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 A1 at high $x_{Bj}$
is of particular interest because valence quark dominate in this region, which makes
it a relatively clean region to study the nucleon spin structure. There are several
theoretical calculations that apply to the high x valence quark region, and here we
will focus on the neutron $A_1^n$. The neutron $A_1^n$ is predited to be 0 in the
naive SU(6) quark model, while both relativistic constituent quark model (RCQM) and
perturbative QCD (pQCD) predict $A_1^n$ to be 1 at $x$=1. Predictions for the quark
polarization in the nucleon also exist: $\Delta d/d$ is predicted to approach $+1$
in pQCD while RCQM prediction remains negative at the $x\to 1$ limit. The $A_1^n$
experiment during the 6 GeV JLab era showed that $a_1^n$ indeed turns positive at
$x\sim 0.5$, while $\Delta d/d<0$ at $x=0.61$. Subsequent theoretical studies based
on our 6 GeV results claimed that quark orbital angular momentum or non-perturbative
nature of the strong interaction plays a significant role in the valence quark
region.
With the 12 GeV upgrade of JLab, a new experiment on $A_1^n$ is being 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 will reach a deeper valence quark
region: $x\sim 0.75$. And once combined with expected data from the upgraded CLAS12
experiment on the proton $A_1^p$, we will finally be able to reveal whether $\Delta
d/d$ turns positive (as in pQCD) or remain negative at high $x$ (as in RCQM).
We will present the physics of $A_1^n$ and review the running status of the
experiment. Performance of an upgraded polarized $^3$He target will be presented.
