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Scientists get a more accurate measurement of the weak force between protons and neutrons

Scientists get a measurement of the weak force with greater accuracy between protons and neutrons

The precision experiment with n-helium-3, conducted at ORNL, measured the weak force between protons and neutrons by detecting the tiny electrical signal produced when a neutron and a helium-3 nucleus combine and then decay as they move through the target cell of the helium gas. Credit: Andy Sproles / ORNL, United States Department of Energy

Through a one-of-a-kind experiment at the Department of Energy̵

7;s Oak Ridge National Laboratory, nuclear physicists accurately measured the weak interaction between protons and neutrons. The result quantifies the weak force theory as predicted by the standard model of particle physics.

The observation of the weak force of the team, detailed in Physical Review Letters, was measured through a precision experiment called n3He, or n-helium-3, which was performed at ORNL’s Spallation Neutron Source or SNS. Their discovery has produced the smallest uncertainty of any comparable weak force measurement in the nucleus of an atom to date, which sets an important benchmark.

The Standard Model describes the basic building blocks of matter in the universe and the fundamental forces that act between them. Calculating and measuring the weak force between protons and neutrons is an extremely difficult task.

“Because the interactions we are looking for are very weak, the effects we want to detect in precision nuclear physics experiments are very small and, therefore, extremely difficult to observe,” said David Bowman, co-author and neutron physics team leader at ORNL.

The weak force is one of the four fundamental forces in nature, along with the strong nuclear force, electromagnetism and gravity, and describes the interactions between subatomic particles called quarks that make up protons and neutrons. The weak force is also responsible for the radioactive decay of an atom. Some weak force mechanisms are among the least understood aspects of the Standard Model.

Detecting the elusive weak interactions requires high-precision experiments, led by large international teams with state-of-the-art apparatus and a first-rate cold neutron source with very high neutron flux, such as the Fundamental Neutron Physics Beamline at SNS . The neutrons produced at the SNS are ideal for precision experiments that address the role that the weak force plays in the reaction between neutrons and other nuclei.

Bowman, a leading scientist in this field, has been studying nuclear physics and subatomic interactions since the early 1960s.

“In the beginning there were phenomenological nuclear models collected from an empirical research point of view. But, in recent years, there have been important advances in the calculations of weak force interactions in the nuclear environment,” he said. “New nuclear techniques have become available with varying degrees of freedom, and the calculations are now at a very advanced level.”

The scientists’ latest experiment focused on helium-3, which is a light and stable isotope made up of two protons and a neutron, the only element in nature that has more protons than neutrons in the nucleus. “When a neutron and a helium-3 nucleus combine, the reaction produces an excited and unstable isotope of helium-4, which decays into a proton and a triton (composed of two neutrons and a proton), both of which produce a small but detectable signal as they move through helium gas into the target cell, “said Michael Gericke, corresponding author and professor of subatomic physics at the University of Manitoba.

The n-helium-3 experiment used the same neutron beam, polarizer and diagnostic line as its predecessor, NPDGamma, which used a liquid hydrogen target that produced gamma rays from neutron-proton interactions. The team found that more gamma rays descend relative to the direction of rotation of the neutrons, which led to the successful measurement of a specular asymmetric component of the weak force.

Similar to NPDGamma, the n-helium-3 experiment is the culmination of a decade of research, preparation and analysis. The experiment setup created an extremely low background environment in which neutrons can be checked before entering a helium-3 gas container. Gericke led the team that built the combined helium-3 detector and target system designed to collect the very small signals and led the subsequent analysis.

In the experiment, a beam of slow or cold neutrons at the SNS entered the target of helium-3. An instrument was designed to control the nuclear rotation direction of helium-3 atoms. When neutrons interact with the magnetic field, another apparatus has reversed their direction of rotation up or down, defining the state of rotation. When the neutrons reached their target, they interacted with the protons inside the helium-3 atoms, sending current signals measured by the sensitive electronics.

“We had to develop a unique target gas cell that simultaneously served as a position-sensitive detector to measure the subatomic products of the reaction,” said Gericke.

“To adapt to the different operating conditions of this experiment, we invented a new apparatus necessary to reverse the direction of rotation of the neutrons just before they reacted with the helium-3 target,” said co-author and professor of nuclear physics Christopher. Crawford of the University. of Kentucky. “This universal spin pinball machine was able to operate in a wide neutron speed range with high efficiency.”

Weak force experiments have to contend with the dominant nature of strong force and background noise that could distort the data. “The n-helium-3 experiment had to be sensitive to very small effects, 100 million times smaller than the background,” Crawford said. “It’s like looking for a 1-inch needle in a 12-meter-high barn full of hay.”

For about a year, the team collected and analyzed data to determine the strength of the parity violation, which is a specific property of the weak force between a neutron and a proton. This phenomenon is unique to the weak force and is not observed in the strong force, electromagnetism or gravity.

N-helium-3 exploited the symmetry of the experimental configuration obtained from the well-controlled neutron polarization, measuring a combination of the spin of the neutrons and the output momentum of the reaction products for both neutron polarizations. “This has a certain handiness,” Crawford said. “Since the left and right hands appear opposite in the mirror, this observation was completely insensitive to the effects of the other three forces.”

The results of n-helium-3, along with NPDGamma, have changed the way nuclear physicists understand the role of the weak force in atomic nuclei. Both help answer pending questions in the standard model due to the ability to perform accurate calculations.

“Now what happens next, we need more measurements, like these very precise measurements that we get at SNS,” Bowman said. “Advances in this field require a dialogue between experimentalists and theorists. As the results of experiments such as ours become available, they compare theories and this allows theorists to improve models that predict new observables that may then be achievable. experimentally”.

First precision experiment to isolate, measure the weak force between protons, neutrons

More information:
M. T. Gericke et al, First precision measurement of parity that violates the asymmetry in cold neutron capture on He3, Physical Review Letters (2020). DOI: 10.1103 / PhysRevLett.125.131803

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Oak Ridge National Laboratory

Scientists Obtain More Accurate Weak Force Measurement Between Protons and Neutrons (2020, Sep 24)
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