Strong QCD Insights from Excited Nucleon Structure Studies with CLAS and CLAS12
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Daniel S. Carman · Kyungseon Joo · Victor I. Mokeev
Strong QCD Insights from Excited Nucleon Structure Studies with CLAS and CLAS12
Received: 1 July 2020 / Accepted: 30 July 2020 / Published online: 5 August 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract Studies of the spectrum of hadrons and their structure in experiments with electromagnetic probes offer unique insight into many facets of the strong interaction in the regime of large quark-gluon running coupling, i.e. the regime of strong QCD. The experimental program within Hall B at Jefferson Laboratory based on data acquired with the CLAS spectrometer using electron and photon beams with energies up to 6 GeV has already considerably extended the scope of research in hadron physics in joint efforts between experiment and phenomenological data analysis. Impressive progress in relating the hadron structure observables inferred from the data to the strong QCD mechanisms underlying hadron mass generation has been achieved in the past decade. These results will be considerably extended with data from the experimental program with the new CLAS12 spectrometer that has begun data taking using electron beams with energies up to 11 GeV. With this extended kinematic reach the structure of nucleon resonances will be probed at the highest photon virtualities ever achieved in the studies of exclusive electroproduction, which will allow for the exploration of the distance scale where >98% of light hadron mass emerges from QCD in the transition of the strong interaction from the regime of quark-gluon confinement to perturbative QCD.
1 Introduction Significant progress has been realized in studies of excited nucleon state (N ∗ ) structure from the data on exclusive meson electroproduction measured with the CLAS detector in Hall B at Jefferson Laboratory (JLab) [1–4]. The nucleon resonance electroexcitation amplitudes (i.e. the γv pN ∗ electrocouplings) have become available for most excited nucleon states in the mass range up to 1.8 GeV for photon virtualities Q 2 < 5 GeV2 . These studies offer unique information on the strong QCD dynamics that govern the generation of N ∗ states with different quantum numbers and distinctively different structural features. The description of the structure of the ground state nucleon and the N (1440)1/2+ resonance within the continuum Quantum Chromodynamics (QCD) approach with a traceable connection to QCD has demonstrated distinctive differences in their parton distribution amplitudes (PDAs) [5]. Studies of the resonant contributions to inclusive electron scattering with the resonance electroexcitation amplitudes from the CLAS data have revealed pronounced differences in their evolution with Q 2 for the invariant mass W of the final state hadrons within the first, second, and third resoD. S. Carman · V. I. Mokeev (B) Jefferson Laboratory, 12000 Jefferson Ave., Newport News, VA 23602, USA E-mail: [email protected] D. S. Carmanat E-mail: [email protected] K. Joo University of Connecticut, Storrs, CT 06269, USA E-mail
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