Barrier control in tunneling e+-e- photoproduction.

Tunneling electron-positron pair production is studied in a new setup in which a strong low-frequency and a weak high-frequency laser field propagate in the same direction and collide head-on with a relativistic nucleus. The electron-positron pair-production rate is calculated analytically in the limit in which in the nucleus rest frame, the strong field is undercritical and the frequency of the weak field is below and close to the pair-production threshold. By changing the frequency of the weak field, one can reduce the tunneling barrier substantially. As a result, tunneling pair production is shown to be observable with presently available technology.

Polarizing mechanisms for stored and beams interacting with a polarized target.

The kinetics of the polarization buildup during the interaction of stored protons (antiprotons) with a polarized target is considered. It is demonstrated that for small scattering angles, when a projectile remains in the beam, the polarization buildup is completely due to the spin-flip transitions. The corresponding cross sections turn out to be negligibly small for a hydrogen gas target as well as for a pure electron target. For the latter, the filtering mechanism also does not provide a noticeable beam polarization.

Bremsstrahlung in alpha decay reexamined.

A high-statistics measurement of bremsstrahlung emitted in the alpha decay of (210)Po has been performed, which allows us to follow the photon spectra up to energies of approximately 500 keV. The measured differential emission probability is in good agreement with our theoretical results obtained within the quasiclassical approximation as well as with the exact quantum mechanical calculation. It is shown that, due to the small effective electric dipole charge of the radiating system, a significant interference between the electric dipole and quadrupole contributions occurs, which is altering substantially the angular correlation between the alpha particle and the emitted photon.

Relativistic corrections to the electric polarizability of compound systems.

The widely used relation alpha; = alpha(0)+Deltaalpha for the electric polarizability, where Deltaalpha is proportional to the r.m.s. charge radius of the system, is revisited within a simple model of two spinless particles bound electromagnetically. A complete set of the relativistic corrections to alpha(0) following from the Breit Hamiltonian is obtained. It is shown that these corrections modify the result for alpha; in comparison to that obtained with alpha(0) taken in the nonrelativistic approximation. We propose that the same situation can take place in the more complicated case of hadrons. Special attention is devoted to the correct definition of the center-of-mass coordinates which are found to be very important.

Radiative corrections and parity nonconservation in heavy atoms.

The self-energy and the vertex radiative corrections to the effect of parity nonconservation in heavy atoms are calculated analytically in orders Zalpha2 and Z2alpha3ln((lambda(C)/r(0)), where lambda(C) and r(0) are the Compton wavelength and the nuclear radius, respectively. The sum of the radiative corrections is -0.85% for Cs and -1.41% for Tl. Using these results, we have performed analysis of the experimental data on atomic parity nonconservation. The values obtained for the nuclear weak charge, Q(W)=-72.90(28)exp(35)theor for Cs, and Q(W)=-116.7(1.2)exp(3.4)(theor) for Tl, agree with predictions of the standard model. As an application of our approach, we have also calculated analytically the dependence of the Lamb shift on the finite size of the nucleus.

Experimental investigation of high-energy photon splitting in atomic fields.

Data analysis of an experiment in which photon splitting in atomic fields was observed is presented. The experiment was performed at the tagged photon beam of the ROKK-1M facility at the VEPP-4M collider. In the energy region of 120-450 MeV, statistics of 1.6x10(9) photons incident on the BGO target was collected. About 400 candidate photon-splitting events were reconstructed. Within the attained experimental accuracy, the experimental results are consistent with the calculated exact atomic-field cross section. The predictions obtained in the Born approximation differ significantly from the experimental results.