E12-06-112: Measurement of neutron spin asymmetry An1 in the valence quark region using 8.8 GeV and 6.6 GeV beam energies and Bigbite spectrometer in Hall A

This document provides an update for experiment E12-06-122 [1]: a precision measurement of the virtual photon asymmetry A1 of the neutron in the Deep Inelastic Scattering (DIS) region (W 2 > 4 GeV) using the Hall A polarized He target and the Hall A Bigbite spectrometer up to x Bj ≈ 0.71. Since the approval of this experiment, the hall A d2 [2] experiment, a DIS experiment, and the hall A transversity experiment [3], a SIDIS experiment, have successfully run using the polarized He target and the Bigbite spectrometer. These two experiments have clearly demonstrated the feasibility of using this setup for DIS studies. We have also been able to determine which parts of the setup need to be improved. Most notably, the need for a Cerenkov detector better designed to operate in a high rate enviorenment. In section 3 of this update we present the plans for the design and construction of a high rate Cerenkov detector for Bigbite. While the experiment can be performed with impresssive statistical accuracy within 550 hours using the existing configuration of the polarized He target and the Bigbite spectrometer, as originally proposed four years ago, several important advances in polarized He target technology and in the availability of detectors in Hall A make it possible to improve the experiment even further. These changes are also described in section 3 of this update. The improved setup will allow us to reduce statistical errors, so that they are comparable to the systematic errors, and complete the experiment within 480 hours.