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Parity mixing of electron states should be extremely strong for heliumlike uranium. We calculate its size and discuss whether it could be determined experimentally. We analyze one specific scheme for such an experiment. The required laser intensities for two-photon spectroscopy of the 23P0–2 1S0level splitting is of the order of 1017 W/cm2. A determination of parity mixing would require at least 1021 W/cm2.
The angular distribution of electrons and positrons emitted in internal pair conversion is calculated. Coulomb-distorted waves are used as electron wave functions. Nuclear transitions of various multipolarities L>0 and of magnetic (ML) and of electric (EL) type are considered as well as E0 conversion. Analytical expressions for the angular correlation are derived, which are evaluated numerically assuming a finite extension of the nucleus and, for the EL and ML conversion, also in the point-nucleus approximation. The calculated angular correlations are compared with results obtained within the Born approximation and, for the E0 case, with experimental data.
We consider the contribution of nuclear polarization to the Lamb shift of K- and L-shell electrons in heavy atoms and quasiatoms. Our formal approach is based on the concept of effective photon propagators with nuclear-polarization insertions treating effects of nuclear polarization on the same footing as usual QED radiative corrections. We explicitly derive the modification of the photon propagator for various collective nuclear excitations and calculate the corresponding effective self-energy shift perturbatively. The energy shift of the 1s1/2 state in 92238U due to virtual excitation of nuclear rotational states is shown to be a considerable correction for atomic high-precision experiments. In contrast to this, nuclear-polarization effects are of minor importance for Lamb-shift studies in 82208Pb.
We investigate the production of heavy quarks in continuum and bound states in nuclear collisions. Creation rates for free bb and tt quark pairs and for bottomonium and toponium in the ground state are computed at energies of the BNL Relativistic Heavy Ion Collider, CERN Large Hadron Collider (LHC), and Superconducting Super Collider. Central and peripheral heavy-ion collisions are discussed. For top-quark creation we assumed a mass range of 90≤mt≤250 GeV. The creation rate for top quarks in peripheral collisions is estimated to be by a factor 40 to 130 smaller compared with corresponding central collisions. For mt=130 GeV we calculated a creation rate of about 4760 top-quark pairs per day at the LHC (3.5 TeV/nucleon) for Pb-Pb collisions.
We discuss the multiplicity distribution of electron-positron pairs created in the strong electromagnetic fields of ultrarelativistic heavy-ion transits. Based on nonperturbative expressions for the N-pair creation amplitudes, the Poisson distribution is derived by neglecting interference terms. The source of unitarity violation is identified in the vacuum-to-vacuum amplitude, and a perturbative expression for the mean number of pairs is given.
Using relativistic Green’s-function techniques we examined single-electron excitations from the occupied Dirac sea in the presence of strong external fields. The energies of these excited states are determined taking into account the electron-electron interaction. We also evaluate relativistic transition strengths incorporating retardation, which represents a direct measure of correlation effects. The shifts in excitation energies are computed to be lower than 0.5%, while the correlated transition strengths never deviate by more than 10% from their bare values. A major conclusion is that we found no evidence for collectivity in the electron-positron field around heavy and superheavy nuclei.
Ionization, pair creation, and electron excitations in relativistic heavy-ion collisions are investigated in the framework of the coupled-channel formalism. Collisions between heavy projectiles and Pb82+ are considered for various bombarding energies in the region E=500 up to 2000 MeV/u. Useful symmetry relations for the matrix elements are derived and the influence of gauge transformations onto the coupled-channel equations is explored.
A calculation of the vacuum-polarization contribution to the hyperfine splitting for hydrogenlike atoms is presented. The extended nuclear charge distribution is taken into account. For the experimentally interesting case 209Bi82+ we predict a delta-lambda- -1.6 nm shift for the transition wavelength of the ground-state hyperfine splitting.