Photo- and Electrocouplings of Nucleon Resonances

Abstract

Advances in the exploration of the spectrum and structure of the excited states of the nucleon from experiments with electromagnetic probes on proton targets are presented. Impressive progress has been achieved in the studies of exclusive meson photoproduction in experiments with continuous electron beams and with detectors of almost $4\pi$ acceptance. The high-quality data, coupled with the advances in the amplitude analyses of exclusive photo- and hadroproduction data, allow for the observation of several long-awaited new baryon states known previously as the ``missing" resonances. Studies of exclusive meson electroproduction in the resonance region with the CLAS detector at JLab have provided the dominant part of the available world information on exclusive meson electroproduction observables. These data offer unique information on the structure of most well-established excited nucleon states in the mass region up to 1.8~GeV in terms of the evolution of their electroexcitation amplitudes with momentum transfer of the virtual photon. We discuss the impact of these results on the insight into the strong interaction dynamics that underlie the generation of the full spectrum of nucleon resonances of distinctively different structure. These results shed light on the emergence of hadron mass, which is one of the most important and still open problems in the Standard Model. The extension of the nucleon resonance studies in the experiments of the 12-GeV era at JLab with the CLAS12 detector are outlined.

23 pages, 4 tables, 14 figures, lecture given at Baryon 21 School, October 18-22, 2021,Seville, https://indico.cern.ch/event/1026059/

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• Spectrum of light- and heavy-baryons
• The melting and abundance of open charm hadrons
• Natural constraints on the gluon-quark vertex
• Empirical Consequences of Emergent Mass
• Model Analysis of the $ep\to e{p}^{\prime }{\pi }^{+}{\pi }^{-}$ Electroproduction Reaction on the Proton
• Strong QCD Insights from Excited Nucleon Structure Studies with CLAS and CLAS12
• Dissecting nucleon transition electromagnetic form factors
• Strong evidence for nucleon resonances near 1900\,MeV
• Evidence for the $N^{\prime }(1720)3/2^{+}$ Nucleon Resonance from Combined Studies of CLAS ${\pi }^{+}{\pi }^{-}p$ Photo- and Electroproduction Data
• Jülich-Bonn-Washington Model for Pion Electroproduction Multipoles
• Dynamical diquarks in the ${\mathbf{\gamma }}^{(\ast )}\mathbf{p}\to \mathbf{N}(1535){\frac{1}{2}}^{-}$ transition
• Amplitude reconstruction from complete experiments and truncated partial-wave expansions
• Resonant contributions to inclusive nucleon structure functions from exclusive meson electroproduction data
• $N^{\ast }$ resonances from $K\Lambda$ amplitudes in sliced bins in energy
• A cryostat to hold frozen-spin polarized HD targets in CLAS: HDice-II
• A dispersive approach to pion photo- and electroproduction

Cited by in corpus (14)

• Precision Studies of QCD in the Low Energy Domain of the EIC
• The Present and Future of QCD
• Proton and pion distribution functions in counterpoint
• Electromagnetic Transition Form Factors of Baryon Resonances
• Schwinger mechanism for gluons from lattice QCD
• Nucleon axial form factor at large momentum transfers
• Strong Interaction Physics at the Luminosity Frontier with 22 GeV Electrons at Jefferson Lab
• Origin of the Proton Mass
• Resonant contributions to polarized proton structure functions
• Global Data-Driven Determination of Baryon Transition Form Factors
• Meson cloud contributions to the Dalitz decays of decuplet to octet baryons
• Combined analysis of the $\gamma n\to K^0{\Sigma }^0$ and $\gamma n\to K^{+}{\Sigma }^{-}$ reactions
• Polarized proton structure in the resonance region
• About the magnitude of the ${\gamma }^{\ast }N\to N(1520)$ transverse amplitudes near $Q^2=0$