A New Experiment Casts Doubt on the Leading Theory of the Nucleus
The original version from this story appeared in Quanta Magazine.
A new measurement of the strong nuclear force that holds protons and neutrons together confirms earlier evidence of an inconvenient truth: we still don’t have a solid theoretical understanding of even the simplest nuclear systems.
To test the strong nuclear force, physicists resorted to the helium-4 nucleus, which consists of two protons and two neutrons. When helium nuclei are excited, they grow like an inflating balloon until one of the protons flakes off. Surprisingly, in a recent experiment, helium cores did not swell as intended: they swelled more than expected before bursting. A measure describing this expansion, called the form factor, is twice the size of theoretical predictions.
“The theory should work,” he said Sonia Baccaa theoretical physicist at Johannes Gutenberg University Mainz and author of the article describing the discrepancy published in Physical Examination Letters. “We are confused.”
The swelling helium nucleus is a kind of mini-lab for testing nuclear theory, according to researchers, because it’s like a microscope — it can magnify flaws in theoretical calculations. Physicists believe that certain peculiarities of this swelling make it extremely sensitive to even the weakest components of the nuclear force – factors so small that they are usually ignored. It also depends on how much the core swells softness of the core matter, a trait that offers glimpses into the mysterious hearts of neutron stars. Before physicists can explain the crushing of matter in neutron stars, however, they must first figure out why their predictions are so far off.
Bira van Kolck, a core theorist at France’s National Center for Scientific Research, said Bacca and her colleagues uncovered a significant problem in nuclear physics. They had, he said, found a case where our best understanding of nuclear interactions – a framework known as chiral effective field theory – was inadequate.
“This transition amplifies the problems [with the theory] “It’s not so relevant in other situations,” said van Kolck.
The strong nuclear power
Atomic nucleons – protons and neutrons – are held together by the strong force. But the strong force theory was not developed to explain how nucleons stick together. Instead, it was first used to explain how protons and neutrons are made up of elementary particles called quarks and gluons.
For many years, physicists didn’t know how to use the strong force to understand the stickiness of protons and neutrons. One issue was the strong force’s bizarre nature – it gets stronger with distance, rather than slowly dying away. This feature prevented them from using their usual calculation tricks. Typically, when particle physicists want to understand a particular system, they decompose a force into more manageable approximative contributions, rank those contributions from most important to least important, and then simply Ignore the less important posts. With the strong power they could not do that.
Then in 1990, Steven Weinberg found a way to connect the world of quarks and gluons with sticky nuclei. The trick was to use an effective field theory—a theory that is only as detailed as it needs to be to describe nature on a given magnitude (or energy) scale. You don’t need to know anything about quarks and gluons to describe the behavior of a nucleus. Instead, a new effective force is emerging on these scales – the strong nuclear force, which is transmitted between nucleons through the exchange of pions.
