Leading Theory of the Nucleus Might Not Really Be Accurate; New Experiment Challenges Our Understanding of the Nuclear Force

A recent measurement of the strong nuclear force, which keeps protons and neutrons together, gives hints of an unsettling truth: We still do not have a solid theoretical grasp of the fundamental forces of nature.

Chiral Effective Theory

Inside the atom, protons and neutrons, collectively called nucleons, are held together by a strong nuclear force. However, this theory was not developed to explain how nucleons stick together. Instead, it was first used to describe how protons and neutrons comprise small particles called quarks and gluons.

For many years, physicists have failed to use strong force to understand the bond between protons and neutrons. The strong staff has a bizarre nature of growing stronger with increasing distance instead of slowly dying off. Because of this, experts are prevented from using the usual calculation methods.

In 1990, Steven Weinberg attempted to connect the world of quarks and gluons to the nuclei bond. He introduced the chiral effective theory, considered the best available theory in computing the forces that govern the behavior of nuclei.

Theoretical physicist Sonia Bacca used this effective field theory to predict the extent to which an excited helium nucleus could swell. She discovered a substantial discrepancy after comparing her calculations to experiments conducted in the 1970s and 1980s.

Bacca encouraged her colleagues to repeat the decades-old experiment. In response, Simon Kegel updated the experimental framework where they excited the nuclei by shooting a stream of electrons at a tank of cold helium gas. They discovered that the nucleus lost grasp of one of its protons during the inflated state. On the other hand, only a specific amount of donated energy allowed the nucleus to swell during the nuclear transitions.

The Discrepancy in Nuclear Behavior

Headed by Bacca herself, scientists from the Johannes Gutenberg University of Mainz conducted an experiment where they tested the strong nuclear force. They used the nucleus of helium-4, a stable isotope of helium element containing two protons and two neutrons.

Experts know that as the helium nuclei get excited, they expand like an inflated balloon until one of the two protons pops off. However, in the experiment made by Bacca and her colleagues, the helium nuclei did not grow as planned. Instead, they ballooned more than the expected growth before they finally burst. The form factor measurement used in describing the expansion was twice as large as the theoretical projections.

According to the researchers, the swelling helium nucleus is like a mini-laboratory to test the nuclear theory as it acts like a microscope. This means that it can magnify the discrepancies in theoretical computations.

The result of the experiment made by Bacca's team has revealed a crucial problem in nuclear physics, according to nuclear theorist Bira van Kolck from the French National Center for Scientific Research. This is because they have found evidence that our best understanding of nuclear interactions, called chiral effective field theory, has fallen short.