Studying Baryon Resonances with the CLAS Detector at Jefferson Lab

Dr. Phillip Cole
Department of Physics
Idaho State University

 An energetic particle, such as a photon, incident on a proton or neutron (nucleon) can interact directly with one of the quarks inside, causing the quark to undergo a flip in spin or endowing the quark with an orbital or radial excitation, and thus, by exciting the quarks to a higher energy state, the nucleon becomes more energetic, and hence more massive.  These excited states are called baryon resonances and are short lived, on the order of a billionth of a billionth of a second. These excited nucleons will dominantly decay into a ground-state nucleon producing other strongly-interacting particles (mesons), for example.  We can measure the final-state particles with detectors arranged about the interaction point. The types of mesons produced and how they are distributed in space in the decay process provides key information on the internal symmetries of the quarks in the nucleon. The study of these excited states is called spectroscopy. And just as ordinary optical spectroscopy proved to be the incisive tool for understanding the electronic structure of the elements, we expect nucleon spectroscopy will reveal many of the basic features of the quark substructure of matter, and, in turn, it will provide a critical testing ground for theoretical models describing these systems. 

The CLAS detector at Jefferson Lab in Newport New, Virginia is a unique instrument, which has provided the lion's share of the world's data on meson photo- and electroproduction in the resonance excitation region.   In this talk I will discuss the underlying physics ideas of baryon resonances and recent experimental results from the CLAS (CEBAF Large Acceptance Spectrometer) detector.