America’s only particle collider shuts down – so a new one could emerge

I took a look at the house-sized particle detector known as STAR just after it took its final snapshot of one of the most extreme types of fireballs ever created. Inside, the conditions following the Big Bang had been recreated in miniature by the collision of gold atomic nuclei, as had been done countless times during the detector’s 25 years of existence. This era was now coming to an end.

Physicist Alex Jentsch took stock of this moment, which he said calls for a kind of ambiguous recognition: “Either celebrate or you cry, one of the two. »

A whirring fan ruffled our hair as we watched STAR, an organized tangle of wires, tubes, electronics, and particle detection systems. Above our heads, a surprisingly thin pipe threaded through the machine, the conduit through which atomic nuclei – positively charged ions – were projected to their demise. In the STAR control room, the alarms sounded slowly like in a hospital. The scientists showed the latest collisions on a monitor, a fireworks display of curved lines in blue, green and cyan.

STAR was designed to capture the consequences of collisions of atomic nuclei moving at close to the speed of light, produced by the Relativistic heavy ion collideror RHIC, at Brookhaven National Laboratory in Upton, New York. Today, RHIC (pronounced “Rick”) collided with its last beams of gold cores as it neared its final shutdown in preparation for a future next-generation collider.

Starting in the 2000s, experiments at RHIC revealed a suspension of particles from the early universe called quark-gluon plasma. The installation then revealed surprising new details about this primordial soup of the universe, from which the particles that make up stars, galaxies, planets – and, ultimately, us – descend. And that’s only half. RHIC also collided with protons, characterizing subatomic particles in exquisite detail and revealing the surprisingly tumultuous inner world of these omnipresent constituents of matter.

On the day of my visit, scientists were switching the collider to proton collisions in an effort to gather all possible data before shutdown. The RHIC was permanently extinguished during a ceremony on February 6.

“RHIC has had a spectacular run…beyond what one could dream,” says Wolfram Fischer, an accelerator physicist at Brookhaven.

The closure of RHIC marks the end of the only operating particle collider in the United States and the only collider of its type in the world. Most particle accelerators are incapable of directing two particle beams so that they collide head-on. That’s what colliders do, and that makes them a rare and valuable commodity. The country’s other collider in recent memory, the Tevatron at Fermilab in Batavia, Illinois, was shut down in 2011.

But the end of RHIC is hopeful. This gives way to Electron-ion colliderwhich is scheduled to start in the mid-2030s. “This is where the future is, and I hope it will be just as spectacular,” says Fischer.

The electron-ion collider will build on RHIC discoveries. It will occupy the same tunnel and will reuse a large part of the RHIC equipment and infrastructure. But instead of colliding protons and heavy atomic nuclei, it will collide electrons with protons or atomic nuclei to produce deep information about the structure of the proton.

“This is 3D imaging of the proton, really in all its glory,” explains Elke-Caroline Aschenauer, a physicist at Brookhaven. The collider could even reveal a mysterious substance called colored glass condensate, thought to be hiding in protons.

The subatomic rabbit hole

I toured the lab, its detectors, and other parts of the facility in early December. I embarked on this journey in part because Brookhaven National Laboratory holds special significance in my life. I grew up not far from the lab, where the collider’s 3.8-kilometer ring is nestled in the pine barrens of Long Island. It was partly through this that I fell in love with physics and, ultimately, writing about it.

As a teenager participating in a student research program at the laboratory, I was fascinated to learn that protons are not simple balls of positive charge as they are described in textbooks. Instead, they are composed of smaller substances called quarks and gluons. It was the 1990s, the time of The matrixand for me as a teenager, this proton revelation was my “red pill,” as they say in the film: I needed to know how deep the rabbit hole went. It turns out it went much further.

In the simplest picture, protons are made up of three quarks – two “up” quarks and one “down” quark – and particles called gluons, which, as their name suggests, act like glue. These particles transmit the strong nuclear force that binds quarks together within protons, neutrons and other particles.

And that’s just a small part of the immense complexity of the proton. The particles foam with the fervor of quantum mechanics, where reality is uncertain and fluctuating. Consequently, they contain a “sea” of short-lived quarks and their antimatter equivalent, antiquarks, with gluons that swarm around them like the cloud of dust that surrounds them. Peanuts Pigpen character.

The strong force is so strong that quarks and gluons cannot be observed individually; they are always bound together into larger particles. “The laws of nature forbid them from being alone,” says Abhay Deshpande, a physicist at Brookhaven.

That is, except during the ephemeral existence of quark-gluon plasma. This state of matter existed just after the Big Bang, when the universe was so hot that a mixture of quarks and gluons were mixing. As this soup of particles cooled, protons, neutrons, and other particles condensed from it about 10 microseconds after the birth of the universe.

While I was thinking about the existence of quarks, scientists at Brookhaven were trying to recreate this quark-gluon plasma.

The facility removed electrons from atoms before blasting them at near the speed of light, directing them clockwise and counterclockwise in circles using 1,740 powerful superconducting magnets and throwing the particles against each other. The idea was that when heavy atomic nuclei collided, they would produce temperatures of billions of degrees that would melt their protons and neutrons into a quark-gluon plasma. Multi-layer detectors would then observe the resulting debris, hoping to identify fingerprints of the substance of interest.

In 2005, scientists from RHIC’s four detectors — STAR, PHENIX, PHOBOS and BRAHMS — jointly announced the discovery of a new state of hot, dense matter in a special issue of Nuclear Physics A. The substance, now confirmed to be a quark-gluon plasma, lasted for about 10 quadrillion nanoseconds and reached billions of degrees Celsius in a region only about 10 trillionths of a millimeter in diameter.

“This was the first time that quarks and gluons were observed, or at least indirectly, to be apart from protons and neutrons,” says Deshpande. “It was a big deal.”

But in a scientific surprise, the state of matter discovered by RHIC was not a gas of free-floating quarks and gluons, as scientists had expected for quark-gluon plasma. Instead, quarks and gluons interact with each other like in a liquid. In fact, RHIC revealed, quark-gluon plasma is a near-perfect liquid, meaning it has extremely low viscosity and can flow with almost no resistance. “He has a very distinct personality,” says Deshpande. “It likes to flow.”

The rise of the SPHENIX

Once the quark-gluon plasma was recreated, scientists wanted to know more. Researchers have improved their detectors several times to better study this ephemeral state of matter. The STAR detector has received new tinkered parts even in recent years. During our visit, newly added components were perched on odd platforms, like books teetering on an overstuffed shelf in a long-lived office.

The PHENIX researchers chose a different tactic. Instead of continuing to upgrade PHENIX, they decided it was better to start from scratch. Scientists knew that RHIC might not continue to function for very long. They therefore designed a new instrument allowing us to live fast and die young. In 2023, sPHENIX lit up.

When we toured the three-story detector during my December visit, the contrast with the aging STAR was immediately apparent. If STAR was a well-used pair of hiking shoes – sturdy, comfortable but showing their age – then sPHENIX was a pair of shoes just off the shelves. It was shiny, modern and bright, freshly painted cornflower blue.

With its new, faster electronics and more sensitive hardware, sPHENIX took off. “We collected more data this year than in the entire 25 years RHIC has been operating,” says physicist Rosi Reed of Lehigh University in Bethlehem, Pennsylvania.

sPHENIX serves as a bridge to the electron-ion collider. On the one hand, the detector used a data collection strategy that will be essential for the new installation.

Most particle collider detectors generate too much information to store it all. They therefore only record events that meet certain conditions to be interesting. But then you have to ask yourself: “What about these things I don’t see? “,” Reed said.

sPHENIX components can collect data continuously through a method called streaming, without throwing anything away. This is how the entire electron-ion collider detector will work, a feat made possible by improved computing and storage capabilities, as well as AI techniques that will help sift through this multitude of data.

In an office adjoining the lobby that houses sPHENIX, leftover party bagels and cream cheese They lay scattered on a table. Not exactly champagne, but I grew up in these parts, so I can give you an idea: bagels are the champagne of Long Island.

SPHENIX still feels fresh, so coming out of it is bittersweet, Reed says. “No one ever wants to see the end of something. I think we could, if we had more time, do more. But I’m really happy and proud of what we’ve managed to accomplish.”

A tour of protons

In the RHIC control room, another stop on my tour, execution coordinator Travis Shrey seemed relaxed, as if swapping gold cores for protons along a 3.8-kilometer accelerator was no problem. “We weren’t planning to run protons this year,” Shrey, the accelerator physicist, said nonchalantly. “It’s kind of like a last minute thing.” Imperturbability is probably a desirable quality for someone in charge of the operations of a machine so large that it is visible from space.

RHIC’s proton beams are special: they can be polarized. This means that protons, which have tiny magnetic fields, are aligned so that their magnetic poles all point in the same direction, like packages with “this way” signs traveling on a conveyor belt. But the packages are subatomic particles, and the conveyor belt moves them along at close to the speed of light.

These polarized beams allowed RHIC to study the proton in a way never before possible. In particular, they brought scientists closer to solving a puzzle so vexing for physicists that it was called a “crisis” when it was first revealed in 1987.

The issue is the the spin of the protonthe quantum property that gives it a magnetic field. I spin a quantum version of angular momentum, a kind of rotational punch. This may seem abstract, but it is as important to a particle as its mass or its electric charge. Spin comes in integer or semi-integer values ​​and determines the role of a particle. The building blocks of matter, such as protons, electrons, and neutrons, have spins of ½ and are called fermions. Particles that transmit forces, such as gluons or photons, have integer spins and are called bosons. If protons were bosons instead of fermions, atomic nuclei – and the universe as we know it – would not exist.

At first, physicists thought that quarks, which each have their own spin, made up the spin of the proton. But experiments indicated that only about 30 percent of the spin came from quarks. “It was a bit of a shock,” says Jentsch, of Brookhaven. “Where does the rest come from?”

RHIC’s polarized proton beams revealed that gluons contribute about 20 to 30 percent of the spin. But that still leaves about half of the rotation unexplained.

This is where the new electron-ion collider comes in. When it starts in the mid-2030s, it will provide maps of the positions and momenta of the particles that make up the proton. And this will allow scientists to study another potential source of spin. In addition to the intrinsic spins of quarks and gluons, their swirling motions within the proton can also contribute to the proton spin.

The “electron” of the electron-ion collider is crucial here. The collider will use electrons to probe protons, rather than colliding protons with protons. This is a game changer because, although protons have smaller constituents, electrons do not. An electron is therefore a more precise probe that provides a finer view of the inner world of protons.

“You can think of it like an electron microscope,” says Aschenauer. “It’s truly a precision machine that will reveal to us all the secrets of visible matter that can be pierced.”

This requires a collider unlike any other built before. The electron-ion collider will have polarized electron beams and polarized ion beams. Polarizing the two is not an easy task: the two types of particles behave very differently in an accelerator. That means the collider is “everything that’s difficult in an electron machine and everything that’s difficult in an ion machine,” Shrey says. “And then you’re going to add them up, which adds a whole new level of complication. It’s the most difficult machine there is.”

To build it, Brookhaven is partnering with Jefferson Lab in Newport News, Virginia. And scientists don’t start from scratch. The RHIC consisted of two rings of equipment to direct, focus, and monitor the two beams, one of which traveled clockwise and the other counterclockwise through the tunnel. The counterclockwise ring will essentially stay as is to accelerate the protons and ions. The other will be removed and replaced with a new electronic ring. Also remaining in place are the multiple stages of pre-accelerators that the protons and ions pass through before entering the collider.

Some of the magnets to steer the electron beams will be recycled from an electron accelerator at Argonne National Laboratory in Lemont, Illinois, called the Advanced Photon Source, which itself was upgraded in 2024, leaving its magnets up for grabs. The bright yellow electromagnets, the size of mini-fridges, are already at Brookhaven, arranged in rows upon rows in a storage room, like a farm growing an unusual crop.

STAR and sPHENIX components will also find new life in the electron-ion collider detector. You can view the closure of RHIC not as an end, but as a metamorphosis.

The quantum essence of the proton

Fittingly, the electron-ion collider could allow scientists to better understand RHIC’s flagship discovery, quark-gluon plasma.

Some of the greatest uncertainties in quark-gluon plasma studies arise from unknowns about the initial states of protons and neutrons in colliding atomic nuclei. One way to better understand this state of matter is therefore to understand the proton itself.

Protons are subject to the laws of quantum physics, in which objects do not exist as concrete entities with fixed properties. This feature “really encapsulates the essence of quantum mechanics,” says Raju Venugopalan, a theoretical physicist at Brookhaven. “What you see depends on how you probe the object.” Like this optical illusion that can look like a rabbit or a duck depending on your point of view, scientists see the proton differently depending on how they look at it.

When studied at low energies, protons appear as simple three-quark objects. At higher energies, a sea of ​​transient quarks and antiquarks comes into play. At the highest energies, such as those at RHIC and possibly the electron-ion collider, scientists believe the proton is obstructed by a multitude of gluons, forming a dense wall called a colored glass condensate.

During collisions of atomic nuclei at RHIC, gluons in their colored glass condensates are thought to have interacted with each other, producing the quark-gluon plasma. But scientists have not been able to fully confirm the existence of this colored glass condensate or study it in detail. The electron-ion collider could make it possible. And this could have repercussions for all of physics.

Protons and quark-gluon plasma are described by a theory called quantum chromodynamics. The mathematics of this theory is so complex that there is still a mystery as to how the quarks and gluons are confined within the proton. How do the gluons in the colored glass condensate know how far to extend from the center of the proton, for example? Understanding the color of glass condensate could shed light on this question of containment.

Perhaps the strangest thing about colored glass condensate is that when the gluons condense in these globules, they somehow shed their quantum nature. “You think of the elements inside a proton as being intensely quantum, right? All these quarks and gluons are sort of fluctuating,” Venugopalan explains. But, he says, the “glue balls” that make up the colored glass condensate behave like classical, not quantum, objects. This means that studying the colored glass condensate could also help scientists determine where the boundary between the quantum and classical worlds lies. another major physics dilemma.

Exposing the colored glass condensate could unlock some of physics’ deepest mysteries. “The electron-ion collider, in this sense, is sort of the ultimate machine,” says Venugopalan.

A refuge for curiosity

When I was in high school, we were asked to write essays about our favorite place in the community. Most people wrote about the beach. A bit cliché, but we lived on an island. I wrote about the Brookhaven National Lab.

However, before my trip in December, I hadn’t visited the site in decades. Why did I feel the need to see RHIC one last time? Maybe it’s because I’m always amazed that this facility exists—a testament to what humans can accomplish when we’re allowed to follow pure scientific curiosity. The exact composition of the universe is perhaps the most fundamental information anyone could want to know. ître. RHIC’s experiences have undoubtedly brought us closer to this. In doing so, they have produced more than 600 doctorates (and inspired at least one journalist).

I also wanted to visit RHIC because I’m worried. At this moment in history, when American science funding is under threat, I fear that a facility like the electron-ion collider will be difficult to complete. It is expected to cost nearly $3 billion to build, with the majority of the money coming from the Department of Energy’s Office of Science. When I asked Deshpande about funding issues at an American Physical Society meeting last March, he noted that the project was an international collaboration, with funding coming from foreign governments and other sources. But, he said at the time, “we are still worried.”

Inside a next-generation collider

In the electron-ion collider, protons and atomic nuclei will be created and pre-accelerated (orange) before being injected into the anti-clockwise ring of the collider (yellow). The electron beam will be created and accelerated in a separate ring before entering the hour ring (red). The particles will collide with one or more detectors (green) which will measure
the results.

And the United States isn’t the only one in this game. China’s proposed electron-ion collider, to be located in Huizhou, is being planned earlier than the U.S. efforts, but if it comes to fruition, it is expected to begin operations in the late 2030s.

During my college years, I spent a summer doing research at Stony Brook University in New York, neighboring Brookhaven, and there were many scientists working on the RHIC experiments. My mentor at the time, physicist Thomas Hemmick, member of the sPHENIX team, was an optimist. He reassured me: “DOE Nuclear Physics’ track record is that their highest priority for new construction has always been construction, which gives a lot of confidence in the field. » According to a 2023 Report of the American Advisory Committee on Nuclear Sciencethe electron-ion collider is the highest priority. In a statement to Scientific newsa DOE spokesperson expressed excitement about the “cutting-edge scientific possibilities that the electron-ion collider will enable.” For Hemmick, the transition to the new collider marks “the birth of hope at the end of an era.”

Recently, my teenage niece started asking me questions about physics, touching on the same kinds of deep questions that captured my attention as a child. I hope the country still has room for a collider that can serve as an inspiration to him and others of his generation.