Pair creation by a photon in a strong magnetic field

The process of pair creation by a photon in a strong magnetic field is investigated basing on the polarization operator in the field. The total probability of the process is found in a relatively simple form. The probability exhibits a "saw-tooth" pattern because of divergences arising when the electron and positron are created at threshold of the Landau energy levels. The pattern will be washed out at averaging over any smooth photon energy distribution. The new results are obtained in the scope of the quasiclassical approach: 1) in the case when the magnetic field $B \ll B_0, (B_0$ is the critical field) the new formulation extends the photon energy interval to the case when the created particles are not ultrarelativistic; 2) the correction to the standard quasiclassical approximation is found showing the range of applicability of the approach at high photon energy as well. The very important conclusion is that for both cases $B \ll B_0$ and $B \geq B_0$ the results of the quasiclassical calculation are very close to averaged probabilities of exact theory in a very wide range of photon energies. The quasiclassical approximation is valid also for the energy distribution if the electron and positron are created on enough high levels.Comment: 21 pages, 6 figure

Comparison of theory with experiment for positron production from high-energy electrons moving along crystal axes

Various positron distributions are obtained using an approach developed earlier for the description of electron-photon showers in axially aligned single crystals. Based on these distributions, characteristics of the positron yield measured in recent experiments are calculated. Theoretical estimations display a rather good agreement with experimental results obtained using 3 to 10 GeV electrons aligned to the - axis of the tungsten crystals.Comment: 10 pages, 6 Postscript figure

Delbr\"uck scattering in combined Coulomb and laser fields

We study Delbr\"uck scattering in a Coulomb field in the presence of a laser field. The amplitudes are calculated in the Born approximation with respect to the Coulomb field and exactly in the parameters of the laser field having arbitrary strength, spectral content and polarization. The case of high energy initial photon energy is investigated in detail for a monochromatic circularly polarized laser field. It is shown that the angular distribution of the process substantially differs from that for Delbr\"uck scattering in a pure Coulomb field. The value of the cross section under discussion may exceed the latter at realistic laser parameters that essentially simplify the possibility of the experimental observation of the phenomenon. The effect of high order terms in the quantum intensity parameter $\chi$ of the laser field is found to be very important already at relatively small $\chi$.Comment: 21 pages, 4 figure

Spectra of radiation and created particles at intermediate energy in oriented crystal taking into account energy loss

The spectral distribution of positron created by photon and the spectral distribution of photons radiated from electron in an oriented single crystal of intermediate thickness is calculated at intermediate energies. The energy loss of charged particles as well as photon absorption are taken into account. The used basic probabilities of processes include the action of field of axis as well as the multiple scattering of radiating electron or particles of the created pair (the Landau-Pomeranchuk-Migdal (LPM) effect)Comment: 11 pages, 5 figures, report at "Channeling 2008" Workshop, Erice, Italy, October 200

Photon splitting in a laser field

Photon splitting due to vacuum polarization in a laser field is considered. Using an operator technique, we derive the amplitudes for arbitrary strength, spectral content and polarization of the laser field. The case of a monochromatic circularly polarized laser field is studied in detail and the amplitudes are obtained as three-fold integrals. The asymptotic behavior of the amplitudes for various limits of interest are investigated also in the case of a linearly polarized laser field. Using the obtained results, the possibility of experimental observation of the process is discussed.Comment: 31 pages, 4 figure

Beam-size effect in Bremsstrahlung

The smallness of the transverse dimensions of the colliding beams leads to suppression of bremsstrahlung cross section. This beam-size effect was discovered and investigated at INP, Novosibirsk. Different mechanisms of radiation are discussed. Separation of coherent and incoherent radiation is analyzed in detail. For linear collider this suppression affects the whole radiation spectrum. It is shown that objections to the subtraction procedure in \cite{BK2} are groundless.Comment: Plenary talk at the Workshop "Quantum Aspects of Beam Physics", Hiroshima, Japan, January 7-11, 2003. To be pulished in the Proceedings (World Scientific, Singapore