Fundamental Studies of Hadronic Structure

Fundamental Studies of Hadronic Structure -- Wolfgang Korsch, University of Kentucky The strong interaction at the most fundamental level is governed by quarks and gluons and quantum chromodynamics is the most successful theory to describe these interactions and its consequences for nucleons and nuclei. The quarks can be considered asymptotically free at high energies, i.e. the interaction strength decreases at smaller and smaller distance scales. However, at lower energies interactions between the partons cannot be neglected anymore and its consequences for the behavior strongly interacting systems is di?cult to predict quantitatively. This proposal focuses on the e?ect of the neutron spin on the possible alignment of intrinsic color ?elds due to the quark-gluon correlations. The results of this research will serve as a very important test of lattice gauge calculations. The main goals of the projects proposed in this document are the advancement of our basic knowledge of the neutron spin structure and the development of new techniques to reduce systematic uncertainties associated with polarized helium-3 targets. The PI has been active in the ?eld of polarization physics for several decades and most recently he participated in two experiments in Hall C at Je?erson Lab which measured inclusive scattering asymmetries in previously poorly studied regions. The ?rst experiment (E12-06-110) measured the virtual photon asymmetries, A1n(x) at large values of x. The idea was to take precision An1 data for 0.33 < x < 0.7 so that theoretical models describing the x-dependence of the nucleon spin structure can be severely constrained. Especially the shape of this asymmetry as x → 1 is currently not well known. The second experiment (E12-06-121) studied the e?ect of quark-gluon correlations in the spin structure function g2n(x, Q2). The PI is a co-spokesperson on this experiment. The objective of this research e?ort is a precision determination of d2(Q2) which is the second moment in x of the twist-3 part of the spin structure function g2n(x, Q2). Data were taken with the HMS and SHMS spectrometers at Q2 values of 3.0, 4.3, and 5.6 GeV2. Both experiments were completed in 2020. The PI and his group have taken on leading roles in the data analysis for both ex- periments. A dedicated graduate student from UK, Ms. Murchhana Roy, is working on the determination of unpolarized cross sections which will be needed for the extraction of g2n(x, Q2) (and also g1(x, Q2)). This work will serve as the basis for Ms. Roy’s PhD thesis. Further e?orts of the PI’s group include the construction of a new magnet system which potentially can be used to provide the spin holding ?eld for the SoLID spectrometer. The design will utilize a ?eld-generating coil which is enclosed in a soft iron housing. Such a system will produce a very uniform ?eld and reduce the impact of external ?eld gradients on the polarization drastically. In addition, a new development e?ort is proposed to measure the helium-3 density and polarization in Je?erson Lab-style target cells. For this purpose Doppler-free absorption spectroscopy and Faraday rotation experiments will be performed on the existing small amounts of rubidium and/or potassium atoms inside the target cell. The idea is to disentangle the pressure and magnetic ?eld dependence of the D1 and D2 transitions and use this information to determine the target cell helium-3 density in a model independent way and also monitor the stability of the ambient magnetic ?eld.