Puzzling Positron Peaks Appear in Heavy‐Ion Collisions at GSI

Searching for the “sparking of the vacuum,” one of the more exotic predictions of quantum electrodynamics, two groups of heavy‐ion experimenters at the Gesellschaft für Schwerionenforschung in Darmstadt have found something quite unexpected, and very puzzling. If one had a nucleus of sufficiently high charge, Z, with a vacancy in the lowest‐lying electron orbit, it would be energetically cheaper to create an electron‐positron pair spontaneously out of the vacuum than to leave the orbit vacant. This is, in fact, the vacuum sparking—or “spontaneous positron emission”—predicted by QED. One might expect that this somewhat bizarre creation of an ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ pair out of nothing but an ultrastrong electrostatic field in the vacuum would occur at $\text{Z\hspace{0.17em}=\hspace{0.17em}137,}$ the reciprocal of the fine‐structure constant, where the bound‐state solutions of the Dirac equation for a point‐charge nucleus disappear. But for realistic, finite nuclear diameters, detailed QED calculations predict that spontaneous positron emission will first occur when Z is about 173.

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