Neutron surprises physicists – oscillation of electromagnetic characteristics does not match theoretical models –

Puzzling oscillation: physicists have re-measured the properties of the neutron – and observed surprising things. Because the electromagnetic form factors of the core component do not change evenly with increasing energy, but fluctuate periodically. This oscillation, which has already been observed in the proton, cannot yet be theoretically explained, as the team explains in “Nature Physics”. However, it indicates an internal structure of the neutron that is more dynamic and complex than expected.

The protons and neutrons, each consisting of three quarks, are the basic building blocks of matter – together they form the atomic nucleus. But despite this fundamental importance, the core components still exist mystery on. Even with such basic features as lifespan, size and internal structure there are great uncertainties. In addition, physicists repeatedly observe phenomena that do not match the theoretical models.

The secret of the form factor

The physicists of the BESIII collaboration have now gained a new insight into the characteristics of the neutron. The aim of the experiment was to determine the so-called electromagnetic form factors of the neutron more precisely. They describe the mean distribution of electrical charge and magnetization within the neutron and thus provide clues for the behavior and arrangement of the three quarks connected via gluons in their interior.

“A single form factor, measured at a certain energy, does not say much at first,” explains Frank Maas from the Helmholtz Institute Mainz (HIM). “Only knowing the form factors at different energies allows conclusions to be drawn about the structure of the neutron.” Because there have been hardly any measurements in the energy range from two to 3.8 gigaelectron volts, the team has now closed this gap with annihilation experiments.

To do this, the physicists examined the properties of neutrons and anti-neutrons that were generated when electrons and positrons collided in the BESIII particle accelerator in China. “In a figurative sense, we have filled a blank area on the map of neutron form factors with new data,” says Maas.

Oscillation contradicts models

The analyzes provided several new findings. One of them: Depending on the energy, the form factor of the neutron does not produce a smooth line, but rather shows an oscillating pattern. With increasing energy, the deflections become smaller and smaller. This surprising behavior represented a clear deviation from the expected behavior, as the physicists explain. However, such an oscillation has recently been observed in the proton as well.

“We are now observing a corresponding behavior with a similar frequency in the neutron as well, but with a large phase shift,” the scientists write. “These results suggest that there are not yet understood intrinsic dynamics in the nucleons that are responsible for these almost orthogonal oscillations.” In other words: Something is going on inside the core components that the current models have not yet captured.

“Now, in theory, our colleagues are asked to develop models for this extraordinary behavior,” says Maas.

Earlier deviation refuted

However, the new measurements also reveal that a discrepancy found in earlier measurements does not seem to exist after all. Researchers had demonstrated a stronger coupling with virtual photons for the neutron than for the proton – the neutron should therefore consistently show a larger form factor than the proton. But that contradicts fundamental theories: “Since the proton is charged, one would expect it to be exactly the other way around,” says Maas.

The data from the BESIII experiment now refute the earlier result: “Our result shows that the photon-proton interaction is stronger than the corresponding photon-neutron interaction – as predicted by most theoretical models,” writes the team . “That clears up a more than 20 year old riddle about this interaction.”

Also important for astrophysics

The new measurement data thus help to solve at least some riddles about these building blocks of matter. Even if many of the fundamental properties of the neutron and proton are by no means fully understood, they at least provide new starting points for researching the core building blocks and their behavior.

These findings not only benefit particle physics and materials research, but can also provide new insights in astrophysics: “By looking at the smallest building blocks of matter, we can also understand phenomena that take place in the largest dimensions – such as the merging of two neutron stars . This extreme physics is very fascinating, ”says Maas. (Nature Physics, 2021; doi: 10.1038 / s41567-021-01345-6)

Source: University of Mainz

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Neutron surprises physicists – oscillation of electromagnetic characteristics does not match theoretical models –