Speaker
Ms
Melissa Cummings
(College of William and Mary)
Description
Jefferson Lab has been at the forefront of a program to study the polarized structure of nucleons. Measurements of the spin-dependent structure functions, $g_1$ and $g_2$, have proven to be powerful tools in testing and understanding Quantum Chromodynamics. To measure $g_2$ a transversely polarized target is needed, which proves to be challenging experimentally. Prior to JLab, the only dedicated experiment to measure $g_2$ was SLAC E155x, which shows consistency with the leading twist $g_2^{WW}$ prediction, but has large uncertainties. The function $g_2^n$ has been measured extensively in Hall A at JLab over a wide range of $Q^2$. The Resonances Spin Structure (RSS) experiment in Hall C gave a precision measurement of $g_2$ for the proton and deuteron at intermediate $Q^2$, providing the first world data for $g_2^{p,d}$ in the nucleon resonance region. The Spin Asymmetries of the Nucleon experiment (SANE), also performed in Hall C, provided a measurement of $g_2$ in the high $Q^2$ (DIS) region. More recently, the $g_2^p$ experiment took data covering the low $Q^2$ region. The 0$^{th}$ moment of $g_2$ provides a test of the Burkhardt-Cottingham sum rule, which states that the integral of $g_2$ over the Bjorken scaling variable x goes to zero. This sum rule, valid for all values of $Q^2$, has been satisfied for the neutron, but a violation is suggested for the proton at high $Q^2$. The 2$^{nd}$ moment allows for a benchmark test of lattice QCD predictions at high $Q^2$ with $d_2$, and a test of Chiral Pertubation Theory at low $Q^2$. Specifically, the behavior of the longitudinally-transverse spin polarizability ($\delta_{LT}$), as $\chi$PT calculations of this quantity deviate significantly from the measured neutron data. This talk will provide an overview of the JLab effort to measure $g_2$, with a focus on the more recent $g_2^p$ experiment; the current status of the analysis will be discussed along with preliminary results.
Primary author
Ms
Melissa Cummings
(College of William and Mary)
