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RESEARCH PRODUCT

Positron Production In Heavy-Ion Collisions

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Atomic systems with a nuclear charge Z much greater than 100 exhibit a number of unique features not otherwise found in nature. Two characteristic properties are illustrated in Figs. 1 and 2. In Fig. 1 we have plotted the binding energy of a K-shell electron around hypothetical nuclei up to Z ≈ 200. For Z > 150 the binding energy exceeds the rest energy m e c 2 of the electron; i.e., adding the electron to the nucleus actually diminishes the total mass of the system. At the critical charge Z c ≈ 170–175 the binding energy reaches twice the electron rest mass, the threshold for spontaneous creation of an electron-positron pair. As has been discussed extensively in the literature (Pieper and Greiner, 1969; Zeldovich and Popov, 1972; Muller, Rafelski, and Greiner, 1972; Greiner, Muller, and Rafelski, 1985), this signals the transition to a new ground state of quantum electrodynamics: for Z > Z c the static electric field surrounding a bare nucleus spontaneously creates two electrons to fill the vacant K shell, while two positrons are emitted to balance the overall charge. Thus, the ground state is not any more a bare nucleus but one that is surrounded by two electrons. Since the new ground state carries charge, one speaks of a charged vacuum (Rafelski, Muller, and Greiner, 1974). This situation offers the unique opportunity to study by laboratory experiments a phase transition, i.e., a discontinuous change, in the ground state according to relativistic quantum field theory.