A cutting-edge experiment that probes the internal structure of the neutron has been done at Jefferson Lab in the US. An international collaboration used the CEBAF Large Acceptance Spectrometer (CLAS12) to study the scattering of high-energy electrons from a deuterium target. The team measured generalized parton distributions, which provide a detailed picture of how the neutron’s constituent quarks contribute to its momentum and spin. A key innovation was the use of the Central Neutron Detector, a specialized instrument enabling the direct detection of neutrons ejected from the target.
“The theory of the strong force, called quantum chromodynamics [QCD], that describes the interaction between quarks via the exchange of gluons, is too complex and cannot be used to compute the properties of bound states, such as nucleons [both protons and neutrons],” explains Silvia Niccolai, a research director at the French National Centre for Scientific Research, who proposed the idea for the new detector. “Therefore, we need to use unknown but experimentally measurable functions called generalized parton distributions that help us connect the properties of the nucleons (for instance their spin) to the dynamics of quarks and gluons.”
The parton model assumes that a nucleon contains point-like constituents called partons – which represent the quarks and gluons of QCD. By measuring parton distributions, physicists can examine correlations between a quark’s longitudinal momentum — how much of the nucleon’s total momentum it carries — and its transverse position within the nucleon. By analyzing these relationships for varying momentum values, scientists create a tomographic-like scan of the nucleon’s internal structure.
“This experiment is important because it directly accesses the structure of the neutron,” says Gerald Miller at the University of Washington, who was not involved in the study. “A neutron [outside of a nuclei] will decay in about 15 min, so it is difficult to study. The experiment in question used a novel technique to directly examine the neutron. They measured the neutron in the final state, which required new detection techniques.”
Separating quark contributions
Protons and neutrons consist of distinct combinations of up and down quarks: up-up-down for protons and down-down-up for neutrons. Each type of quark is associated with its own set of generalized parton distributions, and the overarching aim of the experimental effort is to determine distributions for both protons and neutrons. This would enable researchers to disentangle the distributions by quark type, offering deeper insights into the contributions of individual quark flavours to the properties of nucleons.
While these distributions are vital for understanding the strong interactions within both protons and neutrons, our understanding of protons is significantly more advanced. This disparity arises from the electric charge of protons, which facilitates their interaction with other charged particles, unlike electrically neutral neutrons. Additionally, proton targets are simpler to prepare, consisting solely of hydrogen atoms. In contrast, neutron experiments target deuterium nuclei, which comprise a neutron and a proton. The interaction between these two nucleons within the nucleus complicates the analysis of scattering data in neutron experiments.
To address these problems, the CLAS12 collaboration utilized the Central Neutron Detector, which was developed at France’s Laboratory of the Physics of the Two Infinities Irène Joliot-Curie (IJCLab). This allowed them to detect neutrons ejected from the deuterium target by high-energy electrons for the first time.
By combining neutron detection with the simultaneous measurement of scattered electrons and energetic photons produced during the interactions, the team gathered comprehensive data on particle momenta. This was used to calculate the generalized parton distributions of quarks inside neutrons.
Spin alignment
The CLAS12 team used electron beams with spins aligned both parallel and antiparallel to their momentum. This configuration resulted in slightly different interactions with the target, enabling the team to investigate subtle features of the generalized parton distributions related to angular momentum. By analyzing these details, they successfully disentangled the contributions of up and down quarks to the angular momentum of the neutron.
The team believes their findings could help address the longstanding “spin crisis“. This is the large body of experimental evidence suggesting that quarks and gluons contribute far less to the total spin of nucleons than initially expected.
The spin of a proton
“The sum of both the intrinsic spin of the quarks and gluons still doesn’t add up to the total spin,” says Adam Hobart, a researcher at IJCLab who led the data analysis for this experiment. “The only missing piece to complement the intrinsic spin of the quarks and the gluons is the orbital angular momentum of the quarks.”
The team plan to do a new and more accurate experiment that will involve firing electrons at a polarized target in which the nuclear spins of the deuterium all point in the same direction. This should allow the physicists to extract all possible generalized parton distributions from the scattering data.
“More data are needed to get a fuller picture, but this experiment can be thought of as a big step in a huge experimental program that is needed to get a complete understanding,” concludes Miller. “I think that this work will clearly influence future studies. Others will try to build on this experiment to expand the kinematic reach.”
The research is described in Physical Review Letters.
