Speaker
Description
The virtual photon asymmetry $A_1$ is one of the fundamental quantities that provide information on the spin structure of the nucleon. The value of $A_1$ at high $x_{Bj}$ is of particular interest because valence quarks dominate in this region, which makes it a relatively clean region to study the nucleon structure. Several theoretical calculations, including naive SU(6) quark model, relativistic constituent quark model (RCQM), perturbative QCD (pQCD), predicted the behavior for $A_1$ and the quark polarization in the high $x_{Bj}$ valence quark region. The $A_1^n$ experiment during the 6 GeV JLab era showed that $A_1^n$ turns positive at $x\sim 0.5$, while up to the highest measured $x$ value of 0.61 $\Delta d/d$ remains negative, in contrast to the pQCD prediction. Subsequent theoretical studies following the 6 GeV results claimed that quark orbital angular momentum could delay the upward turn of $\Delta d/d$ to higher $x_{Bj}$ or non-perturbative nature of the strong interaction could keep it negative all the way to $x_{Bj}=1$ as predicted in Schwinger-Dyson approach with di-quark model assumption. With the 12 GeV upgrade of JLab, a new experiment on $A_1^n$ (E12-06-110)$^1$ was carried out using a 10.4 GeV beam, a polarized $^3$He target, and the HMS and the Super-HMS (spectrometers) in Hall C. This measurement reached a deeper valence quark region: $x\sim 0.75$. When combined with the expected data from the upgraded CLAS12 experiment on the proton $A_1^p$, we will be able to reveal whether $\Delta d/d$ turns positive (as in pQCD) or remain negative at high $x$ (as in RCQM or Schwinger-Dyson/di-quark). We will present the physics of $A_1^n$ and report the analysis status for the $A_1^n$ experiment. Performance of the upgraded polarized $^3$He target will also be presented.
$^1$ This work is supported in part by the U.S. Department of Energy, Office of Nuclear Physics, under Contract No. DE-FG02-94ER4084.
