O Presente trabalho propôs o estudo de reações fotonucleares e fotoprodução de pions em energias intermediárias e altas usando o modelo de cascata intranuclear MCMC para a fase rápida de reação e outro algoritmo de Monte Carlo para descrever o processo de decaimento do núcleo composto via evaporação de partículas e fissão nuclear. Dentre as principais inovações implementadas no MCMC, podemos citar: i) a inclusão detalhada do canal do quase-dêuteron no intervalo 20 < Eg < 140 MeV; ii) o desenvolvimento inédito e rigoroso de um mecanismo de bloqueio de Pauli não-estocástico, avaliando as excitações partícula-buraco em tempo real na fase de pré-equilíbrio; iii) a incorporação de um critério energético de parada para a cascata; iv) a utilização de distribuições de momento apropriadas para núcleos leves (Modelo de Camadas), v) a inclusão de canais de fotoprodução do pi^0 na região da Delta (1232) e em altas energias 4,0 < Eg < 6,0 GeV e ângulos frontais e, vi) a metodologia inédita para avaliar as interações de estado final de mésons e ressonâncias com o núcleo via cenário de espalhamentos múltiplos. O mecanismo do quase-dêuteron reproduz satisfatoriamente as seções de choque totais de fotoabsorção e as multiplicidades médias de nêutrons emitidos em núcleos pesados, interpretando qualitativamente as seções de choque para os canais de eletrodesintegração (e,xnyp) do 232Th. A contribuição de um possível processo de fotoabsorção por poucos corpos no 232Th é analisada via canal de fotofissão. Os cálculos das distribuições angulares na fotoprodução incoerente do pi^0 para o 12C e 208Pb em energias intermediárias estão em ótimo acordo com dados experimentais obtidos no Microtron de Mainz (MAMI). Os resultados para as seções de choque das distribuições angulares do pi^0 em altas energias servirão como referência na Colaboração PrimEx no Jefferson Laboratory Facility, propiciando a separação da parte incoerente das demais contribuições nucleares e eletromagnéticas e auxiliando na determinação precisa da meia-vida do decaimento radiativo pi^0 --> GG. / Photonuclear reactions and pion photoproduction at intermediate and high energies have been studied using the multicollisional intranuclear cascade model (MCMC) for the rapid reaction step and another Monte Carlo algorithm to describe the de-excitation of the compound nucleus via particle evaporation and nuclear fission. The new features of the MCMC are: i) the detailed inclusion of the quasideuteron channel in the range 20 < Eg < 140 MeV; ii) the development of a rigorous and completely new non stochastic Pauli blocking mechanism, taking into account particle - hole excitations during the pre-equilibrium stage; iii) the incorporation of an energetic criterion to stop de cascade; iv) the employment of an appropriate momentum distribution for light nuclei (Shell Model); v) the inclusion of pi^0 photoproduction channels in the Delta (1232) region and at high energies 4.0 < Eg < 6.0 GeV and forward angles, and vi) the new methodology to evaluate the pion-nucleus and Delta - nucleus Final State Interactions via a multiple scattering scenario. The quasideuteron mechanism reproduces the total photoabsorption cross sections and neutron multiplicities in heavy nuclei, giving a qualitative interpretation for the cross sections of the electrodisintegration channels (e,xnyp) from 232Th. The contribution of a possible few body photoabsorption process in 232Th is evaluated via fission channel. The calculations of the pi^0 angular distributions for incoherent production at intermediate energies for 12C and 208Pb are in excellent agreement with the experimental data of the Mainz Microtron (MAMI). The results of the pi^0 angular distributions at higher energies will be reference for the PrimEx Collaboration at the Jefferson Laboratory Facility, propitiating the separation of the incoherent part from the remaining nuclear and electromagnetic contributions and helping to perform a high precision measurement of the pi^0 to gamma-gamma radiactive decay width.

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O Presente trabalho propôs o estudo de reações fotonucleares e fotoprodução de pions em energias intermediárias e altas usando o modelo de cascata intranuclear MCMC para a fase rápida de reação e outro algoritmo de Monte Carlo para descrever o processo de decaimento do núcleo composto via evaporação de partículas e fissão nuclear. Dentre as principais inovações implementadas no MCMC, podemos citar: i) a inclusão detalhada do canal do quase-dêuteron no intervalo 20 < Eg < 140 MeV; ii) o desenvolvimento inédito e rigoroso de um mecanismo de bloqueio de Pauli não-estocástico, avaliando as excitações partícula-buraco em tempo real na fase de pré-equilíbrio; iii) a incorporação de um critério energético de parada para a cascata; iv) a utilização de distribuições de momento apropriadas para núcleos leves (Modelo de Camadas), v) a inclusão de canais de fotoprodução do pi^0 na região da Delta (1232) e em altas energias 4,0 < Eg < 6,0 GeV e ângulos frontais e, vi) a metodologia inédita para avaliar as interações de estado final de mésons e ressonâncias com o núcleo via cenário de espalhamentos múltiplos. O mecanismo do quase-dêuteron reproduz satisfatoriamente as seções de choque totais de fotoabsorção e as multiplicidades médias de nêutrons emitidos em núcleos pesados, interpretando qualitativamente as seções de choque para os canais de eletrodesintegração (e,xnyp) do 232Th. A contribuição de um possível processo de fotoabsorção por poucos corpos no 232Th é analisada via canal de fotofissão. Os cálculos das distribuições angulares na fotoprodução incoerente do pi^0 para o 12C e 208Pb em energias intermediárias estão em ótimo acordo com dados experimentais obtidos no Microtron de Mainz (MAMI). Os resultados para as seções de choque das distribuições angulares do pi^0 em altas energias servirão como referência na Colaboração PrimEx no Jefferson Laboratory Facility, propiciando a separação da parte incoerente das demais contribuições nucleares e eletromagnéticas e auxiliando na determinação precisa da meia-vida do decaimento radiativo pi^0 --> GG. / Photonuclear reactions and pion photoproduction at intermediate and high energies have been studied using the multicollisional intranuclear cascade model (MCMC) for the rapid reaction step and another Monte Carlo algorithm to describe the de-excitation of the compound nucleus via particle evaporation and nuclear fission. The new features of the MCMC are: i) the detailed inclusion of the quasideuteron channel in the range 20 < Eg < 140 MeV; ii) the development of a rigorous and completely new non stochastic Pauli blocking mechanism, taking into account particle - hole excitations during the pre-equilibrium stage; iii) the incorporation of an energetic criterion to stop de cascade; iv) the employment of an appropriate momentum distribution for light nuclei (Shell Model); v) the inclusion of pi^0 photoproduction channels in the Delta (1232) region and at high energies 4.0 < Eg < 6.0 GeV and forward angles, and vi) the new methodology to evaluate the pion-nucleus and Delta - nucleus Final State Interactions via a multiple scattering scenario. The quasideuteron mechanism reproduces the total photoabsorption cross sections and neutron multiplicities in heavy nuclei, giving a qualitative interpretation for the cross sections of the electrodisintegration channels (e,xnyp) from 232Th. The contribution of a possible few body photoabsorption process in 232Th is evaluated via fission channel. The calculations of the pi^0 angular distributions for incoherent production at intermediate energies for 12C and 208Pb are in excellent agreement with the experimental data of the Mainz Microtron (MAMI). The results of the pi^0 angular distributions at higher energies will be reference for the PrimEx Collaboration at the Jefferson Laboratory Facility, propitiating the separation of the incoherent part from the remaining nuclear and electromagnetic contributions and helping to perform a high precision measurement of the pi^0 to gamma-gamma radiactive decay width.

One-nucleon halo nuclei are exotic nuclei which can be seen as a core around which orbits a loosely-bound valence nucleon. They are usually studied through reactions such as elastic scattering and breakup. The ratio method has been developed as a tool to study one-neutron halo nuclei at high energies. It consists of the ratio of angular cross sections, breakup and elastic scattering, which removes most of the sensitivity to the reaction mechanism and to the reaction model. In the simple recoil excitation and breakup (REB) model, the ratio simplifies to a form factor dependent solely on the wave function of the projectile. By measuring this observable and comparing it to the REB form factor, i.e. in the ratio method, more detailed information on the structure of the halo could be obtained. For neutron-halo nuclei at high energy, the ratio observable obtained from accurate CDCC and DEA theoretical calculations follows its REB prediction. I study the extension of this method to lower energies of the reaction which could make the measurement appropriate to facilities such as SPIRAL2 (GANIL, Caen, France) and ReA12 at FRIB (Michigan State University) and to proton halos. This is done by comparing the REB form factor to dynamical calculations of the ratio. The reactions investigated are the reaction of 11Be, the archetypical one-neutron halo nucleus, on 12C, 40Ca and 208Pb targets at 20 MeV/nucleon and of 8B, the archetypical one-proton halo nucleus, on 12C, 58Ni and 208Pb targets at44 MeV/nucleon.For these reactions, the adiabatic assumption is no longer valid due to the effect of the Coulomb interaction. This effect is mainly visible at forward angle for 11Be and is aggravated for 8B by the fact that the halo is charged. The ratio works less well than for neutron-halos at intermediate and high energies. Nevertheless, the ratio is shown to be very sensitive to the orbital angular momentum l0 in which the halo is bound and its binding energy E0, i.e. the single-particle structure of the projectile. Variations of l0 and E0 induce visible changes in shape and in magnitude (up to several orders) of the ratio. Also, the agreement of the ratio with its REB prediction is best when the projectile is loosely-bound and for low l0, i.e. for s and p waves. The validity of the method is not affected by the use of energy ranges—or bins— in the projectile continuum. These tend to increase the cross section without changing the agreement of the ratio with its REB prediction. The applicability of the method is finally explored at high energy for proton-rich nuclei 17F, 25Al and 27P. I show that the ratio method works the latter since this nucleus is bound by a mere 0.870 MeV in the s-wave. For the other nuclei, although the agreement of the ratio with its REB prediction is less good than for neutron-halo nuclei at high energy, it still provides estimates of nuclear-structure features, such as l0 and E0 and could be applied in what can be called an approximate application of the ratio method. Heavy nuclei exhibit a neutron skin, i.e. a thin layer around the nucleus where only neutrons are found. The thickness of the skin is highly correlated with the slope of the symmetry energy. The process of coherent neutral-pion photoproduction is used to extract the nuclear density and hence the neutron-skin thickness of heavy nuclei. In order to analyse recent data on the photoproduction on 12C, 40,48Ca, 116, 120, 124Sn and 208Pb, I build a reaction code. My model uses the formalism of Kerman, McManus and Thaler (KMT) which allows to build the photoproduction matrix on a nucleus from the ones describing the elementary process on a single nucleon. Within the impulse approximation, the photoproduction is seen as the coherent sum of the photoproduction on each of the nucleons. In the plane wave impulse approximation (PWIA), no rescattering of the pion is considered after its production and the cross section is directly proportional to the Fourier transform of the density. Such process is taken into account at the distorted wave impulse approximation (DWIA) by considering a potential simulating the pion-nucleus interaction and built from the KMT formalism.The agreement of my model with the data is good, especially for 208Pb. The distortion has a significant impact on the photoproduction process. The sensitivity of the process to the density of the target is analysed by performing the calculations with several different densities calculated in different structure models. The distortion has the effect of deteriorating this sensitivity. In the particular case of a 208Pb target, the impact of variations of the neutron-skin thickness of around 0.1 fm on the photoproduction cross section is ten times smaller than the size of the error bars on the experimental data. These results, although less dramatic, hold for the tin targets, for which preliminary data exists. In the light of these results, the coherent neutral-pion photoproduction process does not seem to be suited in the study of the neutron-skin thickness. This conclusion goes in contrast to the results of recent measurements on 208Pb, for which the method was shown to be sensitive to fine details of the density. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

The parity violating weak decay of hyperons offers a valuable means of measuring their polarization, providing insight into the production of strange quarks and the matter they compose. Jefferson Lab’s CLAS collaboration has utilized this property of hyperons, publishing the most precise polarization measurements for the Λ and Σ in both photoproduction and electroproduction to date. In contrast, cascades, which contain two strange quarks, can only be produced through indirect processes and as a result, exhibit low cross sections thus remaining experimentally elusive. At present, there are two aspects in cascade physics where progress has been minimal: characterizing their production mechanism, which lacks theoretical and experimental developments, and observation of the numerous excited cascade resonances that are required to exist by flavor SU(3)F symmetry. However, CLAS data were collected in 2008 with a luminosity of 68 pb−1 using a circularly polarized photon beam with energies up to 5.45 GeV, incident on a liquid hydrogen target. This dataset is, at present, the world’s largest for meson photoproduction in its energy range and provides a unique opportunity to study cascade physics with polarization measurements. The current analysis explores hyperon production through the γp → K+K+Ξ− reaction by providing the first ever determination of spin observables P, Cx and Cz for the cascade. Three of our primary goals are to test the only cascade photoproduction model in existence, examine the underlying processes that give rise to hyperon polarization, and to stimulate future theoretical developments while providing constraints for their parameters. Our research is part of a broader program to understand the production of strange quarks and hadrons with strangeness. The remainder of this document discusses the motivation behind such research, the method of data collection, details of their analysis, and the significance of our results.