Atom-smashing and Femtoscopy

by Christopher Plumberg

Hi! My name is Christopher Plumberg, and I’m a sixth-year graduate student in nuclear physics at The Ohio State University, studying relativistic heavy-ion collisions. Those last four words may be a bit unfamiliar, so let me break it down:

“Relativistic” = wicked fast. Like, 99%-the-speed-of-light fast. Let’s just say, don’t get caught doing that on the freeway.

“Heavy ion” = large atomic nuclei. You might have heard that everything around us—water, air, kitty cats—is made up of atoms. And if you’ve ever thought about what atoms are made of, you might know they have a big part in the middle (the “nucleus”) and an outer part (the “electrons”), and the whole thing looks a little bit like honeybees buzzing around their hive. The “ion” part is basically just another name for the nucleus after its electrons have been stripped away, and the “heavy” part means we’re looking at relatively big nuclei (like gold and lead), rather than “light” nuclei (like hydrogen and helium).

“Collisions” = smashing things together. But you probably already knew that.

So there you have it. My job is taking large atomic nuclei, and smashing them together, wicked fast. Sounds pretty cool, right? Well, to tell you the truth, my job isn’t quite that glamorous. I don’t actually get to do the “smashing” myself: there are labs that do this for me, like the Relativistic Heavy Ion Collider (RHIC) in Brookhaven, NY, or CERN in Switzerland. My real job, however, is slightly different: I try to understand how these atom-crunching collisions actually work.

Of course, this is what physics really is: trying to figure out how things work. And knowing this helps us to understand why physicists went to all the trouble of smashing these atomic nuclei together in the first place. If you want to understand how a toaster works, one way to do it is to simply take the toaster apart (after it’s unplugged!) and peek inside. It turns out, if you want to understand how the atomic nucleus works, you just take it apart and look inside it!

The only catch is that, in the nuclear case, the object of study is way too small for a screwdriver to pry it apart, so we have to settle for smashing the nuclei together. How does that help? Well, the physical theories that we use to describe how the atomic nucleus works can also tell us what should happen when we smash two of those nuclei together at nearly the speed of light. If what we see in the laboratory matches what our theories predict, we know we’re probably on the right track.

Of course, this doesn’t happen automatically: there’s plenty of math that goes into making the connection between a physical theory (like the theory for an atomic nucleus) and the results of an experiment (like smashing two nuclei together). Working to make that connection between theory and experiment is a large part of the job of a theoretical physicist…in fact, that’s what my job is: I use the theories of nuclear physics to try to predict what experimental physicists (the folks who help run the atom-smashing machines) will measure in their laboratories. Working out exactly what those predictions are requires a good deal of intense math, but fortunately, we have computers to help us do most of it (phew!).

A friendly sidenote: you may be thinking about a career in science, but find the “math” part to be a bit scary at times. And that’s totally okay: most scientists are not math geniuses. In fact, I used to be somewhat terrified by really difficult math myself. The thing I have found is, if you tell yourself that the math will be too hard for you, then you will probably find it overwhelming. But, if you pretend instead that math is like a secret message—written in a coded language, to teach you something about the world that no one else has ever figured out before—then math becomes much more exciting, and it actually becomes a bit easier, too. Of course, like any code or language, math has rules, and it often takes some practice to get really good at using them. Nevertheless, if you are willing to do some hard work, chances are that you will surprise yourself with how much you are capable of doing. So keep at it.

The special focus of my research has to do with ‘taking pictures’ of heavy-ion collisions (via a technique known as “femtoscopy”). Here’s how it works: when two nuclei smash together, they are actually travelling so fast that they crash right on through each other. In the process, some of their energy gets converted into new particles that fly away from the collision—particles that we can measure. These particles can fly away in any direction, but the rules of quantum mechanics tell us that these particles prefer to ‘clump’ together in a way that depends on how far apart they were when they were created. If we can measure the ‘clumpy-ness’ of the emitted particles, we can learn something about the size and shape of the collision.


A rough picture of two atomic nuclei smashing through each other, and the leftover energy which converts to particles that fly away from the collision.

And now comes the really cool part: using femtoscopy has allowed me to explain some puzzling triangular patterns that experimentalists have noticed in their data. Some folks thoughts that these patterns only showed up because the collisions themselves were triangular in shape. However, I was able to show that these “triangles” are actually due to the way that the collision itself expands, giving us some important clues about the behavior of an exotic form of matter—known as the quark-gluon plasma—which is formed during the collisions, and which is one of the hottest topics in nuclear physics today.


Some femtoscopic measurements of size and shape for some typical heavy-ion collisions. The “double-hump” curves on the top (in red) represent a kind of “elliptical” structure in the collisions; the “triple-hump” curves on the bottom (in blue) show that some “triangular” structure is present as well.

But there is still more work to be done, and the next big step might be made by a bright young scientist like you. Nuclear physics is still an active field of research with much to teach us about nature. So what will your contribution be?


About Christopher Plumberg

ChrisAndAllysonMy name is Christopher Plumberg and I am a year or two from completing my PhD in nuclear physics at Ohio State. My undergraduate work was done at Eastern University in astronomy and astrophysics, in cooperation with Villanova University. When I’m not working on my research or learning something new about math or physics, I’m probably reading books on theology and philosophy of science, practicing the piano, or hanging out with my awesome fiancée, Allyson (who is pictured with me to the left).