WHAT IS THE NUCLEAR EMC EFFECT?

With all the things we know today, the things we don’t know are mind boggling. The workings of the EMC effect is one of them. Nuclear physicists think they have their heads wrapped around some of the basic building blocks of matter, like molecules and atoms, but when it comes to the things that those are made up of, the science gets really fuzzy. All they can say is that matter seems to be energy tied up in knots. Getting the merit badges for figuring out how some of those tangles are tied could earn a Nobel Prize to go with it.

EMC effect controversy

Splitting atoms for electricity or bombs is fairly commonplace. At the core of every atom are protons and neutrons. They should be well understood by now but they aren’t. They keep behaving in surprising ways that don’t fit the existing theory models.

Take the EMC effect for instance. Despite the choice of initials for an acronym, it has nothing to do with Einstein or relativity. A strange phenomena was discovered by the European Muon Collaboration.

In October, new evidence was presented which could solve the EMC mystery but critics aren’t done kicking around it’s flaws yet. At the core of the issue, as well as every atom in the universe, are “nucleons.” Protons and neutrons are nucleons. Protons have a positive charge and neutrons are neutral.

Each atom in the periodic table contain these nucleons “tangled together in different ratios.” Various numbers of negatively charged electrons orbit around them in swarms. The simplest atom, hydrogen, is one proton with one orbiting electron. Up the atomic scale, “plutonium has 94 protons, 94 electrons, and up to 150 neutrons.”

While that much is fairly straightforward, each proton “is made up of three quarks that are tightly held together by different forces, including the strong force.” All nucleons are made up of quarks. Each of those comes in six “flavors,” up, down, charm, strange, top, and bottom.

A proton is made of two up quarks and a down. Neutrons have one up, two down. Binding the quarks together is the strong nuclear force, which, like gravity or magnetism, is considered “one of the four fundamental forces.” The EMC effect throws a monkey wrench into all that neatness.

Reality doing something different

Reality is something that isn’t easy to nail down in particle physics. Theory says that the strong force “should keep both the nucleons and their quarks from deforming or changing within an atom’s nucleus.” Things seldom happen as they should. Scientists are baffled that they found “something peculiar when they measured the size and shape” of “nucleons inside the atomic nucleus.”

Ones they captured and measured outside an atomic nucleus behave just fine, the inside ones are warped and twisted in ways they aren’t supposed to be. The European Muon Collaboration was working at the CERN particle accelerator in 1983 when they “noticed something peculiar when they measured the size and shape of these nucleons inside the atomic nucleus.” The EMC effect was named for them.

The observed 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.” According to Dr. Or Hen with MIT, what makes the EMC effect so “particularly troubling” is that “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.” Is there some other force we don’t even know about?

He thinks he came up with an answer, maybe. He’s confidant that “high-energy nucleon pairs may be forming within the nucleus.” Since they’re in such “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.” Not so fast, critics say. the “Nuclear Mean-Field Theory, suggests that this effect may still be caused by the nucleons’ strong nuclear force.” They may be right. Both teams have started playing with “spectator” neutrons.