70 Years of Hyperon Spectroscopy: A review of strange Ξ, Ω baryons, and the spectrum of charmed and bottom baryons.

Abstract

The first hyperon was discovered about 70 years ago, but the nature of these particles, particularly with regard to multistrange hyperons, and many of their properties can still be considered to be literally strange. A dedicated and successful global spectroscopy program in the 1960s and 1970s using K-beams revealed many multistrange candidates, but the available evidence of their existence is statistically limited. For this reason, there is still much to learn about the systematics of the spectrum of excited hyperon states and what they have in common with their non-strange companions, or how they differ from the nucleon and Δ resonances. Results from photo- and electroproduction experiments off the proton and neutron using polarized beams and targets have provided intriguing evidence for new nucleon excitations and shed light on the structure of some of the known nucleon and Δ states. Recent years have also seen a great deal of progress in the field of charmed and bottom baryon spectroscopy. Unprecedented data from the Large Hadron Collider in particular indicate continued rapid progress in the field of bottom baryons. On the theoretical side, baryons with one heavy quark Q and a light qq system serve as an ideal laboratory for studying light qq (diquark) correlations and the dynamics of the light quarks in the colour environment of a heavy quark. In this review, we discuss the status of doubly and triply strange Ξ as well as Ω baryons, and the properties of all the known charmed and bottom states. The comparison of the two heavy sectors reveals many similarities as predicted by heavy-quark symmetries, together with differences in mass splittings easily understood by potential models. The multistrange hyperons bridge the under-explored gap between the light- and the heavy-flavour baryons. How do the properties of a singly charmed Q-qq system change with decreasing mass of the heavy quark in the transition to a doubly strange q-QQ system with a heavier quark-quark system relative to one light quark? Significant progress towards understanding hyperon resonances is expected in coming years from the ongoing experiments at the high-energy collider facilities and planned experiments using K beams at Jefferson Laboratory and J-PARC.