Grant: Subatomic Physics Envelope - Project

A major new experiment to search for physics beyond the Standard Model of fundamental particle interactions is about to start taking data at Jefferson Laboratory, North America's premier electron beam accelerator facility for nuclear and particle physics research. The `Qweak' experiment will measure the proton's weak charge -- a basic 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 born 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 will compare the scattering rate when the electron beam spins are aligned parallel and antiparallel to their direction of motion, incident on a hydrogen (proton) target. The expected dependence on the electron spin direction is very small - only 0.3 parts per million, and we must measure this tiny asymmetry to within +/- 4% in order to test the predictions of the Standard Model theory. We will make use of Jefferson Lab's excellent polarized electron beam to ensure that systematic errors are well under control in order to carry out these very precise measurements. The Qweak collaboration consists of approximately 90 scientists from 24 institutions in the US, Canada, and Mexico. Our Canadian group represents about 1/5 of the collaboration, and is playing a leading role in the Qweak experiment, through development, construction, and deployment of selected key pieces of the instrumentation. We are training a number of highly qualified personnel through this activity.