Measurement of the Generalized Polarizabilities of the Proton by Virtual Compton Scattering at MAMI and Q 2 PDF Download
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Author: Loup Correa
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Languages : en
Pages : 0
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Author: Loup Correa
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Languages : en
Pages : 0
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Author: Jure Beričič
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Languages : en
Pages : 144
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Author: Ruonan Li
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Languages : en
Pages : 0
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Understanding how the visible matter in the universe arises from its elementary quark andgluon constituents is a central question for science. The visible world is founded on the proton, the only composite building block of matter that is stable in nature. Consequently, understanding the formation of matter relies on explaining the dynamics and the properties of the proton's bound state. A fundamental property of the proton involves the system's response to an external electromagnetic (EM) field. It is characterized by the EM polarizabilities that describe how easily the charge and magnetization distributions inside the system are distorted by the EM field. When the polarizabilities are generalized to finite momentum transfer, their Fourier transform can map out the spatial distribution of the polarization densities in a proton subject to an EM field. This thesis focuses on the measurement of the proton generalized polarizabilities (GPs) at low four-momentum transfer in experimental Hall C at the Thomas Jefferson National Accelerator Facility (TJNAF). Among the six independent GPs, we will focus on the electric (aE1) and the magnetic (bM1) scalar GPs. The GPs can be accessed through measurements of the Virtual Compton Scattering reaction, by replacing the incoming real photon of the Compton scattering process with a space-like virtual photon. The outgoing real photon provides the EM perturbation to the system. In addition, the Dispersion Relation Formalism is used for generalized polarizability extraction. In this work, the two scalar GPs measured with unprecedented precision, and the measurements help explore a momentum transfer region where an anomalous enhancement of the electric GP that contradicts the predictions of nuclear theory has been observed.
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Languages : en
Pages : 1
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We report a Virtual Compton Scattering study of the proton at low CM energies. We have determined the structure functions P{sub LL} - P{sub TT}/[epsilon] and P{sub LT}, and the electric and magnetic Generalized Polarizabilities (GPs) [alpha]{sub E}(Q2) and [beta]{sub M}(Q2) at momentum transfer Q2 = 0.92 and 1.76 GeV2. The electric GP shows a strong fall-off with Q2, and its global behavior does not follow a simple dipole form. The magnetic GP shows a rise and then a fall-off; this can be interpreted as the dominance of a long-distance diamagnetic pion cloud at low Q2, compensated at higher Q2 by a paramagnetic contribution from [pi]N intermediate states.
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Languages : en
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Virtual Compton Scattering is studied at the Thomas Jefferson National Accelerator Facility in the energy domain below pion threshold and in the Delta(1232) resonance region. The data analysis is based on the Dispersion Relation (DR) approach. The electric and magnetic Generalized Polarizabilities (GPs) of the proton and the structure functions Pll-Ptt/epsilon and Plt are determined at four-momentum transfer squared Q2=0.92 and 1.76 GeV2. The DR analysis is consistent with the low-energy expansion analysis. The world data set indicates that neither the electric nor magnetic GP follows a simple dipole form.
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Languages : en
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Virtual Compton Scattering is studied at the Thomas Jefferson National Accelerator Facility at low Center-of-Mass energies, below pion threshold. Following the Low Energy Theorem for the ep --> ep gamma process, we obtain values for the two structure functions Pll-Ptt/epsilon and Plt at four-momentum transfer squared Q2=0.92 and 1.76 GeV2.
Author: Christophe Jutier
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Category : Electrons, Compton
Languages : en
Pages : 458
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Author: Helene Fonvieille
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Languages : en
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Virtual Compton Scattering (VCS) {gamma}*p {yields} {gamma}p at low CM energy gives access to the Generalized Polarizabilities of the nucleon. These observables generalize the concept of electromagnetic polarizabilities to the case of a virtual photon. Dedicated VCS experiments have been performed at MAMI, Jefferson Lab and MIT-Bates. The experimental status is reviewed, including analysis methods and physics results. The measurement of absolute (ep {yields} ep{gamma}) cross sections allows the extraction of the two unpolarized VCS structure functions P{sub LL}-P{sub TT}/{epsilon} and P{sub LT}, which are combinations of the Generalized Polarizabilities of the proton. Future prospects in the field of VCS at low energy are also presented.
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Languages : en
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Virtual Compton Scattering (VCS) is an important reaction for understanding nucleon structure at low energies. By studying this process, the generalized polarizabilities of the nucleon can be measured. These observables are a generalization of the already known polarizabilities and will permit theoretical models to be challenged on a new level. More specifically, there exist six generalized polarizabilities and in order to disentangle them all, a double polarization experiment must be performed. Within this work, the VCS reaction p(e, e p)gamma was measured at MAMI using the A1 Collaboration three spectrometer setup with Q2=0.33 (GeV/c)2. Using the highly polarized MAMI beam and a recoil proton polarimeter, it was possible to measure both the VCS cross section and the double polarization observables. Already in 2000, the unpolarized VCS cross section was measured at MAMI. In this new experiment, we could confirm the old data and furthermore the double polarization observables were measured for the first time. The data were taken in five periods between 2005 and 2006. In this work, the data were analyzed to extract the cross section and the proton polarization. For the analysis, a maximum likelihood algorithm was developed together with the full simulation of all the analysis steps. The experiment is limited by the low statistics due mainly to the focal plane proton polarimeter efficiency. To overcome this problem, a new determination and parameterization of the carbon analyzing power was performed. The main result of the experiment is the extraction of a new combination of the generalized polarizabilities using the double polarization observables.
Author: Bruce Everett MacGibbon
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Languages : en
Pages : 296
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The Compton scattering cross section on the proton was measured at laboratory angles of 90$spcirc$ and 135$spcirc$, using 70-100 MeV tagged photons, and simultaneously using 100-150 MeV untagged photons, in order to determine the electric and magnetic polarizabilities. The 135$spcirc$ cross section is sensitive to the difference of the electric and magnetic polarizabilities, $bar alpha - barbeta$, while the 90$spcirc$ cross section is sensitive to the electric polarizability $bar alpha$. Using the model independent sum rule constraint for $bar alpha$ + $bar beta$ and a model dependent dispersion relation calculation, the values for the polarizabilities extracted from the cross sections are:$$bar alpha = (12.5 pm 0.6 pm 0.8 pm 0.6) times 10sp{-4} fmsp3cr bar beta = (1.8 mp 0.6 mp 0.8 mp 0.6) times 10sp{-4} fmsp3$$where the errors shown are statistical, systematic, and model, respectively.
