Scientists have been trying to solve the mystery of the helium core and have been more confused than ever

    An illustration of a helium atom, with two protons and a neutron in its nucleus.

An illustration of a helium atom, with two protons and a neutron in its nucleus.

One of nature’s simplest elements is giving scientists a major headache after new research shows that the protons and neutrons in helium atoms don’t behave as theory suggests they should. The discrepancy between theoretical predictions about how these particles behave and what they’re actually doing could point to new physics beyond the Standard Model, the reigning model that describes the subatomic particle zoo.

In research published in April in the journal Physical Review Lettersphysicists pounded a container of helium atoms with electrons to bring the helium nuclei into an excited state, causing the nucleus to temporarily inflate and deflate, like a chest breath. The team found that the response of protons and neutrons in the nucleus to the electron beam diverged significantly from what theory predicted, confirming conclusions drawn from experiments done decades ago. The new research shows this discrepancy is real, not an artifact of experimental uncertainty. Instead, it appears that scientists simply don’t have a solid enough understanding of the low-energy physics governing the interactions between particles in the nucleus.

The helium nucleus comprises two protons and two neutrons. Equations describing the behavior of the helium nucleus are used for all types of nuclear matter and neutrons, so resolving the discrepancy could help us understand other exotic phenomena, such as neutron star mergers.

The discrepancy between theory and experiment first became apparent in 2013 following calculations of the helium core led by Sonia Berry, then at Canada’s national particle accelerator TRIUMF and now a professor at Johannes Gutenberg University Mainz, and co-author of the new study. Bacca and colleagues used updated techniques to calculate how the protons and neutrons in a helium nucleus behave when excited by an electron beam, resulting in figures that deviate significantly from the experimental data. however, the experimental data used for comparison dates back to the 1980s and was recorded with large uncertainties in the measurements.

Lead author of the new study Simon Kegel, a nuclear physicist who studied the helium nucleus for his doctoral thesis at the Johannes Gutenberg University in Mainz, Germany, pointed out that his university’s current facilities could perform these measurements with very high precision. “We thought, if you can do it a little better, we should at least give it a try,” he told Live Science.

Better but worse

The primary interaction that holds the particles together in the nucleus is called strong force but a cornucopia of effects that arise from the nuances of these interactions complicates calculations of how these particles interact. Theorists had simplified the problem by using “effective field theory” (EFT), which approximates the many forces acting on particles, just as a jpeg file approximates all the data in an uncompressed image file. The updated version of EFT provides a better approximation of the effects that complicate models of the strong interactions in the nucleus, but when the researchers analyzed the numbers, they found that the theoretical predictions strayed even further from the observed phenomena than the roughest approximations.

To test how much discrepancy could be attributed to experimental uncertainty, Kegel and the Mainz team used the MAMI electron accelerator facility at the university to fire a beam of electrons at a canister of helium atoms. The electrons push the helium nuclei into an excited state described as an isoscalar monopole. “Imagine the nucleus as a sphere that changes its radius, swelling and contracting, maintaining spherical symmetry,” Bacca told Live Science.

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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 could be compounded at very high values ​​at the Mainz University facility, Kegel said.

Before they had even finished analyzing the data, it was clear that this new data set was not going to solve the problem. Scientists still do not know the origin of the discrepancy between theory and experiment. But Bacca suggested that “missing or mismatched pieces of the interactions” could be to blame.

Once the new superconducting energy recovery accelerator in Mainz (MESA) will go online in 2024, will produce electron beams orders of magnitude greater in intensity than the current accelerator, albeit still at the low energies required for this type of experiment. This contrasts with accelerators like the Large Hadron Collider, which vie for higher-energy beams to discover new exotic particles at the other end of the energy spectrum. However, the higher intensities of MESA will allow for even higher precision measurements and an even more detailed view of the low-energy frontier of the Standard Model.

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