N* Experiments and what they tell us about Strong QCD Physics

Abstract

I give an overview on experimental studies of the spectrum and the structure of the excited states of the nucleon and what we can learn about their in ternal structure. One focus is on the efforts to obtain a more complete picture of the light-quark baryon exci tation spectrum employing electromagnetic beams that will allow us to draw some conclusions on the symme tries underlying the spectrum. For the higher mass ex citations, the full employment of coupled channel ap proaches is essential when searching for new excited states in the large amounts of data already accumulated in different channels involving a variety of polarization observables. The other focus is on the study of transition form factors and helicity amplitudes and their de pendences on Q2, especially on some of the more promi nent resonances, especially Δ(1232)3/2+, N(1440)1/2+, and negative parity states N(1535)1/2-, and N(1675)5/2-.These were obtained in pion and eta electroproduction experi ments off proton targets and have already led to further insights in the active degrees-of-freedom as a function of the distance scale involved.

Photo- and Electrocouplings of Nucleon Resonances

“…Studies of meson electroproduction in the nucleon resonance region with the CLAS detector provided the first and only available results on the electrocouplings of most nucleon resonances in the mass range up to 1.8 GeV [4,5,16]. These results allow for the evaluation of the resonant contributions into inclusive electron scattering from the experimental data on the electrocouplings [17,18,19].…”

“…The impressive progress achieved in the last decade with the measurements of the photoproduction differential cross sections and polarization asymmetries [4,20,14] has provided a dataset for "complete" measurements. Combined analyses of these observables makes it possible to determine the pseudoscalar meson-baryon photoproduction amplitudes for the πN , KΛ, and KΣ channels directly from the experimental data.…”

“…The continuous, multi-GeV electron beam from the CEBAF accelerator, combined with a detector of nearly 4π acceptance, provides the best opportunity for the exploration of nucleon resonance structure from the results on the evolution of the nucleon resonance electroexcitation amplitudes (γ v pN * electrocouplings) as a function of Q 2 . The extensive research efforts in progress in Hall B at JLab are focused on the extraction of the amplitudes of all prominent nucleon resonances in the mass range W < 2.5 GeV and 0.05 GeV 2 < Q 2 < 10 GeV 2 from both the independent and combined studies of the different meson electroproduction channels [1,2,4,5,6]. Currently, the new CLAS12 detector, installed in Hall B as the part of the JLab 12-GeV upgrade, is the only available and foreseen facility in the world capable of providing information on the electrocouplings of all prominent resonances in the almost unexplored range Q 2 > 5 GeV 2 [1,59].…”

“…The development of such a reaction model for the extraction of the electrocouplings from the available KY electroproduction data represents an urgent need for advancing the exploration of the spectrum and structure of the higherlying nucleon resonances. The exclusive π + π − p and KY channels look very promising in the search for new baryon states in the combined studies of meson photo-and electroproduction data [4,5].…”

“…The experimental results on the N * spectrum shed light on the approximate symmetries of strong QCD that are relevant for the generation of these states. The full N * spectrum, including those states already observed and those that are still to be discovered, defines the rate for the transition from the primordial deconfined mixture of quarks and gluons into the hadron gas phase that took place within the first microseconds after the Big Bang [3,4]. In this phase transition, quark-gluon confinement emerged and the masses of hadrons were generated in close connection with the dynamical breaking of the approximate chiral symmetry of QCD.…”