Accessing little-known nucleons: New neutron measurements help physicists learn about nucleon structure and spin

Protons and neutrons (collectively known as nucleons) are both building blocks of matter, but one of these particles gets a little more attention in certain types of nuclear physics experiments.

Until now. The new results, published in Physical Review Letters, provide the first glimpse into the inner workings of neutrons, thanks to a special detector developed over a decade in Experiment Hall B at the U.S. Department of Energy’s Thomas Research Institute. It says it was done. Jefferson National Accelerator Facility.

“This is the first time we have detected a neutron in a reaction of this kind. This is a very important result for the study of nucleons,” said Sylvia Nicolai, head of research at France’s National Center for Scientific Research (CNRS).

Nicolai proposed an experiment that would allow physicists to make this measurement, which would help them better understand the structure and spin of both neutrons and protons.

A new way to detect neutrons

Nucleons are made up of small particles called quarks and gluons. Physicists still do not fully understand how these constituent particles are distributed inside the nucleon or how they contribute to the nucleon’s overall rotation. Experimenters will study these particles using the Continuous Electron Beam Accelerator Facility (CEBAF), a DOE Office of Science user facility, to scatter electrons from nucleon targets and detect the end products of these reactions. .

One reaction is called deep virtual Compton scattering (DVCS). In DVCS, electrons interact with nucleon targets. The nucleon absorbs some of the electron’s energy and emits a photon, but it does not break down. Ultimately, three particles can be detected: the nucleons that collided, the photons that the nucleons emitted, and the electrons that interacted with the nucleons.

Researchers have extensively studied DVCS using the CLAS12 detector, which stands for CEBAF Large Acceptance Spectrometer, with a 12 GeV beam energy, and its predecessor, CLAS. However, Hall B’s CLAS and CLAS12 detectors are primarily used to investigate DVCS on protons, which are easier to measure than DVCS on neutrons.

The neutrons involved in DVCS tend to be scattered 40 degrees above the beamline, an area inaccessible to CLAS12 and therefore more difficult to detect.

“With standard configurations, detection of neutrons was not possible at these angles,” Nikkolay said. In 2007, she started thinking about how these neutrons could be measured with the CLAS collaboration of nuclear physicists. Her solution? Central neutron detector.

In 2011, Nicolai and her team at the Institut Irène Joliot-Curie (IJCLab), two infinite physics laboratories, are a joint research unit of the CNRS, the University of Paris-Saclay and the University of the City of Paris in Orsay, France. Construction of the detector began in . .

The team completed the detector in 2015, and it was installed on CLAS12 two years later. Dr. Pierre Chatagnon, then a student at the University of Paris-Saclay, joined the IJCLab team at the Jefferson Institute and installed the detector. He also wrote the software to adjust it. Now, he has returned to the Jefferson Institute as a postdoc in Hall B.

The Central Neutron Detector collected data from 2019 to 2020. Although they were able to cover the angles needed for neutron detection, Nikolai and his team encountered an unexpected problem while analyzing the data: proton contamination.

The detector was designed to discard the charged non-neutron signal. However, it was found that the part of the detector responsible for rejecting protons had a dead zone, allowing protons to enter and contaminate the neutron measurements.

Fortunately, IJCLab researcher Adam Hobart, who led the data analysis for this experiment, was able to clean up the data.

“This problem was solved thanks to Adam’s experience using machine learning techniques,” Nikkolay said. “He developed ML-based tools to distinguish fake signals from real neutrons, which was essential to achieving the final result.”

These ML techniques, in conjunction with a central neutron detector, enabled the first measurements of DVCS on neutrons that directly detect the neutrons involved in the reaction. Many processes can occur between the beam electrons and the nucleon target. By directly detecting neutrons, researchers can be confident that they are actually detecting a DVCS.

“If you’re not detecting neutrons, there’s a range of possibilities for what processes are going on, and the observables you measure afterwards are less accurate,” Hobart said.

First flavor separation

A theoretical framework known as the generalized parton distribution (GPD) converts measurements from scattering experiments into information about the distribution of partons (quarks and gluons) inside a nucleon. There are four types of GPD. Neutron measurements from this experiment gave researchers access to one of the least known types, called GPD E.

During the experiment, CEBAF’s beam was polarized. This means that the spins of the electrons are pointing in the same direction. This allowed the researchers to extract observations known as asymmetries that depend on the rotation of the beam. This asymmetry allowed us to extract GPD E with unprecedented accuracy.

“GPD E is very important because it gives us information about the spin structure of the nucleon,” Nikkolay said.

Combining GPD E with other GPDs allows us to quantify how much the constituent quarks contribute to the total spin of the nucleon, which is currently unknown. This calculation will be performed in a future study, but with this study the researchers have taken another step toward solving the so-called nucleon spin crisis.

Nucleons contain two types of quarks (flavors): up and down. A proton has two up quarks and one down quark. A neutron has two down quarks and one up quark. GPD can be divided by quark flavor.

By combining measurements of DVCS on neutrons with previous measurements of DVCS on protons, the researchers were able to separate the imaginary parts of GPD E and H by quark flavor for the first time. Separating the distribution of up and down quarks helps physicists understand how different quarks contribute to the spin of a nucleon.

Although theorists Maria Čuić and Krešimir Kumerički also contributed to this first flavor separation, a collaborative effort across CLAS was essential to achieve these pioneering results.

“I have to thank CLAS for the entire collaboration,” Hobart said. “Data acquisition and processing is a collaborative effort.”

Now that they have a proof of principle, the researchers plan to use CLAS12 and the central neutron detector to collect more data and make even more precise measurements.

“But this first result is significant,” Nikolai said. “This is the first project in my career that I have taken full responsibility for, so I feel like I have completed a cycle and accomplished a lifetime of accomplishments. When it was published, it felt like I had another child.”