The qweak experiment: A search for new physics at the tev scale by measurement of the proton's weak charge

Abstract

The observed coupling of the proton to the Z-boson, i.e. the «weak charge» of the proton, varies with distance scale. The coupling has already been accurately measured at short distances by high energy experiments. Qweak will make the measurement at a momentum transfer of only 0.3 (GeV/c)2. The «running» of the coupling from high to low energy can be calculated by correcting for the effect of clouds of virtual particles in the vacuum. Because the corrections depend on all of nature’s particles, not only those which have been discovered, a difference between the calculated and measured low energy weak charge could signal new physics. A measurement of Qweak to 4% will be sensitive to new physics at the few TeV scale. The Qweak experiment will use the fact that the parity-violating longitudinal analyzing power, Az, is proportional to the proton’s weak charge. The experiment plans to measure the predicted Az of -0.3 ppm with a combined statistical and systematic uncertainty of 2.2%, corresponding to a total uncertainty of 4% in Qweak. This requires a statistical precision of 5 x , which can be achieved in 2200 hours with an 85% polarized, 180 µA electron beam incident on a 0.35 m liquid hydrogen target. A synchronous data acquisition system will integrate the detector current signals over each spin state and extract the helicity correlated, parity violating component.