Search

We consider the model of the self-gravity driven spontaneous wavefunction reduction proposed by L. Diosi, R. Penrose et al. and based on a self-consistent system of Schrödinger and Poisson equations. An analogous system of coupled Dirac and Maxwell-like equations is proposed as a relativization. Regular solutions to the latter form a discrete spectrum in which all the "active" gravitational masses are always positive, and approximately equal to inertial masses and to the mass m of the quanta of Dirac field up to the corrections of order α 2 . Here α = (m / M p l) 2 is the gravitational analogue of the fine structure constant negligibly small for nucleons. In the limit α = 0 the model reduces back to the nonrelativistic Schrödinger-Newton one. The equivalence principle is fulfilled with an extremely high precision. The above solutions correspond to various states of the same (free) particle rather than to different particles. These states possess a negligibly small difference in characteristics but essentially differ in the widths of the wavefunctions. For the ground state the latter is α times larger the Compton length, so that a nucleon cannot be sufficiently localized to model the reduction process. [ABSTRACT FROM AUTHOR]

The electric (α π) and magnetic (β π) Compton polarizabilities of both the charged and the neutral pion are of fundamental interest in the low-energy sector of quantum chromodynamics (QCD). Pion polarizabilities affect the shape of the γ π → γ π Compton scattering angular distribution at back scattering angles and γ γ → π π absolute cross sections. Theory derivations are given for the γ π → γ π Compton scattering differential cross section, dispersion relations, and sum rules in terms of the polarizabilities. We review experimental charged and neutral polarizability studies and theoretical predictions. The π 0 polarizabilities were deduced from DESY Crystal Ball γ γ → π 0 π 0 data, but with large uncertainties. The charged pion polarizabilities were deduced most recently from (1) radiative pion Primakoff scattering π − Z → π − Z γ at CERN COMPASS, (2) two-photon pion pair production γ γ → π + π − at SLAC Mark II, and (3) radiative pion photoproduction γ p → γ π + n from the proton at MAMI in Mainz. A stringent test of chiral perturbation theory (ChPT) is possible based on comparisons of precision experimental charged pion polarizabilities with ChPT predictions. Only the CERN COMPASS charged pion polarizability measurement has acceptably small uncertainties. Its value α π ± − β π ± = (4. 0 ± 1. 8) × 1 0 − 4 fm 3 agrees well with the two-loop ChPT prediction α π ± − β π ± = (5. 7 ± 1. 0) × 1 0 − 4 fm 3 , strengthening the identification of the pion with the Goldstone boson of chiral symmetry breaking in QCD. [ABSTRACT FROM AUTHOR]

Generalized Parton Distributions (GPDs) are nowadays the object of an intense effort of research, in the perspective of understanding nucleon structure. They describe the correlations between the longitudinal momentum and the transverse spatial position of the partons inside the nucleon and they can give access to the contribution of the orbital momentum of the quarks to the nucleon spin. Deeply Virtual Compton scattering (DVCS), the electroproduction on the nucleon, at the quark level, of a real photon, is the process more directly interpretable in terms of GPDs of the nucleon. Depending on the target nucleon (proton or neutron) and on the DVCS observable extracted (cross-sections, target- or beam-spin asymmetries, etc.), different sensitivity to the various GPDs for each quark flavor can be exploited. This article is focused on recent promising results, obtained at Jefferson Lab, on cross-sections and asymmetries for DVCS, and their link to GPDs. These data open the way to a 'tomographic' representation of the structure of the nucleon, allowing the extraction of transverse-space densities of the quarks at fixed longitudinal momentum. The extensive experimental program to measure GPDs at Jefferson Lab with the 12 GeV-upgraded electron accelerator and the complementary detectors that will be housed in three experimental Halls (A, B and C), will also be presented. [ABSTRACT FROM AUTHOR]

This paper reviews the experimental measurements in the field of deeply virtual Compton scattering and related theoretical efforts aimed for the extraction of generalized parton distributions, objects, describing the three-dimensional structure of nucleons and nuclei. The future experiments and theoretical expectations are also considered. [ABSTRACT FROM AUTHOR]