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Register a new userVirtual Compton Scattering from the Proton and the Properties of Nucleon Excited States
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We calculate the $N^*$ contributions to the generalized polarizabilities of the proton in virtual Compton scattering. The following nucleon excitations are included: $N^*(1535)$, $N^*(1650)$, $N^*(1520)$, $N^*(1700)$, $Delta(1232)$, $Delta^*(1620)$ and $Delta^*(1700)$. The relationship between nucleon structure parameters, $N^*$ properties and the generalized polarizabilities of the proton is illustrated.
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We give an overview of low-energy Compton scattering (gamma^(*) p --> gamma p) with a real or virtual incoming photon. These processes allow the investigation of one of the fundamental properties of the nucleon, i.e. how its internal structure deforms under an applied static electromagnetic field. Our knowledge of nucleon polarisabilities and their generalization to non-zero four-momentum transfer will be reviewed, including the presently ongoing experiments and future perspectives.
We investigate the spin-independent part of the virtual Compton scattering (VCS) amplitude off the nucleon within the framework of chiral perturbation theory. We perform a consistent calculation to third order in external momenta according to Weinbergs power counting. With this calculation we can determine the second- and fourth-order structure-dependent coefficients of the general low-energy expansion of the spin-averaged VCS amplitude based on gauge invariance, crossing symmetry and the discrete symmetries. We discuss the kinematical regime to which our calculation can be applied and compare our expansion with the multipole expansion by Guichon, Liu and Thomas. We establish the connection of our calculation with the generalized polarizabilities of the nucleon where it is possible.
Virtual Compton scattering off the nucleon has been studied in the one-loop approximation of the linear sigma model. The three generalized polarizabilities of the nucleon have been calculated and compared with the existing theoretical predictions. In particular, we find that only two of the three scalar polarizabilities are independent observables.
This review gives an update on virtual Compton scattering (VCS) off the nucleon, $gamma^* N to N gamma$, in the low-energy regime. We recall the theoretical formalism related to the generalized polarizabilities (GPs) and model predictions for these observables. We present the GP extraction methods that are used in the experiments: the approach based on the low-energy theorem for VCS and the formalism of Dispersion Relations. We then review the experimental results, with a focus on the progress brought by recent experimental data on proton generalized polarizabilities, and we conclude by some perspectives in the field of VCS at low energy.
We report on new measurements of the electric Generalized Polarizability (GP) of the proton $alpha_E$ in a kinematic region where a puzzling dependence on momentum transfer has been observed, and we have found that $alpha_E = (5.3 pm 0.6_{stat} pm 1.3_{sys})~10^{-4} fm^3$ at $Q^2=0.20~(GeV/c)^2$. The new measurements, when considered along with the rest of the world data, suggest that $alpha_E$ can be described by either a local plateau or by an enhancement in the region $Q^2=0.20~(GeV/c)^2$ to $0.33~(GeV/c)^2$. The experiment also provides the first measurement of the Coulomb quadrupole amplitude in the $N rightarrow Delta$ transition through the exploration of the $p(e,ep)gamma$ reaction. The new measurement gives $CMR = (-4.4 pm 0.8_{stat} pm 0.6_{sys})~%$ at $Q^2=0.20~(GeV/c)^2$ and is consistent with the results from the pion electroproduction world data. It has been obtained using a completely different extraction method, and therefore represents a strong validation test of the world data model uncertainties.
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