One of nature’s simplest elements is giving scientists a major headache after new research shows that protons and neutrons in helium atoms don’t behave as theory suggests they should. The mismatch between theoretical predictions of how these particles behave and what they actually do could point to new physics beyond the Standard Model, the reigning model that describes the zoo of subatomic particles. particles.
In research published in April in the journal Letters of Physical Examination, the physicists zapped a container of helium atoms with electrons to knock the helium nuclei into an excited state, causing the nucleus to temporarily expand, like inhaling a chest. The team found that the response of protons and neutrons in the nucleus to the electron beam differed significantly from what the theory predicts – confirming conclusions drawn from experiments made decades ago. New research confirms that this mismatch is real, not an artifact of experimental uncertainty. Rather, it seems that scientists do not have sufficient knowledge of the low-energy physics that governs the interactions between particles in the nucleus.
The helium nucleus consists of two protons and two neutrons. The equations that describe the behavior of the helium nucleus are used for all types of nuclear and neutron matter, so resolving the discrepancy may help us understand other strange phenomena, such as neutron fusions star.
The discrepancy between theory and experiment first became apparent in 2013 following calculations of the helium nucleus led by Sonia Berry, then at Canada’s national TRIUMF particle accelerator and now a professor at Johannes Gutenberg University Mainz, and co-author of the new study. Bacca and colleagues used upgraded techniques to calculate how the protons and neutrons in a helium nucleus behave when excited by a beam of electrons, yielding numbers that differ greatly from the data of experimental. However, the experimental data used for comparison dated to the 1980s and recorded with large uncertainty in measurements.
The lead author of the new study, Simon Kegel, a nuclear physicist who studied the helium nucleus for his doctoral dissertation at Johannes Gutenberg University Mainz, in Germany, pointed out that the current facilities at his university can perform these measurements with very high accuracy. “We thought, if you can do that a little better we should try,” he told Live Science.
Better but worse
The fundamental interaction that holds the particles in the nucleus together is called strong force — but a cornucopia of effects arising from the nuances of these interactions complicates calculations of how these particles interact. Theoreticians simplified the problem using “effective field theory” (EFT), which approximates the many forces acting on particles, much like a jpeg file approximates all the data in an uncompressed image file. The upgraded version of EFT provides a better estimate of the effects that complicate models of strong interactions in the nucleus, but when the researchers crunched the numbers, they found that the theoretical predictions are further removed from observed phenomena than cruder approximations.
To examine how much of the difference could be attributed to experimental uncertainty, Kegel and the Mainz team used the MAMI electron accelerator facility at the University to shoot a beam of electrons at a container of helium atoms with electron. The electrons push the helium nuclei into an excited state described as an isoscalar monopole. “Think of the nucleus like a sphere that changes its radius, swelling and shrinking, maintaining spherical symmetry,” Bacca told Live Science via email.
Two parameters improved the accuracy of the measurements – the density of helium atoms in the container and the intensity of the beam of low-energy electrons. Both can be dialed in at a very high cost at the University Mainz facility, Kegel said.
Before they even finished analyzing the data it was clear that this new data set was not going to solve the issue. Scientists still do not know the origin of the difference between theory and experiment. But Bacca suggested that “missing or poorly calibrated pieces of interactions,” could be to blame.
Once the new Mainz Energy-recovering Superconducting Accelerator (TABLE) will come online in 2024, it will produce electron-beams of orders of magnitude greater intensity than the current accelerator, although still at the low energy required for this type of experiment. This is in contrast to accelerators like the Large Hadron Collider, which scramble for higher energy beams to discover exotic new particles at the other end of the energetic spectrum. However, the higher intensity of MESA will allow higher precision measurements, and a more detailed view of the low-energy boundary of the Standard Model.
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