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Illustration by Sandbox Studio, Chicago with Corinne Mucha

What’s so hard about measuring the strong force?

The ATLAS collaboration recently measured the strength of the strong force to a record level of precision, but there’s still a long way to go toward understanding this fundamental force.

In September, members of the ATLAS experiment at the Large Hadron Collider announced that they had measured the strength of the strong force with unprecedented precision.

What does that mean, and why do physicists find it exciting?

Let’s start with the strong force. You probably know that the world all around you—your phone, your house, your cat—is made of atoms. And that those atoms are made of protons and neutrons, tightly bound in a nucleus with electrons whizzing around them. Perhaps you also know that those protons and neutrons are made of even smaller particles, called quarks, which cannot be broken down into any smaller components, as far as we know. 

“It’s quite counterintuitive, but when you try to take two quarks apart, the coupling actually gets stronger.” 

What is holding those quarks together? What keeps them glued to each other, so that they can form the nucleons inside the atoms that create the molecules that you and I are made of? That would be the strong force. 

“The strong force is one of what we consider the four fundamental forces,” explains Stefano Camarda, a researcher on the ATLAS experiment at the Large Hadron Collider at CERN. 

The four fundamental forces include gravity, the electromagnetic force, the weak nuclear force, and finally, the strong force. These forces are what hold together or push apart everything in our universe. For example, gravity binds our galaxies together; the electromagnetic force attracts the electrons and protons in an atom; the weak force enables the thermonuclear reactions in the sun; and the strong force holds together the matter that makes up our bodies and the world around us.

Each of these forces have differing strengths. Gravity can reach between galaxies that are thousands of lightyears apart, whereas the strong force acts only on the scale of a proton, which has a volume of a quadrillionth of a meter. 

But that miniscule stage is where the strong force earns its name. Inside an atom, gravity has virtually no impact on how protons or quarks interact with each other; the strong force, on the other hand, is incredibly powerful.

Too small to observe

It is the strength of those strong force bonds between quarks that make up most of the mass of the proton.

“The mass of the quarks are responsible for only about a percent of the nucleon mass,” says Katerina Lipka, an experimentalist working at the German research center DESY, where the gluon—the force-carrying particle for the strong force—was first discovered in 1979.

“The rest is the energy contained in the motion of the gluons. The mass of matter is given by the energy of the strong interaction.”

Given the importance of the strong force to our very existence, it makes sense that scientists want to understand how it works. To do this, they’re measuring the fundamental parameter of the strong interaction, called “alpha strong,” or αs. Alpha strong tells us about the strength of the force, depending on the energy or distance at which matter is probed.

But studying the strong force is especially difficult. “The reason for this is because of the nature of the strong force,” says Tevong You, a theorist at King’s College London.

As you can observe by experimenting with magnets, the strength of the electromagnetic force increases as you bring two particles together and gets weaker as you pull them apart. This makes sense to us. The strong force, however, exhibits the opposite behavior. “It’s quite counterintuitive, but when you try to take two quarks apart, the coupling actually gets stronger,” You says.

Imagine two quarks, connected by the strong force, as beads attached to opposite sides of a rubber band. The farther apart you pull the two beads, the stronger the tension of the rubber band between them. The force you would need to break that rubber band—to, for example, remove a quark from a proton—is equal to about 1.6 metric tons, the weight of a small car, Lipka says.

After a certain distance, the potential between the two quarks gets so large that it actually requires less energy to pull a new quark or antiquark out of the quantum background than to maintain the connection between the original quarks. This process of “hadronization” literally pulls quarks out of thin air, and it means that we can only ever observe quarks in pairs or clumps.  

To deal with this, scientists use different methods to indirectly study the strong force.

At the LHC, scientists study the collisions of protons. In their recent record-breaking work, the ATLAS experiment measured alpha strong using proton-proton collisions and the production of particles called Z bosons.

When quarks in colliding protons annihilate, they typically produce Z bosons, among other particles. The Z bosons do not interact via the strong force, but the strong force gives them a kick as they escape particle collisions. The ATLAS experiment measured the transverse momentum of the Z bosons to extract alpha strong.

Lipka and her colleagues on the CMS experiment at the LHC are studying the strong force using sprays of particles called jets. When quarks are kicked out of high-energy proton-proton collisions, they produce collimated “jets” of particles made of strongly bound quarks.

“These jets preserve the properties of the original quark or gluon, and their rates are proportional to the value of the strong coupling,” Lipka says.

Her team uses the jets to measure the precise strength of the strong force.

It’s important to develop multiple methods of studying the strong coupling, Lipka says. “If we combine the results from different measurements, we could get even higher precision,” she says. “The ATLAS and CMS experiments are designed to be independent of each other so we can cross-check results and eventually combine them.”

More and more precise

We are a long way from fully understanding the strong force. We know the strength of the electromagnetic force to about one part in a billion, but we know the strength of the strong force to only about one in a hundred.

Camarda explains why working to close this gap is important: “When we evolve the strengths of the electromagnetic, weak and strong forces up to a very high energy scale, like there was in the very early universe, they become very similar.”

This similarity hints to a possible unification of these three forces in the time right after the Big Bang. “The details of this grand unification theory depend very much on the precise values of the coupling constants, and so here we are limited by the precision of the strong coupling,” Camarda says.

Understanding alpha strong is also key to studies at the LHC and the upcoming upgrade to the LHC, the High-Luminosity LHC, he says. “If we want to do precision physics, for example, in the Higgs sector, we need to improve the determination of alpha strong.”

Doing precision studies, of any kind, can help physicists test their theories. For example, scientists precisely measuring the orbit of the planet Mercury found a slight discrepancy between prediction and reality, “and it turned out to change our understanding of gravity completely,” You says.

Precision studies could give us hints of physics that might exist beyond the Standard Model, outside of what we already know about physics. “It is possible that known elementary particles, including quarks, could be composed of even more elementary objects,” Lipka says. “These objects could be subject to new forces at much higher energies than we can probe in current experiments.”

The more precise the measurement, the better scientists are able to put their predictions to the test. Camarda, Lipka and You continue to look for hints of interactions beyond the Standard Model in their work.

“It’s always exciting to push at the next frontier,” You says. “Sometimes measuring to one more digit of precision leads to a radical transformation in our understanding of the universe at a fundamental level.”