Neutrino-antineutrino pair production by a photon in a dense matter

The possibility of radiative effects that are due to interaction of fermions with a dense matter is investigated. Neutrino-antineutrino photo-production is studied. The rate of this process is calculated in the Furry picture. It is demonstrated that this effect does not disappear even if the medium refractive index is assumed to be equal to unity. The rate obtained strongly depends on the polarization states of the particles involved. This leads to evident spatial asymmetries, which may have certain consequences observable in astrophysical and cosmological studies.Comment: 10 pages, Late

Radiative transitions of high energy neutrino in dense matter

The quantum theory of the ``spin light'' (electromagnetic radiation emitted by a massive neutrino propagating in dense matter due to the weak interaction of a neutrino with background fermions) is developed. In contrast to the Cherenkov radiation, this effect does not disappear even if the medium refractive index is assumed to be equal to unity. The formulas for the transition rate and the total radiation power are obtained. It is found out that radiation of photons is possible only when the sign of the particle helicity is opposite to that of the effective potential describing the interaction of a neutrino (antineutrino) with the background medium. Due to the radiative self-polarization the radiating particle can change its helicity. As a result, the active left-handed polarized neutrino (right-handed polarized antineutrino) converting to the state with inverse helicity can become practically ``sterile''. Since the sign of the effective potential depends on the neutrino flavor and the matter structure, the ``spin light'' can change a ratio of active neutrinos of different flavors. In the ultra relativistic approach, the radiated photons averaged energy is equal to one third of the initial neutrino energy, and two thirds of the energy are carried out by the final ``sterile'' neutrinos. This fact can be important for the understanding of the ``dark matter'' formation mechanism on the early stages of evolution of the Universe.Comment: 7 pages, latex, one misprint in eq. 12 correcte

Longitudinal and transverse velocity fields in parsec-scale jets

Radio-loud AGN typically manifest powerful relativistic jets extending up to millions of light years and often showing superluminal motions organised in a complex kinematic pattern. A number of physical models are still competing to explain the jet structure and kinematics revealed by radio images using the VLBI technique. Robust measurements of longitudinal and transverse velocity field in the jets would provide crucial information for these models. This is a difficult task, particularly for transversely resolved jets in objects like 3C 273 and M87. To address this task, we have developed a new technique for identifying significant structural patterns (SSP) of smooth, transversely resolved flows and obtaining a velocity field from cross-correlation of these regions in multi-epoch observations. Detection of individual SSP is performed using the wavelet decomposition and multiscale segmentation of the observed structure. The cross-correlation algorithm combines structural information on different scales of the wavelet decomposition, providing a robust and reliable identification of related SSP in multi -epoch images. The algorithm enables recovering structural evolution on scales down to 0.25 full width at half maximum (FWHM) of the image point spread function (PSF). We present here examples of applying this algorithm to obtain the first detailed transverse velocity fields and to study the kinematic evolution in the parsec-scale jets in 3C 273 and M87.Comment: 6 pages, proceeding of the 12th European VLBI Network Symposium and Users Meeting - EVN 201