Pulsed production of cold protonium in Penning traps

Precision comparison experiments on bound states of matter and antimatter rely on the production of corresponding systems at low temperatures and in sufficient numbers. In this paper we propose a scheme for the pulsed production of highly excited protonium (Pn) in a Penning-Malmberg trap at low kinetic energies of tens of meV. The scheme relies on the resonant-charge-exchange reaction H*$+\overline p →$Pn*$+ e^−$ where Rydberg excited hydrogen and antiprotons ($\overline p$) interact to form Pn*. The reagent H($n$=30,$l$=2) is created from laser photodetached and excited hydrogen anions (H$^−$), which are initially trapped and mixed in a plasma together with electrons and antiprotons at low kinetic energies. We discuss a three-step pulsed laser excitation using rate equations. A semiclassical Monte Carlo approach leads to a formation rate of 10$^5$ Pn per 20 s when assuming a production temperature of 100 K. The formed Pn are internally excited in states with average principal quantum number $〈n〉≈$1200 having lifetimes that can reach seconds. The proposed scheme is therefore particularly interesting for experiments aiming at the study of cold antimatter and purely baryonic systems for precision experiments (charge neutrality, gravity, spectroscopy), as performed at the antiproton decelerator facility at CERN.