[A1n_d2n] Abstract for the A1n talk in DIS2022

[mingyu at jlab.org]

Hi All,
Here is my draft for the abstract for the A1n talk in the upcoming
DIS2022. Feel free to leave comments and suggestions. (in LaTex math
formulae for indico.cern.ch abstract submittion)

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 $A_1^n$
(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: $x\sim 0.75$. 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 report the analysis results
from the $A_1^n$  experiment. Performance of an upgraded polarized $^3$He
target will 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.

Sincerely,
Mingyu Chen