Even small children know that opposite charges attract and like charges repel. Yet somehow, atomic nuclei easily pack together dozens of positively charged protons. How?
It has to do with the strong force, which is roughly 100 times stronger than the electromagnetic force trying to pull those protons apart. The strong force, carried by particles called gluons, clumps point-like quarks into protons and neutrons, and protons and neutrons into atoms. It is the reason we can exist at all.
But that strength is exactly what makes the strong force so hard to study. Inside a proton, it’s so strong that our usual calculation methods break down. So how do you study a force you can’t even fully describe mathematically? This is something my team of researchers at Tufts is currently working on.
Luckily, we aren’t limited to studying stationary protons—we have a big collider. Specifically, the Large Hadron Collider at CERN, which we use to do this research. When two protons collide inside the LHC, something strange happens: the strong force actually gets weaker. That’s because the strong force pulls harder when the quarks are far apart (such as inside a proton), but relaxes at extremely close distances (such as when two protons merge during a collision, and all the quarks are forced together). It’s completely counterintuitive, but you can imagine it like a jealous lover with terrible abandonment issues (the LHC collisions are basically one big subatomic soap opera).
This relaxed grip gives us enough leeway to study and test the mathematical theory that describes the strong force: quantum chromodynamics, or QCD. But the role QCD plays in the LHC collisions also gives many particle physicists a headache. Here’s why:
When two protons collide, their inner components interact to produce new and rare forms of matter—the Higgs boson, for instance. But the protons inside the LHC aren’t the simple three-quark structures you’d picture from a textbook. As we accelerate them, they bubble with virtual quarks and gluons, which are quantum fluctuations momentarily pulled from the vacuum of empty space to soak up the extra energy. (Weird, I know.)
Then, when the protons collide, all of that “stuff” rapidly decelerates, briefly comes to a near standstill, and then recoils in every direction. This pulls even more quarks and gluons out of the vacuum, which then clump into hadrons. These hadrons avalanche into even more hadrons, which then clog up our detectors. Now try to find a single Higgs boson buried inside that mess. A headache indeed.
My group at Tufts, along with our colleagues in experimental and theoretical physics, have decided to stop lamenting the chaos QCD causes and start embracing it. QCD is a beautiful, mathematically robust theory and it deserves our full attention. But studying it at the LHC means confronting two distinct challenges.
The first is the enormous range of energies we need to account for. Picture an engineer running calculations for the force needed to hammer a nail into a wall, and then the force needed to launch a rocket to the moon. That’s roughly the range we’re working with for the strength of the strong force across the different moments of a single LHC collision.
The second is physical distance: we start at scales far smaller than a proton, then have to map out how the byproducts transform and travel over several meters.
We tackle these challenges by picking a class of LHC collision events and then slowly teasing out their secrets. Right now, we’re studying subatomic processes that touch nearly every aspect of QCD (for the physicists in the audience, it’s called vector boson plus jet). These are simple processes that also capture the entire range of the weird QCD complications, thus making them a stringent test of our mathematical theory.
By measuring these patterns in the detector and comparing them to QCD-based simulations, we can see exactly where the theory succeeds and where it needs to be pushed further. That precision feeds directly into predictions about other particles’ behavior. For example, take the muon’s magnetic wobble. The Muon g-2 experiment famously measured a mismatch with theory—a potential sign of new physics. But the theory community then reassessed their calculations and decided that this discrepancy was mostly likely the result of yet another misunderstanding of QCD.
We also need to understand QCD processes to identify and strip out the backgrounds that can bury the delicate signatures of new physics searches, such as searches for dark matter.
Even though the quest for a full mastery of QCD is still ongoing, this work has already paid off enormously. When I entered the field and started working on QCD in the mid-2000s, our community held a workshop to draw up a “wish list” of QCD measurements we wanted. As we worked through that list, we kept finding massive disagreements between data and theory, and had to push the theoretical framework further and further to close the gaps.
Today, we’re far beyond what we could have imagined 20 years ago. It’s been a revolution—and our measurements at Tufts are part of it. By pushing QCD to its limits in some of the messiest events the LHC produces, we’re turning a long-standing headache into a precision tool. If we want to be confident that a strange bump in the data is truly a sign of new physics—and not just the strong force playing one of its usual tricks—we need QCD to be more than a beautiful theory on paper. We need it fully mastered. That mastery, built by many US researchers working at the LHC, is ensuring that we keep the door to discovery open.
