What is the Nuclear EMC effect? Scientists shed light on a physics-defying mystery

Nuclear science is many things: it’s powerful, it’s potentially dangerous, and it’s ultimately still not totally understood by scientists — at least when it comes to its constituent parts.

Controlling the splitting of atoms to fuel a nuclear reaction is old hat these days, but while scientists understand how these reactions take place what they don’t understand completely is something much more fundamental: the behavior of protons and neutrons.

This should be simple stuff. After all, we’re taught in school that protons and neutrons make up the center of all atoms in the universe. But what your science books didn’t tell you is that scientists have observed some peculiar and physics-defying traits of these subatomic particles that can’t be explained by existing theory.

One of the most puzzling is called the EMC effect. Scientists have been on a long journey to solve this mystery and new evidence presented this October could lead to the answer.

What’s a nucleon?

Protons and neutrons are subatomic particles (that is, particles smaller than the size of an atom) that make up a class of particles called nucleons. These nucleons are respectively positively and neutrally charged and are tangled together in different ratios — along with a mix of negatively charged electrons in orbit around them — to create every atom we observe in the natural world.

A hydrogen atom, for example, is simply made of one proton and one electron while an atom of plutonium has 94 protons, 94 electrons, and up to 150 neutrons.

Easy enough, except these nucleons are made up of even smaller and even stranger subatomic particles called quarks. First physically observed at CERN in 1968, quarks come in six “flavors”:

• Up
• Down
• Charm
• Strange
• Top
• Bottom

A proton, for example, is made up of two “up” quarks and one “down” quark while neutrons have one “up” quark and two “down” quarks. To hold everything together, both the nucleons and their quarks experience something called the strong nuclear force.

Along with forces we can perceive, like gravity or magnetism, the strong force is one of the four fundamental forces. Instead of acting on the human scale, this force works on the subatomic scale to bind quarks to each other and to then bind protons and neutrons together in the nucleus of an atom.

In theory, this strong force should keep both the nucleons and their quarks from deforming or changing within an atom’s nucleus. However, to scientists' confusion, something very different is happening in reality.

What is the EMC effect?

When scientists have observed protons and neutrons outside of an atomic nucleus, they’ve been able to measure a definite size and shape of protons and neutrons. However, in 1983, a team of scientists at CERN noticed something peculiar when they measured the size and shape of these nucleons inside the atomic nucleus.

Instead of maintaining their shape — as would be expected given how the strong force resists tampering — the research team observed that the nucleons appeared to be much larger than they should be. As a result, scientists inferred that their internal quarks were moving much more slowly than usual.

“Something's going to be in the textbook and the ball game is over.”

This phenomenon was named the EMC effect after the group that discovered it: the European Muon Collaboration.

Or Hen, an MIT nuclear physicist, explained to Live Science in 2019 that this second aspect is particularly troubling because the force holding the quarks together is incredibly strong. Protons and neutrons are bound together by a force of about 8 million electron volts whereas their quarks are held together by a force of 1000 million electron volts.

The idea that anything affecting a nucleon could disrupt something so tightly wound as its internal quarks simply don’t add up with existing theories, Hen told Live Science.

The question then becomes, what forces don’t we know about?

Is the EMC effect solved?

As for whether or not scientists have officially cracked the code on what’s causing the EMC effect, that depends on who you ask. One confident stance, proposed by Hen and colleagues, is that high-energy nucleon pairs may be forming within the nucleus. Under these close quarters and high-energy environments, Hen and colleagues speculate that quarks from different nucleons may be able to interact with each other as well with even more energy.

As a result of these quark-quark interactions, the nucleons could appear to change size for short periods of time — voila, the EMC effect.

Other groups, however, aren’t so sure. Another prevailing theory, called Nuclear Mean-Field Theory, suggests that this effect may still be caused by the nucleons’ strong nuclear force.

Experimental results announced this October from Jefferson Lab have also taken the search for a solution to the EMC effect another step further. For the first time, this group has tagged and observed “spectator” neutrons to better understand the EMC effect on nucleons. Preliminary observations of these data suggest that local density and momentum fluctuations may be driving this effect.

“We present results from a new transformative measurement of a novel observable that provides direct insight into the origin of the EMC effect,” Tyler Kutz, a postdoctoral researcher at MIT and coauthor on the paper, said in a statement.

The race to understand this strange science may be far from over, but that’s just part of the fun Gerald Miller, nuclear physicist at the University of Washington told Live Science.

"Eventually, something's going to be in the textbook and the ball game is over,” Miller said. “The fact that there's... competing ideas means that it's exciting and vibrant. And now finally we have the experimental tools to resolve these issues."

With any luck, answering this question could completely transform our understanding of matter and fundamental forces.