The element of the sun god is giving scientists a headache

Helium is a simple element on the periodic table, named after the sun god. Yet modern theory and experiments on it have produced puzzlingly different results.

Helium is one of nature's simplest elements, just a little more complex than hydrogen. But the element of the Sun is giving scientists a headache after new research showed that the protons and neutrons in helium atoms don't behave the way they're supposed to. The discrepancy between theoretical predictions of how these particles should behave and what's happening to helium atoms could hint at new physics beyond the Standard Model, the dominant theory for describing the world of subatomic particles.

In a study published in April in the journal Physical Review Letters, physicists bombarded a cube of helium atoms with electrons to push the helium nuclei into an excited state, causing the nucleus to temporarily inflate, like a chest inhaling. The team found that the protons and neutrons in the nucleus responded significantly to the electron beam than what scientists had predicted from decades of experiments.

The new research shows that this discrepancy is real, not an artifact of experimental error. Instead, it appears that scientists previously simply did not have a solid enough grasp of the low-energy physics that governs interactions between particles in nuclei.

Helium atomic nucleus simulation.

The nucleus of a common helium atom consists of two protons and two neutrons . The equations that describe the behavior of a helium nucleus are used for all types of nuclear and neutron matter, so solving the discrepancy could help us understand other exotic phenomena, such as the merger of neutron stars.

The discrepancy between theory and experiment first became apparent in 2013, following calculations of helium nuclei by Sonia Bacca, then at Canada's TRIUMF particle accelerator (Bacca is now a professor at Johannes Gutenberg University Mainz in Germany and a co-author of the new study) . Bacca and her colleagues used improved techniques to calculate how protons and neutrons in helium nuclei behave when excited by a beam of electrons, yielding figures that differed significantly from previous experimental data. However, the previous experimental data used for comparison dates back to the 1980s and was recorded with a precision that is not up to today's standards.

Lead author of the new study Simon Kegel, a nuclear physicist who studied helium nuclei for his PhD thesis at Johannes Gutenberg University Mainz, pointed out that the equipment his university owns can repeat these measurements with very high precision.

The more modern, the more harmful to electricity

The force that holds the subatomic particles in an atom's nucleus together is called the strong force —but myriad effects stemming from aspects of these interactions complicate calculations of how these particles interact. Theorists have simplified the problem using "effective field theory" (EFT) , which approximates many of the forces acting on particles, much like a popular jpeg file approximates all the data in an uncompressed image file. An upgraded version of EFT gives a better approximation of the effects that complicate simulations of the strong force in nuclei. But when the researchers crunched the numbers, they found that the new theory's predictions were even further from the observed phenomena than the previous crude approximations.

To test how much of the difference could be due to experimental error, Kegel and the Mainz team used the MAMI electron accelerator facility at the University to fire a beam of electrons at a container of helium atoms. The electrons pushed the helium nucleus into an excited state described as an isotropic monopole. 'Imagine the nucleus as a sphere that changes radius, expanding and contracting, maintaining spherical symmetry,' Bacca describes.

Two parameters improved the accuracy of the measurements – the density of helium atoms in the container and the intensity of the low-energy electron beam. Both can be achieved at the Mainz University facility, Kegel said.

Before they finished analyzing the data, it was clear that this new dataset wouldn't solve the problem. Scientists still don't know the source of the discrepancy between theory and experiment. But Bacca suggests that 'missing or poorly calibrated interaction parts' could be to blame.

Once the ultramodern Mainz Energy Recovery Superconducting Accelerator (MESA) comes online in 2024, it will produce electron beams of greater intensity than current accelerators, although still at the low energies required for this type of experiment. MESA's higher intensity will allow for even more precise measurements and an even more detailed look at the low-energy frontier of the Standard Model.