Qweak: Weak Charge of the Proton and New Physics at Jefferson Laboratory (JLab E02-020, E05-008)

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A search for physics beyond the Standard Model of fundamental particle interactions has been underway for the past decade at Jefferson Laboratory in Newport News, VA. The 'Qweak' experiment will measure the proton's weak charge -- a fundamental property that determines how a proton responds to the weak interaction. One of the Standard Model's key predictions is that the strengths of all interactions, characterized by 'coupling constants', are in fact energy scale - dependent. This property has not yet been tested to high precision for the weak interaction. By measuring the proton's weak charge, we will be able to determine whether the anticipated 'running' of the weak interaction with energy scale is in fact borne out by experiment. If it is not, some new physics processes must be taking place, which would have profound implications for our understanding of the fundamental interactions of matter. The key to our experimental test is to isolate the weak interaction contribution to electron-proton scattering by comparing the scattering rates when the electron beam is polarized in two mirror-image states, referred to as parity-violation. This means that we compare the scattering rate when the electron beam spins are aligned parallel and antiparallel to their direction of motion, incident on a liquid hydrogen (proton) target. The expected dependence on the electron spin direction is very small - only 0.2 parts per million, and we are measuring this tiny asymmetry to within +/- 4% of its value in order to test the predictions of the Standard Model theory. After 7 years of development, the Qweak experiment took data almost full time at Jefferson Laboratory's continuous electron beam accelerator facility for 2 years, beginning in mid-2010. A substantial data set has been acquired, from which the weak charge of the proton will be determined. We made use of Jefferson Lab's excellent quality polarized electron beam to help ensure that systematic errors would be very small, in order to carry out these very precise measurements. Our group has played a key role in the Qweak experiment since it was first conceived in 2001; our contributions, with NSERC support, have been and will continue to be pivotal in ensuring the success of this challenging effort.