A new measurement of the strong nuclear force confirms previous hints of an uncomfortable truth: We still don’t have a solid theoretical grasp of even the simplest nuclear systems. Physicists turned to the helium-4 nucleus.
Weinberg’s work helped physicists understand how the strong nuclear force emerges from the strong force. It also made it possible for them to perform theoretical calculations based on the usual method of approximate contributions. The theory—chiral effective theory—is now widely considered the “best theory we have,” Bacca said, for calculating the forces that govern the behavior of nuclei.
Sonia Bacca, a physicist at the Johannes Gutenberg University of Mainz, found that our best theoretical understanding of the strong nuclear force is at odds with experimental results.In 2013, Bacca used this effective field theory to predict how much an excited helium nucleus would swell. But when she compared her calculation to experiments performed in the 1970s and 1980s, she found a substantial discrepancy.
In their experiment, Kegel and his colleagues excited the nuclei by shooting a beam of electrons at a tank of cold helium gas. If an electron zipped within range of one of the helium nuclei, it donated some of its excess energy to the protons and neutrons, causing the nucleus to inflate. This inflated state was fleeting—the nucleus quickly lost grasp of one of its protons, decaying into a hydrogen nucleus with two neutrons, plus a free proton.
As with other nuclear transitions, only a specific amount of donated energy will allow the nucleus to swell. By varying the electrons’ momentum and observing how the helium responded, scientists could measure the expansion. The team then compared this change in a nucleus’s spread—the form factor—with a variety of theoretical calculations. None of the theories matched the data.
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