RUSTON, La. – Physicists from Louisiana Tech University are part of an international team of researchers which has reported first results for the proton's weak charge based on precise new data from Jefferson Laboratory, the nation's premier electron beam facility for nuclear and particle physics research in Newport News, Va.
The results will be published in an upcoming issue of "Physical Review Letters," a prestigious and widely-referenced international journal that focuses on major advances in physics and cross disciplinary developments.
The "Q-weak" experiment used a high energy electron beam to measure the weak charge of the proton – a fundamental property that sets the scale of its interactions via the weak nuclear force. This is distinct from, but analogous to its more familiar electric charge (Q), hence the experiment's name: Q-weak. Following a decade of design and construction, Q-weak had a successful experimental run in 2010-12 at Jefferson Lab with data analysis having been underway ever since.
"This experiment establishes the equivalence between the electric force, which is responsible for a wide range of every day phenomenon from powering your microwave, to the structure of water and its impact on everyday life, and the weak force, both of which have a 'charge' which determines how well it interacts with, in this case, the proton, but to every other fundamental particle," said Dr. Steven Wells, associate professor of physics at Louisiana Tech and contributor to the Q-weak experiment.
In order to measure the proton's weak charge, experimenters had to exploit the weak interaction's unique property of parity violation, closely related to mirror symmetry. The Q-weak collaboration built an apparatus to detect the scattered electrons with unprecedented sensitivity, allowing them to measure the tiny asymmetry in the electron scattering rate that depends on the longitudinal polarization of the electron beam.
The success of the experiment relied on Jefferson Lab's world renowned "parity quality" beam properties. When the spin of the beam particles is reversed with respect to their direction of motion, the changes to its other properties are amazingly small, for example, the beam moves less than the width of an atom, on average.
"The future impacts of this potential discovery, should we find something outside the Standard Model, are huge," says Wells. "It could launch a big research effort, and attract students, teachers and others to seek answers to the big questions of the public such as 'Where are we from?' and 'Who are we?'"
According to the research team, to achieve the required statistical precision for Q-weak, the CEBAF accelerator at Jefferson Lab was pushed to new limits of high intensity polarized beam delivery, and the liquid hydrogen (proton) target built for Q-weak was able to absorb 1.7 kW of beam power while maintaining uniform density at a temperature of only 20 degrees above absolute zero, making it the world's highest power liquid hydrogen target to date.
"Nobody has ever attempted a measurement of the proton's weak charge before due to the extreme technical challenges to reach the required sensitivity," says Roger Carlini, Q-weak's spokesperson at Jefferson Lab. "The first four percent of the data have now been fully analyzed and already have an important scientific impact, although the ultimate sensitivity awaits analysis of the complete experiment."
Wells believes that Louisiana Tech's contributions to the experiment speak highly of the research taking place at the university.
"This is a world class institution at which we are preforming our research, and we have been involved in this experiment from the very beginning," Wells said. "In fact, Dr. Neven Simicevic designed the magnet and detectors of the experiment, and I designed many of the ancillary measurements necessary to minimize the uncertainty of the experiment, yet yielded interesting physics in their own right."
The Q-weak experiment and its findings were achieved by a team of 97 researchers from 23 institutions in the US, Canada, and Europe. It was made possible by funding from the US Department of Energy and National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, university matching contributions from The College of William and Mary, Virginia Tech, George Washington University, and Louisiana Tech University, and technical and engineering support from Jefferson Lab as well as TRIUMF (Canada) and MIT-Bates laboratories.
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