Collaborators ’ commitment to the 12 GeV upgrade of Jefferson Lab

We are proposing a comprehensive program to map out the xand Q2-dependence of the helicity structure of the nucleon in the region of moderate to very large x where presently the experimental uncertainties are still large. The experiment will use the upgraded CLAS12 detector, 11 GeV highly polarized electron beam, and longitudinally polarized solid ammonia targets (NH3 and ND3). Thanks to the large acceptance of CLAS12, we will cover a large kinematical region simultaneously. We will detect both the scattered electrons and leading hadrons from the hadronization of the struck quark, allowing us to gain information on its flavor. Using both inclusive and semi-inclusive data, we will separate the contribution from up and down valence and sea quarks in the region 0.1 ≤ x ≤ 0.8. These results will unambiguously test various models of the helicity structure of the nucleon as x → 1. A combined Next-to-Leading Order (NLO) pQCD analysis of our expected data together with the existing world data will significantly improve our knowledge of all polarized parton distribution functions, including for the gluons (through Q2–evolution). High statistics data on the deuteron in the region of moderate x and with a fairly large range in Q2 are crucial for this purpose. Finally, we will be able to improve significantly the precision of various moments of spin structure functions at moderate Q2, which will allow us to study duality and higher-twist contributions. We request 30 days of running on NH3 and 50 days of running on ND3 (or possibly 6LiD), including about 20% overhead for target anneals, polarization reversal, and auxiliary measurements.