The experiment, called “Qweak”, scatters a high energy electron beam from a liquid hydrogen target to measure the “weak charge” of the proton-a fundamental property, distinct from its more familiar electric charge, which in physics is given the symbol “Q.”
Page, a spokesperson for the research teams, explains: “Many of these systems have never been tested before, since we only had access to the beam after our experiment was installed this summer. Everything is custom made, state of the art-pushing the envelope.”
However, Page says the potential payoff is huge, as Qweak is probing what experts call “physics beyond the Standard Model.” A successful result could reveal the existence of a new fundamental force in nature that physicists have so far only dreamed of, and would be complementary to searches at the highest energy scales such as the Large Hadron Collider project in Geneva, Switzerland.
Page explains that the trick to detecting the proton’s weak charge is to zoom in on the weak interaction’s unique property of mirror-symmetry violation, which means that two identical experiments, reflected in a mirror, yield results that differ in a tiny but reproducible way. No other force in nature does this.
The Qweak experiment uses JLab’s electron beam to achieve the desired sensitivity. When the beam is reflected in a mirror, changes to its other properties are vanishingly small-its position shifts by only a few nanometers, a few times the diameter of a large atom. Producing such beautifully stable electron beams is not just luck, but rather the culmination of many years of development work by the lab’s polarized electron source group, the best in the world.
Since the Qweak experiment was conceived in 2001, University of Manitoba physicists have been a driving force in designing and building it, through a collaboration that has been steadily growing and now stands at 111 researchers from the U.S., Canada and overseas.
The Canadian team, which also includes researchers from the Universities of Winnipeg, Northern British Columbia, and TRIUMF, Canada’s National Laboratory for Particle and Nuclear Physics, is the largest external group in the international collaboration. It plays a key role in developing virtually all of the major subsystems of the Qweak experiment, including an enormous magnetic spectrometer, the main detectors, a sensitive position scanner, and a novel set of diamond strip detectors used to confirm the electron beam is tuned up for maximum sensitivity. Canada has also provided the lion’s share of the sensitive readout electronics, designed and built at the TRIUMF Lab in Vancouver.
The Canadian contributions have been funded by the Natural Sciences and Engineering Research Council of Canada (NSERC), with infrastructure support from the Canada Foundation for Innovation (CFI) and TRIUMF; other elements have been funded by the U.S. Department of Energy and the National Science Foundation.
How is it going so far? “There’s a buzz of excitement in the counting room, where we gather the data,” says Page. “It runs 24/7. We had to limit the number of people so that the shift workers would have room to think. This is the ‘commissioning’ part of the experiment where we turn things on, find a problem, fix it, move on to the next one. We are learning new things about the experiment every day, refining our systems and approaches, solving problems, acquiring and analyzing test data and pushing ahead.”
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l-r: Michael Gericke, Shelley Page and Jim Birchall, Physics and Astronomy