23 September 2020
A deep look inside the neutron
Published online 27 January 2020
The internal structure of the neutron is closer to being understood thanks to novel scattering experiments.
The exact three-dimensional structure of nucleons (neutrons and protons) remains unclear, despite many experimental and theoretical efforts since the 1960s. One strong theoretical framework for solving this problem is called generalized parton distributions (GPD), which describes nucleons through various properties of the quarks and gluons that make them up.
Now, a large international team led by Meriem Benali and Malek Mazouz from Faculté des Sciences de Monastir in Tunisia, and Carlos Munoz Camacho from Institut de Physique Nucléaire d’Orsay in France, has observed, for the first time, rare events of a process called deeply virtual Compton scattering (DVCS) off the neutron. DVCS involves firing very high-energy electrons at nucleons in order to probe the quarks, which in turn emit high-energy photons. These photons carry information on the quark dynamics encoded by GPD.
“The neutron and the proton are actually two manifestations of the same object: the nucleon,” explains Benali. “Experimentally, the study of the neutron is very challenging, but essential. Otherwise it would be like studying humans by focusing only on men!”
Benali and co-workers used data from DVCS experiments performed at Thomas Jefferson National Accelerator Facility in the USA, where the targets were liquid hydrogen (with single-proton nuclei) and liquid deuterium (with nuclei of one proton and one neutron). By comparing the data from both targets, the team deduced the DVCS reactions occurring on quasi-free neutrons. Thanks to the different quark flavour content of protons and neutrons, and prior knowledge of proton structure, the team were able to deduce the likely contributions of the two main quark flavours to the scattering from neutrons.
The study demonstrates the strong potential of these novel techniques to impose new constraints on GPD theory.
Benali, M. et al. Deeply virtual Compton scattering off the neutron. Nature Physics (https://doi.org/10.1038/s41567-019-0774-3) (2020).