194 research outputs found
Spin polarization of electrons induced by strong collisional magnetic fields
We calculate the spin polarization of 1sσ vacancies and emitted δ electrons induced by the strong magnetic field (|Bmax|∼1016G) in collisions of very heavy ions (Z1+Z2=178). The electron excitations are determined by the solution of coupled-channel equations within the quasimolecular basis states including the vector potential. The formulation is extended to the many-electron case. Spin polarizations of the order of 5-10% for impact energies below the Coulomb barrier are predicted
Comment on "New atomic mechanism for positron production in heavy-ion collisoins"
A Comment on the Letter by W. Lichten and A. Robatino, Phys. Rev. Lett. 54, 781 (1985). See Also: W. Lichten and A. Robatino, New atomic mechanism for positron production in heavy-ion collisions, Phys. Rev. Lett. 54, 781 (1985). http://prola.aps.org/abstract/PRL/v54/i8/p781_
Theory of positron production in heavy-ion collisions
Collisions of very heavy ions at energies close to the Coulomb barrier are discussed as a unique tool to study the behavior of the electron-positron field in the presence of strong external electromagnetic fields. To calculate the excitation processes induced by the collision dynamics, a semiclassical model is employed and adapted to describe the field-theoretical many-particle system. An expansion in the adiabatic molecular basis is chosen. Energies and matrix elements are calculated using the monopole approximation. In a supercritical (Z1+Z2≳173) quasiatomic system the 1s level joins the antiparticle continuum and becomes a resonance, rendering the neutral vacuum state unstable. Several methods of treating the corresponding time-dependent problem are discussed. A projection-operator technique is introduced for a fully dynamical treatment of the resonance. Positron excitation rates in s1/2 and p1/2 states are obtained by numerical solution of the coupled-channel equations and are compared with results from first- plus second-order perturbation theory. Calculations are performed for subcritical and supercritical collisions of Pb-Pb, Pb-U, U-U, and U-Cf. Strong relativistic deformations of the wave functions and the growing contributions from inner-shell bound states lead to a very steep Z dependence of positron production. The results are compared with available data from experiments done at GSI. Correlations between electrons and positrons are briefly discussed
Description of atomic excitations in heavy-ion reactions
Excitations of the atomic shell in heavy-ion collisions are influenced by the presence of a nuclear reaction. In the present Rapid Communication we point out the equivalence between a semiclassical description based on the nuclear autocorrelation function with an earlier model which employs a distribution of reaction times f(T). For the example of U+U collisions, results of coupled-channel calculations for positron creation and K-hole excitations are discussed for two schematic reaction models
Delta-electron emission in deep-inelastic heavy-ion collisions
This paper reports calculations of the influence of a reaction time T>10-21 s in deep-inelastic Xe-Pb collisions on the energy spectrum of δ electrons ejected in the same collision. It is shown that the lifetime of the superheavy composite system causes pronounced oscillations of width ε=h/T in the electron distribution, which survive the inclusion of multistep excitations and the folding with a lifetime distribution function. This effect may serve as an atomic clock for deep-inelastic collisions
Induced decay of composite JPC=1++ particles in atomic Coulomb fields
The electron-positron pairs observed in heavy-ion collisions at Gesellschaft für Schwerionen-forschung Darmstadt mbH have been interpreted as the decay products of yet unknown particles with masses around 1.8 MeV. The negative results of resonant Bhabha scattering experiments, however, do not support such an interpretation. Therefore we focus on a more complex decay scenario, where the e+e- lines result from a two-collision process. We discuss the induced decay of a metastable 1++ state into e+e- pairs. For most realizations of a 1++ state such a decay in leading order can only take place in the Coulomb field of a target atom. This fact has the attractive consequence that for such a state the Bhabha bounds are no longer valid. However, the absolute value of the e+e- production cross section turns out to be unacceptably small
Phenomenological consequences of a hypothetical light neutral particle in heavy ion collisions
We discuss the possibility that the line structure observed in the spectrum of the positrons produced in heavy ion collisions is due to the decay of a new neutral elementary particle. We argue that this can be ruled out unless one is willing to accept fine tuning of parameters, or to assume the dominance of nonlinear effects
Role of multistep processes in heavy-ion inner-shell excitations
This Letter discusses inner-shell excitation in collisions of very heavy ions (Z1+Z2≳140) in the framework of the quasimolecular model. The importance of multistep excitations and of coupling between continuum states is demonstrated. The 1sσ vacancy probabilities resulting from coupled-channels calculations exceed perturbation theory by a factor 3-5, thus giving good agreement with recent experimental results
Shakeoff of the vacuum polarization in quasimolecular collisions of very heavy ions
The theory of direct electron-positron pair production in the collision of heavy ions is formulated in the framework of the quasimolecular model. The pair production process acquires a collective nature for (Z1+Z2)α>1 and can be understood as the shakeoff of the strong vacuum polarization cloud formed in the quasimolecule. The total cross section is, e.g., 76 μb for Pb + Pb at Coulomb barrier energies
In-medium vector meson properties and low-mass dilepton production from hot hadronic matter
The in-medium properties of the vector mesons are known to be modified
significantly in hot and dense hadronic matter due to vacuum polarisation
effects from the baryon sector in the Walecka model. The vector meson mass
drops significantly in the medium due to the effects of the Dirac sea. In the
variational approach adopted in the present paper, these effects are taken into
account through a realignment of the ground state with baryon condensates. Such
a realignment of the ground state becomes equivalent to summing of the baryonic
tadpole diagrams in the relativistic Hartree approximation (RHA). The
approximation scheme adopted here goes beyond RHA to include quantum effects
from the scalar meson and is nonperturbative and self--consistent. It includes
multiloop effects, thus corresponding to a different approximation as compared
to the one loop approximation of including scalar field quantum corrections. In
the present work, we study the properties of the vector mesons in the hot and
dense matter as modified due to such quantum correction effects from the baryon
as well as scalar meson sectors. These medium modifications of the properties
of the vector mesons are reflected, through the shifting and broadening of the
respective peaks, in the low mass dilepton spectra. There is broadening of the
peaks due to corrections from scalar meson quantum effects as compared to the
relativistic Hartree approximation. It is seen to be rather prominent for the
meson in the invariant mass plot.Comment: 22 pages (revtex style) including 12 figures (ps files). Final
version, accepted by Phys. Rev. C. The text has been shortened and a
calculational error has been correcte
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