Plasmas, protons, and particle physics
A trip to Brookhaven National Laboratory
Our era of big science requires big instruments, and Brookhaven National Laboratory certainly fits the bill. The entire complex, tucked away in eastern Long Island, is over eight square miles in area, and many of the lab’s flagship experiments are equally monstrous: the Relativistic Heavy Ion Collider is 2.4 miles in circumference, and the National Synchrotron Light Source II is wider than Yankee Stadium.
Why do physicists need such large instruments? To study the smallest things in the universe, of course. Probing strange particles and exotic states of matter requires imbuing beams of other particles with lots and lots of energy. Energetic particles move fast, and while these fast-moving particles can be contained by powerful magnets, the magnets can only bend the beams so much. Increasing a particle accelerator’s radius (a) reduces how strong the magnets need to be, and (b) minimizes energy losses, making it possible for us to build more powerful accelerators.
The size of such a particle accelerator doesn’t really hit you until you get up close and personal with it. I was fortunate enough to visit Brookhaven this week and get a tour of several of the accelerators, and I thought I’d share what I found — and what these machines are finding every day.
RHIC: Going back to the beginning of time
If you’ve heard of any of the instruments currently operating at Brookhaven, chances are you know about the Relativistic Heavy Ion Collider, or RHIC (yes, pronounced “Rick”). RHIC is almost unique in the world of particle physics in that it’s a heavy ion collider, meaning that it can smash many different types of massive nuclei together, from hydrogen to uranium. Two particle beams moving in opposite directions in concentric circles, already traveling at high speeds thanks to a series of booster accelerators, complete 80,000 circuits every second. The nuclei then collide in one of six points where the circles touch, producing a shower of exotic subatomic particles that mass through detectors.
Although the beams could collide at any of the six points, there are only two major experiments currently running at RHIC: the Solenoidal Tracker at RHIC (STAR) and the Pioneering High Energy Nuclear Interaction eXperiment (PHENIX). Both are intimately involved in the collider’s two primary science goals:
- To investigate matter that existed in the immediate aftermath of the Big Bang. Quarks are the fundamental particles that make up protons, and gluons are the force-carrying particles that bind them together. Under extreme conditions, nuclei turn into a liquid-like state of matter called a quark-gluon plasma, which would have formed in the very early universe.
- To try to understand why protons have the spin that they do. Spin is a quantum mechanical form of angular momentum with no macroscopic analog. It turns out that only a small amount of a proton’s spin is due to its constituent quarks — a mystery that remains unsolved. As a heavy ion collider, RHIC is a prime candidate to provide inside into this dilemma.
Now that we’ve covered the science, let’s talk about STAR, the detector I was lucky enough to see. Shielded from the nearby high bay by a wall of concrete blocks, it looks like a tangled mess of wires, magnets, and cables, but this mess is actually carefully planned out and designed.
When nuclei collide, lots of different secondary particles are produced. To identify as many as possible, a detector needs to have multiple components, each designed to catch a different type of subatomic particle. In the case of STAR, there are layers and layers of electronics designed to catch particles like muons and pions. Calorimeters, silicon strip detectors, muon detectors provide information that allows the physicists to learn more about the quark-gluon plasmas they create.
If you exit the hall — it feels like a cavern — containing STAR and head up to the control room, you go through a room containing racks and racks of computers which I’m sure could put the clusters at Nevis to shame. STAR produces enormous amounts of data from, on the order of petabytes per year, and most of that data needs to be processed to reconstruct the collisions. That, in turn, requires a lot of computing power, as is true at accelerators across the globe. As you’d expect, even though it’s been hot in the northeast lately, the room is nice and cool, with fans ensuring nothing overheats.
The STAR control room itself is the end of our stop at RHIC. Rows and rows of terminals and monitors line the room, and the table in the middle, with a seat for the physicist in charge during a run, reminds me a little of the bridge of the USS Enterprise, straight out of Star Trek. Although it’s almost empty for the moment, I’m told it’s full of life when the accelerator’s running. I’m not exactly an expert when it comes to particle physics, least of all on detectors, but standing in the control room, or in the massive building containing STAR, sends shivers down my spine. RHIC might not have the catchiest name, but it’s still one of the world leaders in experimental nuclear physics.
NSRL: Applied physics saves lives
After a tour of several research and development labs in the main physics building, we stop at the NASA Space Radiation Laboratory (NSRL, a much less pronounceable acronym). If RHIC is the poster child for fundamental physics, NSRL is the exact opposite: an accelerator that runs for almost purely practical purposes.
Brookhaven has plenty of exotic experiments with straightforward names; the Space Radiation Laboratory studies, of course, radiation found in space. Cosmic rays, fast-moving nuclei typically from extragalactic sources, irradiate astronauts continuously. As we send more and more humans to space, it’s crucial that we understand the radiation they’re exposed to. The NSRL mimics these particles with protons and heavy nuclei from the Alternating Gradient Synchrotron and its boosters, the same system that feeds the beams used by RHIC.
The test subjects at NSRL aren’t astronauts, though — they’re mice and rats, just like the ones used by the RARAF radiological biophysics group at Nevis. Placed at the end of a 100-meter long tunnel connected to the AGS and the boosters, the rodents are irradiated to a high degree of precision by the heavy ion beams, which can reach several GeV in energy. In addition to mice and rats, both non-rodent biological samples and shielding materials are used as targets, to test how well cells can be protected by the sharp influx of cosmic rays in outer space.
Like RHIC, NSRL isn’t exactly new; it’s been operating since 2003, and continues to produce valuable data that helps protect humans currently on the International Space Station, and those who will go even further in the decades to come. Unlike RHIC, its goals are purely practical — saving lives.
The takeaway
Brookhaven National Laboratory will turn 75 years old in 2022. It’s been operating since the golden age of experimental nuclear physics began, years before even CERN started up. If you stop by to visit — and the lab is open to the public at certain times — no doubt a tour guide will happily point to the seven Nobel Prizes across multiple disciplines which have gone to different Brookhaven scientists. It’s been doing cutting-edge science for decades, and with upgrades to different detectors in various stages of completion, there’s a lot to be excited for in the future.
Visiting Brookhaven was a demonstration of just how difficult experimental science can be. It’s one thing to sit at my laptop and run analyses on data from a telescope on the other side of the country that I’ve never been to. It’s another thing entirely to stand underneath a world-class particle accelerator, albeit one I’ll never fully appreciate the science behind — a reminder of how much work it takes to learn about the most fundamental parts of our universe.
You can read my introduction to my research this summer here. Earlier this week, I wrote about some of the technical hurdles we’ve been dealing with lately at Nevis. Tomorrow’s blog post topic? Probably the next thing that breaks.
