

Search in the rear view—and beyond

When RHIC finally began full operations in 2000, its initial heavy-ion collisions almost immediately ejected the quark–gluon plasma. But demonstrating it beyond a shadow of a doubt proved in some ways even more challenging than actually creating the elusive plasma, with the chances of success becoming stronger as RHIC’s number of collisions increased.

By 2010, RHIC scientists were confident enough to declare that the hot soup they had been studying for a decade was hot and sour enough to create a quark-gluon plasma. And it was even weirder than they thought. Instead of the quark and gluon gas expected by theorists, the plasma acted like a swirling liquid unprecedented in nature. It was nearly “perfect” with zero friction, and set a new record for spin, or “vortex”.

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

But Collider had much more to offer. In 2023, based on RHIC’s trillions of spin-aligned proton collisions, BNL physicists announced that they were a major step closer to solving the proton spin puzzle. He accurately calculated the spins of both quarks and gluons. But a larger piece remains unexplained, mysteriously arising from the combined motion of the two components.

RHIC’s final success isn’t really the end; Even when its collisions stop, its science will live on.

“Most of our scientific productivity is ahead of us,” says David Morrison. sphenix collaborationwhich used a eponymous detector that had begun collecting data at BHL just three years earlier to squeeze out the last set of answers before RHIC was shut down. sPHENIX was focused on how particularly energetic particles burst through the mess of quarks and gluons, and it proved so abundant that it generated most of the hundreds of petabytes of data collected during the final run of RHIC – more than all of RHIC’s previous missions combined.

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

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

The end of RHIC is meant to mark the beginning of something bigger. Its successor, the Electron-Ion Collider (EIC), is expected to be constructed in the next decade. That project will use 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 little knives to cut larger gold ions. Physicists will get an unprecedented look at the workings of quarks and gluons and another chance to grapple with the strongest force in nature.

“We knew that for the EIC to happen, RHIC needed to be finished,” says Wolfram Fischer, head of BNL’s collider-accelerator department. “It’s bittersweet.”

The EIC will be the first new collider built in the US after RHIC. For some, it marks the country’s re-entry into the particle physics landscape that has largely been ceded to Europe and Asia over the past two decades. “For at least 10 or 15 years, this will be the number one place in the world for (young physicists),” says Abhay Deshpande, BNL’s associate laboratory director for nuclear and particle physics.