Updates on the Studies of $${\varvec{N}}^\mathbf{*}$$ Structure with CLAS and the Prospects with CLAS12

Abstract

The recent results on γ v pN * electrocouplings from analyses of the data on exclusive meson electroproduction off protons measured with the CLAS detector at Jefferson Lab are presented. The impact of these results on the exploration of the excited nucleon state structure and non-perturbative strong interaction dynamics behind its formation is outlined. The future extension of these studies in the experiments with the CLAS12 detector in the upgraded Hall-B at JLab will provide for the first time γ v pN * electrocouplings of all prominent resonances at the still unexplored distance scales that correspond to extremely low (0.05 GeV 2 < Q 2 < 0.5 GeV 2 ) and to the highest photon virtualities (5.0 GeV 2 < Q 2 < 12.0 GeV 2 ) ever achieved in the exclusive electroproduction measurements. The expected results will address the most important open problems of the Standard Model: on the nature of more than 98% of hadron mass, quark-gluon confinement and the emergence of the excited nucleon state structure from the QCD Lagrangian, as well as allowing a search for the new states of hadron matter predicted from the first principles of QCD, the so-called hybrid baryons.Keywords exclusive meson electroproduction · nucleon resonance structure · non-perturbative strong interaction

Nucleon resonance contributions to unpolarized inclusive electron scattering

“…For those resonances which decay preferentially into the ππN final states, ∆(1620) 1/2 − , ∆(1700) 3/2 − , N (1720) 3/2 + , the values of β r. obtained in this way are in good agreement with the branching fractions for resonance decays into the ππN final states used in the extraction of the γ v pN * electrocouplings from π + π − p CLAS data [39,40,48]. Furthermore, as mentioned above, we include the candidate baryon state N (1720) 3/2 + , suggested in the analysis of the CLAS π + π − p photo-/electroproduction data off protons [39].…”

Baryons as relativistic three-quark bound states

“…2 led to significant activity aimed at understanding and resolving this discrepancy. It was noted early [16] that there was significant scatter between the results of different Rosenbluth extractions [11,31,32,33,34], as illustrated in Fig. 3, suggesting that the problem was related to the cross section measurements.…”

“…By comparison, the γ * p → N (1520) helicity amplitudes look rather ordinary and suggest a dominant three-quark nature; they are well described by quark models although quantitative agreement is only achieved when meson cloud effects are included. Results for several higher-lying resonances are also available, and the extension to the mass range up to 2.5 GeV as well as Q 2 up to 12 GeV 2 is part of the experimental N * program with CLAS12 at Jefferson Lab [32,33].…”

Structure of the Nucleon and Its First Radial Excitation

“…factors of the γ * p → R + reaction and observe that, while the mismatch in the domain of Q 2 2m 2 N may plausibly be attributed to meson-cloud effects, the agreement on Q 2 2m 2 N owes fundamentally to the QCD-derived momentum-dependence of the propagators and vertices employed in solving the bound-state and scattering problems. Let us finish this manuscript highlighting that novel experiments are approved at JLab 12, and others are either planned or under consideration as part of an international effort to measure transition electro-couplings at large photon virtualities of all prominent nucleon resonances [15,60,61]. Therefore, our predictions herein but also forthcoming analyses that will involve low-lying negative-parity baryons and excited states of the ∆-resonance will be thoroughly tested in the foreseeable future, and such efforts have the potential to deliver empirical information that would address a wide range of issues, including, e.g.…”