New Research Sheds More Light on Electromagnetic Structure of Neutron

by johnsmith

The neutron is a bound system of three valence quarks and a neutral sea consisting of gluons and quark-antiquark pairs. Although the proton was discovered in 1919 and the neutron in 1932, their structure is still not fully understood. Physicists use electromagnetic form factors to describe the dynamic inner structure of the neutron and the proton. These form factors represent an average distribution of electric charge and magnetization. In new research, physicists from the BESIII Collaboration measured electron-positron annihilation reactions into a neutron and antineutron pair to determine the effective form factor of the neutron.

An artist’s impression of the neutron and its internal structure. Image credit: Xiaorong Zhu / University for Science and Technology, China.

An artist’s impression of the neutron and its internal structure. Image credit: Xiaorong Zhu / University for Science and Technology, China.

“A single form factor, measured at a certain energy level, does not say much at first,” said Professor Frank Maas, a researcher at the PRISMA+ Cluster of Excellence, the Helmholtz Institute Mainz, and GSI Helmholtzzentrum für Schwerionenforschung Darmstadt.

“Measurements of the form factors at various energies are needed in order to draw conclusions on the structure of the neutron.”

“In certain energy ranges, which are accessible using standard electron-proton scattering experiments, form factors can be determined fairly accurately.”

“However, so far this has not been the case with other ranges for which so-called annihilation techniques are needed that involve matter and antimatter mutually destroying each other.”

Using the Beijing Spectrometer III (BESIII) at the Beijing Electron-Positron Collider II (BEPCII), Professor Maas and his colleagues studied neutron and antineutron pairs produced in electron-positron annihilations at centre-of-mass energies between 2 and 3.08 GeV (gigaelectronvolts).

Their results improve the statistics on the neutron form factor by more than a factor of 60 over previous measurements.

They also demonstrate that the neutron form factor data from annihilation in the time-like regime is on par with that from electron scattering experiments.

“With these new data, we have, so to speak, filled a blank space on the neutron form factor ‘map,’ which until now was unknown territory,” Professor Maas said.

“These data are now as precise as that obtained in corresponding scattering experiments.”

“As a result, our knowledge of the form factors of the neutron will change dramatically and as such we will get a far more comprehensive picture of this important building block of nature.”

The team’s results were published in the journal Nature Physics.


The BESIII Collaboration. 2021. Oscillating features in the electromagnetic structure of the neutron. Nat. Phys 17, 1200-1204; doi: 10.1038/s41567-021-01345-6

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