Physicists from the NPDGamma Experiment at the DoE’s Oak Ridge National Laboratory (ORNL) have measured the weak interaction between protons and neutrons in the atom’s core, predicted in the Standard Model that describes the elementary particles and their interactions. The findings appear in the journal Physical Review Letters (arXiv.org preprint).
Blyth et al analyzed the gamma rays emitted during the NPDGamma Experiment and found parity-violating asymmetry, which is a specific change in behavior in the force between a neutron and a proton. They measured a 30 parts per billion preference for gamma rays to be emitted antiparallel to the neutron spin when neutrons are captured by protons in liquid hydrogen. After observing that more gammas go down than up, the experiment resolved for the first time a mirror-asymmetric component or handedness of the weak force. Image credit: Andy Sproles / Oak Ridge National Laboratory, U.S. Department of Energy.
Protons and neutrons are made of smaller particles called quarks that are bound together by the strong interaction, which is one of the four known forces of nature: strong force, electromagnetism, weak force and gravity.
The weak force exists in the tiny distance within and between protons and neutrons; the strong interaction confines quarks in neutrons and protons.
The weak force also connects the axial spin and direction of motion of the nuclear particles, revealing subtle aspects of how quarks move inside protons and neutrons.
“The goal of the NPDGamma 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,” said Dr. David Bowman, team leader for neutron physics at ORNL.
The experiment channeled cold neutrons toward a target of liquid hydrogen.
The apparatus was designed to control the spin direction of the slow-moving neutrons, ‘flipping’ them from spin-up to spin-down positions as desired.
When the manipulated neutrons smashed into the target, they interacted with the protons within the liquid hydrogen’s atoms, sending out gamma rays that were measured by special sensors.
After analyzing the gamma rays, Dr. Bowman and colleagues found parity-violating asymmetry, which is a specific change in behavior in the force between a neutron and a proton.
“If parity were conserved, a nucleus spinning in the righthanded way and one spinning in the lefthanded way — as if they were mirrored images — would result in an equal number of gammas emitting up as emitting down,” Dr. Bowman said.
“But, in fact, we observed that more gammas go down than go up, which lead to successfully isolating and measuring a mirror-asymmetric component of the weak force.”
The team ran the experiment numerous times for about two decades, counting and characterizing the gamma rays and collecting data from these events based on neutron spin direction and other factors.
The results filled in a vital piece of information, yet there are still theories to be tested.
“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. That’s still an unsolved problem,” said team member Professor W. Michael Snow, Indiana University.
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,” Professor Snow said.
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D. Blyth et al (NPDGamma Collaboration). 2018. First Observation of P-odd γ Asymmetry in Polarized Neutron Capture on Hydrogen. Phys. Rev. Lett 121 (24); doi: 10.1103/PhysRevLett.121.242002
