Anomalies in the decay rates of antiprotonic helium-atom states

Abstract

Six resonance transitions of the antiprotonic helium atom in helium gas at densities of $3\ifmmode\times\else\texttimes\fi{}{10}^{20}--3\ifmmode\times\else\texttimes\fi{}{10}^{21} {\mathrm{cm}}^{\ensuremath{-}3}$ were studied at the antiproton decelerator (AD) of CERN. The decay rates of the daughter states of these transitions were determined either from the time distributions of the resonance spikes or from the widths of the resonance lines. Whereas most of the observed decay rates agree with theoretical calculations of Auger rates, two states, $(n,l)=(37,33)$ and (32,31), were found to have decay rates two orders of magnitude larger than predicted by these calculations. The effect of coupling with near-lying electron-excited states is considered to be the reason for the anomaly of the (37, 33) state, as pointed out by Kartavtsev et al. [Phys. Rev. A $61,$ 062507 (2000)].