Kaonic hydrogen - past puzzles

Abstract

The experimental study of low energy kaon–nucleon interactions began in the ’50s when accelerators reached an energy at which kaons could first be produced in the laboratory. Hydrogen-, deuterium-, and helium-filled bubble chambers were used to study both negative and positive kaon scattering and absorption reactions. Where available, beams of negative kaons were stopped in targets ranging from helium to carbon – and later up to uranium – for the purpose of studying the X-rays resulting from the de-excitation of the kaon from a high capture orbit down to the lowest orbit reached. The energy of this lowest orbit of these so-called exotic atoms was shifted in energy and absorption-broadened by the kaon–nucleus strong interaction. Hence, the X-ray spectrum could be used to measure both the shift and width of this lowest level. X-ray intensity ratios of the feeding transition to this lowest transition could also be used to deduce the absorption width of the higher level. However, in the late ’70s, no experimental data yet existed for the simplest and most fundamental of these – the exotic kaonic hydrogen isotopes – for reasons discussed below. The optical model theory of exotic hadronic atoms relating low-energy scattering data to shifts and widths was well developed for pions in the ’60s by the Ericsons [1], thus the formalism for kaons, which are also spinless particles, was put in place. Knowing the shift and width, the complex scattering length could then be deduced and compared with scattering data. Physicists embroiled in the excitement of both the experiments and the theoretical underpinnings prepared many excellent reviews. They do far better justice to this period of kaonic hydrogen pre-history than would I. The reader is referred to Burhop [2] and the references therein for the early period up to the late ’60s, and to Seki and Wiegand for kaonic and other exotic atom data up to the mid-’70s [3]. Chris Batty reviewed both theory and experimental results on a broad range of exotic atoms in the late ’70s (and updated it recently [4]). H. Poth [5] provided a valuable compilation of hadronic atom data in 1979. A bibliography of exotic atoms from 1939 to 1982 was produced by Dezso Horvath and Richard Lambrecht [6] in 1984. The state of theory up to the early ’80s has been well presented by Florian Scheck [7].