Volume 29, Issue 5, September 1998
29(1998); http://dx.doi.org/10.1134/1.953085View Description Hide Description
The channeling of high-energy charged particles in bent crystals is being used more and more extensively for steering particle beams at accelerators, for extracting a beam or its halo from an accelerator, and for splitting the extracted beam. The use of the strong intracrystalline fields of bent crystals in high-energy physics, for example, to measure the magnetic moments of short-lived particles from the spin precession angle in a bent crystal, appears to be very promising. The results of studies on the channeling of high-energy particles in a bent crystal are reviewed. The features of channeling and quasichanneling, the ionization energy losses, and the radiation emitted by channeled particles are studied. The possibility of polarization effects for channeled particles is discussed.
29(1998); http://dx.doi.org/10.1134/1.953087View Description Hide Description
This review is devoted to the covariant method of calculating matrix elements of processes in quantum electrodynamics in the diagonal spin basis, which has been developed in the last few years. Its application for calculating the differential cross sections of real processes involving polarized particles is studied. In the diagonal spin basis the spin 4-vectors of particles before and after the interaction are expressed in terms of their 4-momenta. This forms a realization of the Lorentz little group, which is the same for the initial and final states. It causes the spin operators of the initial and final particles to coincide, which can be used to separate covariantly the interactions with and without change of the spin states of the particles involved in the reaction, thereby making it possible to trace the dynamics of the spin interaction. In contrast to the method of the CALCUL group and others, this approach is valid in both the massive and the massless cases. No difficulties arise in calculating the spin-flip amplitudes, and it is not necessary to introduce auxiliary vectors. For constructing the mathematical formalism used to calculate the matrix elements in the diagonal spin basis it is sufficient to know only the 4-momenta of the particles participating in the reaction. The method has been used to study the following QED processes: (1) Mo/ller and Bhabha bremsstrahlung in the ultrarelativistic (massless) limit for the case where the initial particles and the photon are helically polarized; (2) the Compton back-scattering of photons by an intense, circularly polarized laser wave focused on a beam of longitudinally polarized ultrarelativistic electrons (3) -pair creation by a hard photon in a collision with several laser photons simultaneously (4) the Bethe–Heitler process in the case of emission of a linearly polarized photon by an electron, taking into account the proton recoil and form factors; (5) the reaction taking into account the proton polarizability in the kinematics where proton bremsstrahlung dominates; (6) the three-photon annihilation of orthopositronium The results obtained by this method indicate that it is useful for calculating multiparticle processes involving polarized particles.
29(1998); http://dx.doi.org/10.1134/1.953088View Description Hide Description
The results of studies of wire chambers with gas gain corresponding to streamer production in the electronavalanche are reviewed. The dynamics of the development of an avalanche and its transformation into plasma production are demonstrated experimentally for the first time. Some qualitatively new features appear which allow the determination of the necessary conditions for streamer formation and growth toward the cathode. The results of these studies can be used to develop techniques for physics experiments and in gas-discharge physics.
29(1998); http://dx.doi.org/10.1134/1.953089View Description Hide Description
The critical behavior of some of the simplest -dimensional field-theoretic models with four-fermion interaction at nonzero temperature, chemical potential, and external gauge fields is studied. It is shown that an external magnetic field catalyzes spontaneous symmetry breaking. The chromomagnetic gluon condensate in can also spontaneously break chiral symmetry. The thermodynamics of these effects is studied in detail. The conditions under which superconducting phase transitions and also dynamical generation of a Chern–Simons term are possible in three-dimensional models of the Gross–Neveu type are derived.