Polarization effects for pair creation by photon in oriented crystals at high energy

Pair creation by a photon in an oriented crystal is considered in a frame of the quasiclassical operator method, which includes processes with polarized particles. Under some quite generic assumptions the general expression is derived for the probability of pair creation of longitudinally polarized electron (positron) by circularly polarized photon in oriented crystal. In particular cases $\vartheta_0 \ll V_0/m$ and $\vartheta_0 \gg V_0/m$ ($\vartheta_0$ is the angle of incidence, angle between the momentum of initial photon and axis (plane) of crystal, $V_0$ is the scale of a potential of axis or a plane relative to which the angle $\vartheta_0$ is defined) one has constant field approximation and the coherent pair production theory correspondingly. Side by side with coherent process the probability of incoherent pair creation is calculated, which differs essentially from amorphous one. At high energy the pair creation in oriented crystals is strongly enhanced comparing with amorphous medium. In appendixes the integral polarization of positron is found in external field and for coherent and incoherent mechanisms.Comment: 24 pages, 4 figure

On propagation of high-energy photon in a medium in presence of an external field

The polarization tensor is calculated which originates from interaction of a photon with the electron-positron field. The effects of multiple scattering of electrons and positrons in a medium side by side with an external field are included. The corresponding general representation of polarization tensor is found in the form of two-dimentional integral. The both effects may be essential for propagation of high-energy photons in oriented single crystals.Comment: 12 pages, 1 figur

Coherent and incoherent pair creation by a photon in oriented single crystal

The new approach is developed for study of electron-positron pair production by a photon in oriented single crystal. It permits indivisible consideration of both coherent and incoherent mechanisms of pair creation and includes the action of field of axis (or plane) as well as the multiple scattering of particles of the created pair (the Landau-Pomeranchuk-Migdal (LPM) effect). From obtained integral probability of pair creation, it follows that multiple scattering appears only for relatively low energy of photon, while at higher photon energy the field action excludes the LPM effect. The found results agree quite satisfactory with recent CERN experiment.Comment: 10 pages, 3 figure

Quantum theory of transition radiation and transition pair creation

Theory of the transition radiation and the transition pair creation is developed in the frame of QED. The spectral-angular distributions of probability of the transition radiation and of the transition pair creation are found. The total energy losses of and the total probability of pair creation are calculated and analyzed. Features of radiation and pair creation processes in a superdence medium (typical for white dwarfs) are discussed.Comment: LaTeX, 12 pages, 3 eps figure

Gluon Radiation off Hard Quarks in a Nuclear Environment: Opacity Expansion

We study the relation between the Baier-Dokshitzer-Mueller-Peigne-Schiff (BDMPS) and Zakharov formalisms for medium-induced gluon radiation off hard quarks, and the radiation off very few scattering centers. Based on the non-abelian Furry approximation for the motion of hard partons in a spatially extended colour field, we derive a compact diagrammatic and explicitly colour trivial expression for the N-th order term of the kt-differential gluon radiation cross section in an expansion in the opacity of the medium. Resumming this quantity to all orders in opacity, we obtain Zakharov's path-integral expression (supplemented with a regularization prescription). This provides a new proof of the equivalence of the BDMPS and Zakharov formalisms which extends previous arguments to the kt-differential cross section. We give explicit analytical results up to third order in opacity for both the gluon radiation cross section of free incoming and of in-medium produced quarks. The N-th order term in the opacity expansion of the radiation cross section is found to be a convolution of the radiation associated to N-fold rescattering and a readjustment of the probabilities that rescattering occurs with less than N scattering centers. Both informations can be disentangled by factorizing out of the radiation cross section a term which depends only on the mean free path of the projectile. This allows to infer analytical expressions for the totally coherent and totally incoherent limits of the radiation cross section to arbitrary orders in opacity.Comment: 22 pages, LaTeX, eps-figures included, typos corrected, Comment on (5.13) added, to appear in NP

Spectrum and polarization of coherent and incoherent radiation and the LPM effect in oriented single crystal

The spectrum and the circular polarization of radiation from longitudinally polarized high-energy electrons in oriented single crystal are considered using the method which permits inseparable consideration of both the coherent and the incoherent mechanisms of photon emission. The spectral and polarization properties of radiation are obtained and analyzed. It is found that in some part of spectral distribution the influence of multiple scattering (the Landau-Pomeranchuk-Migdal (LPM) effect) attains the order of 7 percent. The same is true for the influence of multiple scattering on the polarization part of the radiation intensity. The degree of circular polarization of total intensity of radiation is found. It is shown that the influence of multiple scattering on the photon polarization is similar to the influence of the LPM effect on the total intensity of radiation: it appears only for relatively low energies of radiating electron and has the order of 1 percent, while at higher energies the crystal field action excludes the LPM effect.Comment: 13 pages, 7 figure

Addendum to "On the pλp^{\lambda} problem"

We prove that the conditions $\lambda<5/19$ and $L\le T^{1/2}$ in Theorems 3 and 4 of our recent paper "On the $p^{\lambda}$ problem" can be omitted.Comment: 7 page

Jet Quenching

A short summary of the physics underlying jet quenching is givenComment: 10 pages, LaTex, 11 postscript figures, Proceedings of Quark Matter 2002, Nantes, France, July 18-24, 200

Perturbative Hot Gauge Theories - Recent Results

Current results in high temperature gauge theories obtained in the context of the perturbative method of resumming hard thermal loops are reviewed. Beyond leading order properties of the gluon excitation, and the recent (controversial) calculations of the damping rates are discussed. QCD predictions on plasma signatures are exemplified by the thermal production rates of energetic as well as soft photons. [Talk given at the 3rd Workshop on Thermal Field Theories and their Applications, August 15--27, 1993, Banff, Alberta, Canada]Comment: 11 pages, typeset using LATEX, BI-TP 93/6