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The G0 spectrometer

The G0 experiment will measure the weak neutral vector current analogs of the elastic charge and magnetic form factors of the proton. By comparing the weak and electromagnetic form factors the quark contributions to each can be determined. These quark contributions are important for two reasons. First, they are the first direct measure of the sea at low energy corresponding to distance scales on the order of the proton size. Second, they are among the small set of fundamental proton ground state matrix elements which also includes the axial-vector current (Ellis-Jaffe sum rule and elastic neutrino scattering) and the scalar current ( sigma term). The experiment is sensitive to contributions to quark contributions to the charge and magnetic form factors at the few %level.

The G0 collaboration consists of approximately 40 physicists from 10 institutions. The experiment will utilize parity-violating electron scattering to determine the neutral weak form factors. Measurements of both the forward and backward angle asymmetries will be made to separate the charge and magnetic form factors. In order to obtain the highest accuracy, a dedicated spectrometer has been designed. The spectrometer utilizes a superconducting toroidal magnet which will focus recoil protons for the forward asymmetry and the scattered electrons for the backward asymmetry. It has been designed to run at high luminosity with a large solid angle acceptance and still count individual particles. In order to reduce the cost, all coils reside in a common cryostat and simple detectors are located outside the vacuum system. The spectrometer will be located in Hall C upstream of the main pivot point and will be able to remain in place while other experiments are performed. The experiment has been approved by the PAC after having been reviewed by a Technical Advisory Panel.

cardman@cebaf.gov