Dispersive contributions to e+p/e−pe^+p/e^-p cross section ratio in forward regime

Two-photon exchange (TPE) contributions to elastic electron-proton scattering in the forward regime are considered. The imaginary part of TPE amplitude in these kinematics is related to the DIS nucleon structure functions. The real part of the TPE amplitude is obtained from the imaginary part by means of dispersion relations. We demonstrate that the dispersion integrals for the relevant elastic $ep$-scattering amplitude converge and do not need subtraction. This allows us to make clean prediction for the real part of the TPE amplitude at forward angles. We furthermore compare $e^+p$ and $e^-p$ cross sections which depends on the real part of TPE amplitude, and predict the positron cross section to exceed the electron one by a few per cent, with the difference ranging from 1.4% to 2.8% for electron $lab$ energies in the range from 3 to 45 GeV. We furthermore predict that the absolute value of this asymmetry grows with energy, which makes it promising for experimental tests.Comment: 10 pages, 6 figure

Beam normal spin asymmetry in the quasi-RCS approximation

The two-photon exchange contribution to the single spin asymmetries with the spin orientation normal to the reaction plane is discussed for elastic electron-proton scattering in the equivalent photon approximation. In this case, hadronic part of the two-photon exchange amplitude describes real Compton scattering (RCS). We show that in the case of the beam normal spin asymmetry, this approximation selects only the photon helicity flip amplitudes of RCS. At low energies, we make use of unitarity and estimate the contribution of the $\pi N$ multipoles to the photon helicity flip amplitudes. In the Regge regime, QRCS approximation allows for a contribution from two pion exchange, and we provide an estimate of such contributions. We furthermore discuss the possibility of the quark and gluon GPD's contributions in the QRCS kinematics.Comment: 10 pages, 5 figures, revtex, submitted to Phys. Rev. C; new version: references adde

Probing Dark Matter with AGN Jets

We study the possibility of detecting a signature of particle dark matter in the spectrum of gamma-ray photons from active galactic nuclei (AGNs) resulting from the scattering of high-energy particles in the AGN jet off of dark matter particles. We consider particle dark matter models in the context of both supersymmetry and universal extra-dimensions (UED), and we present the complete lowest-order calculation for processes where a photon is emitted in dark matter-electron and/or dark matter-proton scattering, where electrons and protons belong to the AGN jet. We find that the process is dominated by a resonance whose energy is dictated by the particle spectrum in the dark matter sector (neutralino and selectron for the case of supersymmetry, Kaluza-Klein photon and electron for UED). The resulting gamma-ray spectrum exhibits a very characteristic spectral feature, consisting of a sharp break to a hard power-law behavior. Although the normalization of the gamma-ray flux depends strongly on assumptions on both the AGN jet geometry, composition and particle spectrum as well as on the particle dark matter model and density distribution, we show that for realistic parameters choices, and for two prominent nearby AGNs (Centaurus A and M87), the detection of this effect is in principle possible. Finally, we compare our predictions and results with recent gamma-ray observations from the Fermi, H.E.S.S. and VERITAS telescopes.Comment: 23 pages, 13 figures, submitte

Pion form factors

We consider the electromagnetic and transition pion form factors. Using dispersion relations we simultaneously describe both the hadronic, time-like region and the asymptotic region of large energy-momentum transfer. For the latter we propose a novel mechanism of Regge fermion exchange. We find that hadronic contributions dominate form factors at all currently available energies.Comment: 4 pages, 3 figure

Doubly virtual Compton scattering and the beam normal spin asymmetry

We construct an invariant basis for Compton scattering with two virtual photons (VVCS). The basis tensors are chosen to be gauge invariant and orthogonal to each other. The properties of the corresponding 18 invariant amplitudes are studied in detail. We consider the special case of elastic VVCS with the virtualities of the initial and final photons equal. The invariant basis for VVCS in this orthogonal form does not exist in the literatur. We furthermore use this VVCS tensor for a calculation of the beam normal spin asymmetry in the forward kinematics. For this, we relate the invariant amplitudes to the helicity amplitudes of the VVCS reaction. The imaginary parts of these latter are related to the inclusive cross section by means of the optical theorem. We use the phenomenological value of the transverse cross section $\sigma_T\sim0.1$ mbarn and the Callan-Gross relation which relates the longitudinal cross section $\sigma_L$ to the transverse one. The result of the calculation agrees with an existing calculation and predicts the negative values of the asymmetry $B_n$ of the order of 4-6 ppm in the energy range from 6 to 45 ppm and for very forward angles.Comment: 13 pages, 2 figures, revtex, submitted to Phys. Rev. C; new version: two figures added, typos correcte

Light-front interpretation of Proton Generalized Polarizabilities

We extend the recently developed formalism to extract light-front quark charge densities from nucleon form factor data to the deformations of these quark charge densities when applying an external electric field. We show that the resulting induced polarizations can be extracted from proton generalized polarizabilities. The available data for the generalized electric polarizabilitiy of the proton yield a pronounced structure in its induced polarization at large transverse distances, which will be pinned down by forthcoming high precision virtual Compton scattering experiments.Comment: 4 pages, 4 figures, reference added, minor typos correcte