In experiment E154 at the Stanford Linear Accelerator Center the spin dependent structure function $g\sbsp{1}{n}(x,Q\sp2)$ of the neutron was measured by scattering longitudinally polarized 48.3 GeV electrons off a longitudinally polarized $\sp3$He target. The high beam energy allowed us to extend the kinematic coverage compared to the previous SLAC experiments to $0.014\leq x\leq0.7$ with an average $Q\sp2$ of 5 GeV$\sp2.$ We report the integral of the spin dependent structure function in the measured range to be $\int\sbsp{0.014}{0.7}dx\ g\sbsp{1}{n}(x,5\ {\rm GeV}\sp2)={-}0.036\pm0.004({\rm stat}.)\pm0.005({\rm syst}.).$ We observe relatively large values of $g\sbsp{1}{n}$ at low x that call into question the reliability of data extrapolation to $x\to0.$ Such divergent behavior disagrees with predictions of the conventional Regge theory, but is qualitatively explained by perturbative QCD. We perform a Next-to-Leading Order perturbative QCD analysis of the world data on the nucleon spin dependent structure functions $g\sbsp{1}{p}$ and $g\sbsp{1}{n}$ paying careful attention to the experimental and theoretical uncertainties. Using the parameterizations of the helicity-dependent parton distributions obtained in the analysis, we evolve the data to $Q\sp2$ = 5 GeV$\sp2,$ determine the first moments of the polarized structure functions of the proton and neutron, and find agreement with the Bjorken sum rule.

Experiment E690 at Fermilab recorded 5.5 billion $p + p \to p + X$ events using an 800 GeV/c proton beam and a liquid hydrogen target, during the 1991 fixed target run. We use a 0.5 billion subset of this sample, to study the reaction $p + p \to p\sb{s}(\pi\sp+\pi\sp-)p\sb{f}$ for dipion invariant mass between threshold and 2.3 GeV/c$\sp2.$ We perform a partial wave analysis for dipion invariant mass between threshold and 1.5 GeV/c$\sp2.$ The assumption of S-wave dominance near threshold is sufficient to determine a single, continuous solution throughout the considered mass spectrum. Precision measurement of the production amplitude aids the mapping the low lying meson spectrum. Other possible studies using this data sample and analysis technique are: (1) the extension of the amplitude analysis above the 1.5 GeV/c$\sp2$ mass region using a the full event sample, and (2) the study of the produced amplitudes as a function of the relative angle between the two proton planes.

Results are presented on a study of resonant structure in the $\omega\pi\sp0$ and $\omega\eta$ final states produced in $\pi\sp-p$ interactions at 18 GeV/c where $\omega\to \pi\sp+\pi\sp-\pi\sp0,\ \pi\sp0\to 2\gamma,$ and $\eta\to 2\gamma.$ Final states such as $\omega\pi\sp0$ and $\omega\eta,$ which contain a vector meson are allowed in the extended flux-tube model for the decay of hybrid mesons. For the $\omega\eta$ final state, the data exhibit the well-known $b\sb1$(1235) and $\rho\sb3$(1690) resonances. A one-pion exchange mechanism is clearly seen for the $\rho\sb3$(1690). The result of a partial wave analysis for the $\omega\eta$ final state finds a new $1\sp{+-}$ meson state $h\sb1$(1540) with a mass $M = 1542\pm 13$ MeV and a width $\Gamma = 285\pm 30$ MeV, which shows phase motion relative to the $\omega$(1600). The decay mode of $\omega(1600)\to \omega\eta$ has not been previously reported.

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The longitudinal relaxation time T$\sb1$ is calculated for a dilute Fermi gas. We include the possibility of arbitrary degrees of polarization P, and uniform external magnetic fields. At low temperatures T$\sb1$ is proportional to T$\sp{-2}$ for the unpolarized gas in agreement with earlier calculations. For a polarized system in zero applied field the separation between up and down Fermi spheres provides an open channel in phase space for spin flipping processes. As temperature is reduced to zero T$\sb1$ levels off and approaches a constant value which depends sensitively on P, leading to much smaller values of T$\sb1$ than in the unpolarized case. For relaxation in a uniform external field H, Zeeman energy conservation during spin flipping interactions essentially shifts the location of the T$\sp{-2}$ behavior from zero polarization to the polarization P(H) which would be produced in equilibrium with the external field H. When the initial polarization of the system is different from P(H), T$\sb1$ again approaches a constant value which is equally sensitive to the initial polarization as temperature is reduced to zero. We apply our calculations to the case of dilute $\sp3$He-HeII solutions. We also consider the calculation of density dependent corrections to T$\sb1$ for unpolarized, or weakly polarized $\sp3$He-HeII using the long wavelength limit of the Bardeen-Baym-Pines interaction as a form for the spin independent part of the $\sp3$He-$\sp3$He interaction. Owing to the positive sign of the antisymmetric Landau parameter $\rm F\sbsp{o}{a}$ in dilute $\sp3$He-HeII Fermi Liquid interactions act to screen the magnetic dipole interactions which cause relaxation. To lowest order in density we find that the relaxation rate 1/T$\sb1$ is reduced by factor $\rm (1 - 3F\sbsp{o}{a})$ which compares favorably with recent calculations of T$\sb1$ for pure $\sp3$He.

Two areas of nonleptonic weak interactions are examined: parity-violation in nucleon-nucleon interactions including $\Delta$ excitations, and strangeness-changing hypernuclear decay processes. In both cases we utilize SU(6)$\sb{w}$-based quark model pictures to describe the weak hadronic physics. In the first part of this dissertation we develop an effective parity-violating Hamiltonian for the weak interactions of $\Delta$'s with hadrons and mesons. We evaluate weak parity-violating $\Delta$-$\Delta$-meson and $\Delta$-nucleon-meson vertices for $\pi$, $\rho$, and $\omega$ mesons, thus extending previous work involving only nucleons and mesons. The second part of this dissertation presents the results of calculations for several observables involving the decay of hypernuclei. We examine the model dependence of these observables by considering several variations of the model and its parameters.

The current data for the nucleon-virtual photon longitudinal spin asymmetry A1 on the proton and neutron have shown that the ratio of the polarized-to-unpolarized down-quark parton distribution functions, Δ d/d, tends towards -1/2 at large x, in disagreement with the perturbative QCD prediction that Δ d/d approaches 1 but more in line with constituent quark models. As a part of experiment E06-014 in Hall A of Jefferson Lab, double-spin asymmetries were measured in the scattering of a longitudinally polarized electron beam of energies 4.74 and 5.89 GeV from a longitudinally and transversely polarized 3He target in the deep inelastic scattering and resonance region, allowing for the extraction of the neutron asymmetry A1n and the ratios Δ d/d and Δ u/u. We will discuss our analysis of the data and present results for A1 and g1/F1 on both 3He and the neutron, and the resulting quark ratios for the up and down quarks in the kinematic range of 0.2 2 2 for our deep inelastic scattering data. Invoking duality, we also extract A1n and g1n/F1n in the resonance region, characterized by 0.6 2 2. Our measurements are compared to the world data and various theoretical models and more recent predictions using the Dyson-Schwinger Equation approach. We also present analysis of the unpolarized cross section data, which contributes to the g1 spin structure function and eventually the a2 matrix element, an x2-weighted moment of g1. The extracted a2 data are compared to a Lattice QCD calculation. / Physics

In undergraduate physics courses, we are all taught that a proton and a neutron both have a quantity called 'spin' which has a simple value of 1/2 in units of ℏ. It was first naively understood that this value comes from the addition of the spins of the basic constituents, quarks and gluons. However, experiments revealed that it is not enough. The value of 1/2 has yet to be reconciled with the addition of not only the spins of the constituents but also their orbital motion. Clearly, the spin structure in terms of the basic constituents needs more investigation. This thesis aims to describe the polarization and calibration of a polarized ³He target, which can be used for probing the spin structure of the neutron in terms of its constituents, quarks and gluons. / Physics

Les projets de réacteurs du futur permettent l'utilisation de nouveaux combustibles et montrent leur capacité a incinérer des déchets nucléaires. Ils ont redonne de l'intérêt aux sections efficaces d'un grand nombre d'éléments dans une large gamme d'énergie. Les désaccords observes entre les bases de données requièrent de nouvelles mesures dans certains cas. Les mesures complètes des sections efficaces avec une résolution élevée réclament des temps importants de faisceau et d'analyse. Cependant il est possible de déterminer un bon profil de section efficace de capture dans la région d'énergie comprise entre 0,1eV et 30keV grâce a un spectromètre a temps de ralentissement au plomb associe a un générateur de neutrons pulse. Ces mesures ont été effectuées a l'ISN de Grenoble. Le flux de neutrons est mesure avec un détecteur de fissions a l'233U et un détecteur a gaz 3He puis compare a des simulations Monte Carlo (code MCNP) très précises du dispositif expérimental utilisant différentes bases de données du plomb. Nous avons montre par cette méthodologie que nous pouvons tester la section efficace de diffusion du plomb avec une précision de l'ordre de 1,5%. Les taux de capture, mesures avec un scintillateur CeF3 couple a un photomultiplicateur, sont compares a des simulations utilisant différentes bases de données pour les échantillons. Des matériaux de référence (Au, Ag, In, Ta) ont fait l'objet de mesures, des matériaux présents dans les c?urs de réacteurs ont pu être testes: 232Th, U, 99Tc, ainsi que des matériaux de structure: W, Mo, Ni, Mn. La précision sur la validation des bases de données est de 5%. Ces tests sont enrichis par des mesures sur plusieurs épaisseurs de cible. Les accords et les désaccords des bases de données avec l'expérience sont systématiquement décrits. Enfin, pour certains éléments, une table de correction de la section efficace de capture extraite de la base de données appropriée est proposée, nous en présentons les résultats.