IU physicist co-leads first observation of the weak interaction between protons and neutrons

Michael Snow is co-leader of a 20-year physics experiment conducted in collaboration with institutions across the globe

BLOOMINGTON, Ind. -- An international group of scientists including physicists at Indiana University are reporting the first observation of an elusive force whose detection verifies a prediction of the Standard Model, the most widely accepted model used to explain the behavior of three of the four known fundamental forces in the universe.

A lead author on the paper reporting the first measurements of the weak interaction between protons and neutrons inside an atom is W. Michael Snow, a professor in the IU Bloomington College of Arts and Sciences' Department of Physics and the director of the IU Center for Spacetime Symmetries. The paper was published Dec. 13 in the journal Physical Review Letters.

"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," said Snow, who is also a co-spokesperson on the NDPGamma Experiment at Oak Ridge National Laboratory, where the experiments were conducted.

"The result deepens our understanding of one of the four fundamental forces of nature," he added.

These four forces are the strong force, electromagnetism, the weak force and gravity. Protons and neutrons are made of smaller particles called quarks that are bound together by the strong force. 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.

To detect the weak interaction inside protons and neutrons, the experiment's leaders used a device called NPDGamma at Oak Ridge National Laboratory designed to control the spin direction of cold neutrons generated by the laboratory's Spallation Neutron Source. After the angular momentum, or spin, of these neutrons was lined up, the team smashed them into protons in a liquid hydrogen target to produce gamma rays.

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 influence of the weak interaction was observed. The small size of this lopsidedness, about 30 parts per billion, is the smallest gamma asymmetry ever measured.

The experiments required to detect the weak force in the study were conducted 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," said Snow, whose work on the project has spanned two major phases, including an initial phase that took place at Los Alamos National Laboratory.

"IU played a major role in designing, constructing and operating the liquid hydrogen target which was the source of protons used in this work," he added. "We also procured and developed the detector array which detected the gamma rays from the neutron-proton reaction we measured to resolve the weak interaction effect. Finally, we made important contributions to the analysis of systematic errors in the measurement."

The Spallation Neutron Source at Oak Ridge National Laboratory
An aerial photograph of the Spallation Neutron Source at Oak Ridge National Laboratory. Photo by the U.S. Department of Energy, Oak Ridge National Laboratory

The majority of this work took place at the IU Center for the Exploration of Energy and Matter, or CEEM, at the Multidisciplinary Engineering and Sciences Hall in Bloomington. The site of the former IU Cyclotron Facility, the building contains large workspaces for the construction of highly specialized equipment. Work on the 20-liter liquid hydrogen target used in the Oak Ridge experiment, and related cryogenic components, made essential use of this infrastructure.

Other IU researchers involved in the work were IU Professor Emeritus Hermann Nann; CEEM mechanical and cryogenic engineers Walter Fox and John Vanderwerp; former IU postdoc Satyaranjan Santra; and former IU graduate students Chris Blessinger, Jason Fry, Michael Gericke, Chad Gillis, Jiawei Mei, Zhaowen Tang and Tony Tong, who contributed to the work over the years, including efforts on site at both the Los Alamos and Oak Ridge labs.

Next, Snow is eager to delve deeper into new questions prompted by the recently reported study, 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, IU physicists and collaborators plan to search for the effect of the weak interaction on slow neutron spin rotation in liquid helium.

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 co-spokesperson for NOPTREX.

Snow is the corresponding author on the paper in Physical Review Letters. Other IU authors are Fry, Gericke, Gillis, Mei, Nann, Santra, Tang and Tong. A total of 64 people from 28 institutions worldwide contributed to the study.

The study was supported in part by the U.S. Department of Energy, the National Science Foundation and the IU Center for Spacetime Symmetries.