Quo vadis electron-positron lines?

Abstract

In strong electric fields the neutral electron-positron vacuum state becomes unstable and decays through pair creation. In a hypothetical atom with a charge exceeding the critical value Zcr ≃ 173 a charged vacuum will be formed, signalled by the spontaneous emission of positrons. In the laboratory sufficiently strong electric fields can be created transiently in heavy-ion collisions with total nuclear charge Z1 + Z2 > Zcr. Detailed coupled channel calculations for the time dependent two-centre Dirac equation have been performed to describe the dynamics of the electron shell in superheavy quasimolecules. K-hole production as well as δ-electron and positron emission are found to be sensitively dependent on the strong field in a collision. The emission of positrons is strongly enhanced in high-Z collisions. Clear experimental signals for supercritical positron creation could be expected in collisions above the nuclear Coulomb barrier if a "gaint" dinuclear system is formed with a lifetime larger than about 10−20 sec.In the spectra of positrons produced in heavy-ion collisions narrow structures with unexpected properties have been found in several experiments performed at GSI (Darmstadt). The subsequent observation of correlated e+ e− emission compatible with the two-body decay of a neutral particle state with mass around 1.8 MeV has led to widespread theoretical speculations on the existence and the nature of these objects. However, despite intense experimental and theoretical research no satisfactory explanation has been found. New elementary particles are clearly ruled out and also models of composite extended objects are beset with problems.