An international team including physicists from The University of Winnipeg has reported first results for the proton’s weak charge in Physical Review Letters (October 18, 2013 issue). The results are based on precise new data from Jefferson Laboratory, the US’s premier electron beam facility for nuclear and particle physics research, located in Newport News, VA.
Protons are, electrically speaking, positively charged, unlike the negatively charged electrons that circulate around them in atoms. 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 from 2010-12 at Jefferson Lab. Data analysis has been underway ever since.
“Nobody has ever attempted a measurement of the proton’s weak charge before,” says Roger Carlini, Q-weak’s spokesperson at Jefferson Lab, “due to the extreme technical challenges to reach the required sensitivity. The first 4% 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.” The first result is in good agreement with the theoretical prediction based on the current understanding of fundamental particle interactions, which physicists refer to as the Standard Model.
The University of Winnipeg group, led by Physics Professor Dr. Jeffery Martin, built two significant pieces of equipment which were crucial for the success of the experiment. One was a particle detector capable of sensing the very highest rates of electrons scattering from protons in the experiment. The other was a new kind of detector, built from synthetic diamond, that was used to analyze electron spins in the experiment. The challenging project provided an excellent training ground for Martin’s PhD student, Dr. Jie Pan, two postdoctoral fellows, and, over the years, ten undergraduate students from The University of Winnipeg.
“It has been a privilege that our group, and the other Canadian groups were invited to participate in this ground-breaking experiment; it’s wonderful that we can finally reveal these exciting results to the rest of the world,” said Dr. Martin. Dr. Martin now leads another new international experiment in Canada, seeking to make to world’s most precise measurement of the neutron’s electric dipole moment. He is also the Canada Research Chair in Subatomic Physics.
Q-weak now has 25 times more data in hand, which is undergoing analysis. This will allow experimenters to search for new physics beyond the Standard Model at unprecedented levels of precision. It is possible that if a discrepancy is found, it could signify new particles beyond the Higgs boson, which was recently discovered at the Large Hadron Collider in Geneva, Switzerland.
This first determination of the proton’s weak charge was carried out 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.
For more information, see www.jlab.org/qweak or contact:
Professor Jeff Martin
Diane Poulin, Communications Officer, The University of Winnipeg
P: 204.988.7135, E: email@example.com