Session X15: Spin Structure of the Nucleon II

Distribution of Angular Momentum in the Transverse Plane

Fourier transforms of GPDs describe the distribution of partons in the transverse plane. The 2nd moment of GPDs has been identified by X.Ji with the angular momentum (orbital plus spin) carried by the quarks - a fundamental result that is being widely utilized in the spin decomposition of a longitudinally polarized nucleon. However, I will demonstrate that, despite the above results, the Fourier transform of the 2nd moment of GPDs does not describe the distribution of angular momentum in the transverse plane for a longitudinally polarized target.   

Single Target-Spin Asymmetries in Inclusive Hadron Electroproduction Reaction Using a Transversely Polarized $^3$He Target

Single-spin asymmetries (SSAs) are an invaluable tool to study the transverse-spin and transverse-momentum structure of the hadron. First measurements of transverse SSA were performed in $pp^{\uparrow}\rightarrow hX$ reactions in mid-70s followed by much more precise measurements from early 90s onwards. More recently transverse SSA were explored using semi-inclusive deep inelastic scattering (SIDIS) reactions using transversely polarized targets. A different type of reaction which is sensitive to transverse spin structure of nucleon, and experimentally least explored, is the inclusive hadron electroproduction on a transversely polarized nucleon ($eN^{\uparrow}\rightarrow hX$). We report the first measurement of such SSAs, measured using a transversely polarized $^3$He target and the 5.9 GeV electron beam at Jefferson Lab. The Hall-A high-resolution spectrometer was used to detect hadrons ($\pi^{\pm},K^{\pm},p$) which had an average hadron transverse momentum (p$_T$) of 0.64 GeV/c. In this talk we will present the preliminary results of this experiment.   

Single and Double Spin Asymmetries for Semi-Inclusive Deep Inelastic Scattering on Proton and Deuteron

Transverse momentum dependent (TMD) parton distribution functions encode information on the transverse motion of quarks and gluons inside the nucleon, and may help us understand their orbital angular momentum. The TMDs can be accessed from the target and double spin asymmetries of semi-inclusive deep inelastic scattering (SIDIS) reactions, where the asymmetries, $A_{UL}$ and $A_{LL}$ are convolutions of the fragmentation functions and the TMDs. The EG1-DVCS experiment with CLAS at Jefferson Lab measured semi-inclusive pion production on longitudinally polarized proton and deuteron targets with polarized electrons of 6 GeV. We will show preliminary results on target single spin asymmetries and target-beam double spin asymmetries for these reactions.   

Leading- and higher-twist double-spin asymmetries in proton-proton scattering

Extensive work has been done over the last four decades on the leading-twist (twist-2) double-longitudinal spin asymmetry $A_{LL}$. The main goal of this research has been to understand how the spin of the nucleon can be explained in terms of the partons that compose it. Similarly, the twist-3 longitudinal-transverse double-spin asymmetry $A_{LT}$, like the one studied in inclusive DIS and Drell-Yan, also provides important information on the spin structure of hadrons. This talk will focus on the analysis of $A_{LT}$ for photon, hadron, and jet production from nucleon-nucleon collisions, which gives one access to a complete set of collinear twist-3 functions for a transversely polarized nucleon. Moreover, we will discuss how this observable could give insight into both the $A_{LL}$ and transverse single-spin asymmetry $A_{UT}$ domains.   

Spectrometer Optics Calibration for Jefferson Lab $g_2^p$ Experiment

The proton spin-dependent structure function $g_2^p$ in the resonance region has been measured in a recent experiment at Jefferson Lab's Hall A. The goal of this experiment is to determine the $g_2^p$ structure function in the $Q^2$ region of 0.02-0.20 GeV$^{2}$ by using a transversely polarized NH$_3$ target. The Hall A High Resolution Spectrometers (HRS) were used to provide an inclusive measurement of scattered electrons at $5.69^{\circ}$. A pair of septum magnets were used to bend the $5.69^{\circ}$ scatted electrons into the $12.5^{\circ}$ spectrometers. A full optics calibration of the spectrometer is required to extract the kinematic quantities at the interaction vertex. However, the calibration procedure is challenging due to the complications from the strong transverse target field (2.5 T and 5.0 T) and the varied field configurations of the septum magnets. The details of the optics calibration and the progress will be described in this talk.   

A Measurement of g2p and the Longitudinal-Transverse Spin Polarizability

The JLab Hall A E08-027 (g2p) experiment aims at measuring the proton structure function g$_2$ in the low Q$^2$ region (0.02$<$Q$^2$$<$0.2 GeV$^2$), which has never been explored. The measured g2p data also allow us to extract the longitudinal-transverse spin polarizability of the proton, which will provide a benchmark test of $\chi$PT calculations. The results from this experiment can be used to test the Burkhardt-Cottingham sum rule. As a fundamental property of the proton, the measured g$_2^p$ will provide crucial inputs for precision studies involving the proton, such as the hydrogen hyperfine splitting and the proton charge radius measurements. The proton charge radius has been shown to be significantly different between electron-proton elastic scattering and hydrogen Lamb shift measurements, and that determined from the Lamb shift of muonic hydrogen. The g2p experiment took data in the spring of 2012. In this talk, I will present the physics motivation, followed by an overall introduction of the experiment and the status of the data analysis.   

Polarized Proton Target for the g2p Experiment

The $g_2^p$ experiment was completed in Jefferson Lab's Hall A in the spring of 2012. These data will provide the first measurement of the proton's $g_2$ structure function in the region $0.02~<~Q^{2}~<~0.2~GeV^{2}$. A large scale installation was required for the $g_2^p$ experiment, including a polarized ammonia (NH$_3$) target never before used in Hall A. Based on the principle of Dynamic Nuclear Polarization (DNP), microwave pumping is used to polarize irradiated ammonia in a high magnetic field at a temperature of 1K. In order to achieve the proposed kinematic range, the target magnet field was used at both 2.5\,T and 5\,T. Few studies of DNP have been done previously with a target field of 2.5\,T, making these data of particular interest. In addition, an experiment to measure the proton form factor ratio ($G_E$/$G_M$) ran concurrently with the $g_2^p$ experiment and required a different target polarization direction. Therefore, a rotatable target chamber was needed for transition between configurations. This talk will introduce the concept of DNP and describe the unique challenges faced in the design of the $g_2^p$ target. Performance indicators such as polarization decay rate and anneal time will be presented along with preliminary polarimetry results.