Exciting baryons: Now and in the future

Abstract

Abstract. This is the final talk of NSTAR2011 conference. It is not a summary talk, but rather a looking forward to what still needs to be done in excited baryon physics. In particular, we need to hone our tools connecting experimental inputs with QCD. At present we rely on models that often have doubtful connections with the underlying theory, and this needs to be dramatically improved, if we are to reach definitive conclusions about the relevant degrees of freedom of excited baryons. Conclusions that we want to have by NSTAR2021.Keywords: Baryons, spectrum, decays, coupled channels, mesons, QCD PACS: 13.30.Eg, 11.80.Et, 14.40.Be, WHERE WE AREOne cannot look to the future of the baryon physics program without reviewing where we are at present. For the past 50 years we have sought to understand the spectrum of baryons. It is sometimes thought that spectroscopy is nothing more than stamp collecting, making pretty patterns, but providing few insights into the workings of the world. This is to misunderstand its fundamental importance. The spectrum of states of any system are determined by the constituents that make up that system and the forces that bind them together. Thus the spectrum provides us with insights into the fundamental degrees of freedom and into the nature of strong coupling QCD.Baryons have a special place in the panoply of hadrons, as their structure is most obviously related to the color degree of freedom. While a color singlet quark-antiquark system is basically the same however many colors there are, the minimum number of quarks in a baryon is intimately tied to the number of colors. If N c were five, the world would be quite different. Moreover the flavor pattern of baryons was a key ingredient in the development of the quark model. This simple model has long served as the paradigm for what we expect the baryon spectrum, both nucleons and ∆'s, to look like [1]. The most quoted template, Fig. 1, was provided by Capstick and Roberts [2], based on three independent quark degrees of freedom, Fig 2. While the lower lying states have been well determined by experiment, many of those above 1.6 GeV or so are missing. A possible reason for this could be that most of the early evidence was accumulated from πN scattering, and decays into the same channel. Perhaps these states are missing because we have not looked in the right place. Consequently, there has been a major effort to investigate other channels like ππN and KY . These are an increasing part of the πN total cross-section as the energy goes up. However, for the most part they have only contributed hints and glimpses of missing states and not yet too much new definitive evidence.