Spin Transfer to $\Lambda$ Hyperons and Dihadron Spin Asymmetry Projections in Semi-Inclusive Deep Inelastic Scattering with CLAS12

Abstract

Spin is a fundamental property of matter, yet its dynamic origin in hadronic matter is not fully understood. The interaction of quarks is governed by the strong force, one of the four fundamental forces of nature. This is described via the well-established theory of Quantum Chromodynamics (QCD). Due to its eponymous strong coupling, QCD is not perturbatively calculable. Fortuitously however, the coupling falls off at high energies, allowing internal hadron dynamics to be accessed experimentally. $\Lambda$ hyperons have the useful property that their polarization is imprinted in the angular distribution of their decay products via the channel $\Lambda \rightarrow p\pi^-$. This allows one to easily study the spin structure of the $\Lambda$, whereas a proton does not have this unique feature. Similarly, dihadron pairs offer an additional degree of freedom in their relative orbital angular momentum that is both sensitive to hadronic spin structure and experimentally observable. Semi-Inclusive Deep Inelastic Scattering (SIDIS) events, where a high energy lepton interacts with a nucleon target and one or more hadrons and the scattered lepton are detected in the final state, provide an ideal laboratory to probe hadron spin structure. In particular, the probability of spin transfer from the incoming polarized lepton to a produced $\Lambda$ may be related to the longitudinal light quark spin structure of the $\Lambda$. Currently, limited experimental data cannot discriminate between different models of $\Lambda$ spin structure. This thesis reports on the measurement of the longitudinal spin transfer $D^{\Lambda}_{LL'}$ to the $\Lambda$ using data taken by the CLAS12 spectrometer at Jefferson Lab with a $10.6$~GeV polarized electron beam and an unpolarized hydrogen target. This measurement is the most precise to date, and, in comparison with theory predictions, it offers valuable insight into the relative dominance of $\Lambda$ production off the struck quark and the target remnant. Additionally, the transverse $\Lambda$ polarization along two orthogonal transverse axes is reported. The polarization along one axis, $D_{LL'}^{x\Lambda}$, is sensitive to the higher-order distribution $e(x)$ within the proton and has received little experimental attention.Another focus of this thesis is the dihadron $\pi^{+}\pi^{-}$ channel. The transverse target spin asymmetry in this channel is sensitive to the transversity distribution $h_1$, describing the distribution of transversely polarized quarks within the transversely target nucleon. This thesis reports on the projected statistical uncertainties attainable with a proposed transversely polarized NH$_3$ target configuration at CLAS12. These projections offer promising improvements to phenomenological estimates in important kinematic regions and the potential to resolve conflicting estimates.