Researchers have reported the first measurements of the weak interaction between protons and neutrons inside an atom.
The detection of the elusive force verifies a prediction of the Standard Model, the most widely accepted model explaining the behavior of three of the four known fundamental forces in the universe.
“This observation determines the most important component of the weak interaction between the neutron and the proton—and also between the neutron and all other nuclei,” says lead author W. Michael Snow, a professor in the Indiana University-Bloomington College of Arts and Sciences’ physics department and the director of the university’s Center for Spacetime Symmetries. Snow is also a cospokesperson on the NDPGamma Experiment at Oak Ridge National Laboratory, where researchers conducted the experiments.
“You have to detect a lot of gammas to see this tiny effect.”
“The result deepens our understanding of one of the four fundamental forces of nature,” he adds.
These four forces are the strong force, electromagnetism, the weak force, and gravity. Protons and neutrons are made of smaller particles called quarks that the strong force binds together. The weak force exists in the distance inside and between protons and neutrons. The goal of the experiment was to isolate and measure one component of this weak interaction.
Inside the atom
To detect the weak interaction inside protons and neutrons, the experiment’s leaders used a device called NPDGamma at Oak Ridge National Laboratory that controls the spin direction of cold neutrons the laboratory’s Spallation Neutron Source generates. After the angular momentum, or spin, of these neutrons lined up, the team smashed them into protons in a liquid hydrogen target to produce gamma rays.
“The goal of the experiment was to isolate and measure one component of this weak interaction, which manifested as gamma rays that could be counted and verified with high statistical accuracy,” says coauthor David Bowman, team leader for neutron physics at Oak Ridge. “You have to detect a lot of gammas to see this tiny effect.”
Any “lopsidedness” in the direction of the resulting rays can only come from the weak force between the protons and neutrons. By counting more gamma ray emissions opposite to the neutron spin than along the neutron spin, the researchers observed the influence of the weak interaction. The small size of this lopsidedness, about 30 parts per billion, is the smallest gamma asymmetry ever measured.
Researchers conducted the experiments to detect the weak force over nearly 20 years, with Snow playing a role in the work since the beginning.
“I’ve been involved in the experiment since the original proposal almost two decades ago,” says Snow, whose work on the project has spanned two major phases, including an initial phase that took place at Los Alamos National Laboratory.
Next, Snow is eager to delve deeper into new questions the recently reported study prompted, including exploring the connection between the weak force between the neutrons and protons and the strong force between the quarks inside them. As part of this effort, researchers plan to search for the effect of the weak interaction on slow neutron spin rotation in liquid helium.
“There is a theory for the weak force between the quarks inside the proton and neutron, but the way that the strong force between the quarks translates into the force between the proton and the neutron is not fully understood,” says Snow. “That’s still an unsolved problem.”
He compared the measurement of the weak force in relation with the strong force as a kind of tracer, similar to a tracer in biology that reveals a process of interest in a system without disturbing it.
“The weak interaction allows us to reveal some unique features of the dynamics of the quarks within the nucleus of an atom,” Snow adds.
The NPDGamma result also helps enable a new search for possible violations of time reversal symmetry. This experiment, called the Neutron OPtics Time Reversal EXperiment, NOPTREX, will address the mystery of why there is more matter than antimatter in the universe. Snow is the cospokesperson for NOPTREX.
The paper appears in the journal Physical Review Letters.
Additional lead coauthors came from Indiana University, Arizona State University, Argonne National Laboratory, the University of Virginia; the University of Tennessee, Knoxville; and Los Alamos National Laboratory. In total, 64 individuals from 28 institutions worldwide contributed to this research. The US Department of Energy, the National Science Foundation, and the IU Center for Spacetime Symmetries partially supported the work.