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This is a contribution to the review 50 Years of Quantum Chromodynamics edited by F. Gross and E. Klempt, to be published in Journal EPJC. This contribution reviews the nucleon resonance transition form factors determined from meson electro-production experiments at electron accelerator facilities, i.e. this contribution focuses on space-like transition form factors and amplitudes. Comparisons are made when available to LQCD and to approaches with traceable links to strong QCD and to advanced quark model calculations, Comment: 14 pages, 12 figures

In this talk the role of spin and polarization is discussed in experimental search for new excited baryon states and in the study of the internal quark-gluon structure of the proton and neutron. Also the perspective of spin physics at the 12 GeV CEBAF electron accelerator is discussed and what we hope to learn about fundamental properties of hadrons such as their multi-dimensional structure through the momentum and spatial imaging, and about the forces on the quarks in the proton and how quark confinement may be realized through the spatial distribution of such forces., 8 pages, 12 figures, Invited talk at SPIN 2016

For half a century, the Roper resonance has defied understanding. Discovered in 1963, it appears to be an exact copy of the proton except that its mass is 50% greater. The mass is the first problem: it is difficult to explain with any theoretical tool that can validly be used to study quantum chromodynamics [QCD]. In the last decade, a new challenge has appeared, viz. precise information on the proton-to-Roper electroproduction transition form factors, reaching $Q^2\approx 4.5\,$GeV$^2$. This scale probes the domain within which hard valence-quark degrees-of-freedom could be expected to determine form factor behavior. Hence, with this new data the Roper resonance becomes a problem for strong-QCD [sQCD]. An explanation of how and where the Roper resonance fits into the emerging spectrum of hadrons cannot rest on a description of its mass alone. Instead, it must combine this with a detailed understanding of the Roper's structure and how that is revealed in the transition form factors. Furthermore, it must unify all this with a similarly complete picture of the proton. This is a prodigious task, but a ten-year international effort, drawing together experimentalists and theorists, has presented a solution to the puzzle. Namely, the Roper is at heart the proton's first radial excitation, consisting of a dressed-quark core augmented by a meson cloud that reduces the core mass by approximately 20% and materially alters its electroproduction form factors on $Q^2, 23 pages, 19 figures

An overview is presented of the upgrade of JLab's cw electron accelerator from a maximum beam energy of currently 6 GeV to 12 GeV. Construction of the 12 GeV upgrade project has started in 2008. A broad experimental program has been developed to map the nucleon's intrinsic correlated spin and momentum distribution through measurements of deeply exclusive and semi-inclusive processes, and to probe the quark and gluon confinement by studying the spectrum of mesons with exotic quantum numbers. Other programs include the forward parton distribution function at large xB, the quark and gluon polarized distribution functions, the measurements of electromagnetic form factors of the nucleon ground state and of nucleon resonance transitions at high Q2, and the exploration of physics beyond the Standard Model in high precision parity violating processes. The 12 GeV electron beam is also well suited to explore quark hadronization properties using the nucleus as a laboratory., 29 pages, 25 figures, Lecture given at the International School of Physics 'Enrico Fermi', 2011, Varenna, Italy

$\gamma_{v}NN^*$ transition helicity amplitudes (electrocouplings) of several prominent excited proton states are determined for the first time in independent analyses of $\pi^+n$, $\pi^0p$, and $\pi^+\pi^-p$ electroproduction off protons. Analysis of $\pi^+\pi^-p$ electroproduction has extended considerably information on electrocouplings of high lying N* states, which decay preferentially to the $N\pi\pi$ final states., Comment: Contributed talk at XIV International Conference on Hadron Spectroscopy, Hadron 2011, June 13-June 17 2011, Munich, Germany

Recent measurements with CLAS at Jefferson Lab of nucleon resonance transition form factors for several lower mass states are discussed., 5 pages, 4 figures, invited talk at the CIPANP 2009 conference, San Diego, CA

A brief overview of strangeness physics with the CLAS detector at JLab is given, mainly covering the domain of nucleon resonances. Several excited states predicted by the symmetric constituent quark model may have significant couplings to the K-Lambda or K-Sigma channels. I will discuss data that are relevant in the search for such states in the strangeness channel, and give an outlook on the future prospects of the N* program at JLab with electromagnetic probes., Comment: 9 pages, 7 figures, Conference on Strangeness in Nuclei, SENDAI 2008

An overview of the CLAS12 detector is presented and the initial physics program after the energy-doubling of the Jefferson Lab electron accelerator. Construction of the 12 GeV upgrade project has started October 2008. A broad program has been developed to map the nucleon's 3-dimensional spin and flavor content through the measurement of deeply exclusive and semi-inclusive processes. Other programs include forward distribution function to large $x_{B} \le 0.85$ and of the quark and gluon polarized distribution functions, as well as nucleon ground state and transition form factors at high $Q^2$. The 12 GeV electron beam and the large acceptance of CLAS12 are also well suited to explore hadronization properties using the nucleus as a laboratory, 9 pages, 14 figures, Workshop on RICH for CLAS12

An overview is presented on the program to study the nucleon structure at the 12 GeV JLab upgrade using the CLAS12 detector. The focus is on deeply virtual exclusive processes to access the generalized parton distributions, semni-inclusive processes to study transverse momentum dependent distribution functions, and inclusive spin structure functions and resonance transition form factors at high Q^2 and with high precision., Comment: 7 pages, 12 figures, NSTAR 2007 conference, Bonn, September 5-8, 2007

An overview of recent results with CLAS is presented with emphasis on nucleon resonance studies, nucleon spin structure, and generalized parton distributions., Comment: Plenary talk presented at NSTAR 2007, Bonn, Germany

The program to study the Generalized Parton Distributions in deeply virtual exclusive processes with CLAS at Jefferson Lab is discussed., added references

I discuss recent results on the electroproduction and photoproduction of mesons in the region of non-strange baryon resonances. Results on the quadrupole transition from the ground state nucleon to the Delta(1232) show the importance of pion cloud contributions to explain the observed quadrupole transition multipoles. The excitation of the "Roper" resonance is being studied in single pion production using unpolarized cross sections measurements and beam spin asymmetries. New results on the S_{11}(1535) transition formfactor give now a consistent picture over a large Q2 range. First results on electroproduction of p pi^+ pi^- show intruiging resonance structure. which is difficult to explain in an isobar model fit using known resonance properties, and may hint at excitation of a "missing" resonance. Searches for resonances in K^+Lambda also reveal resonance-like behavior. I briefly address a new avenue of N* physics using exclusive deeply virtual Compton scattering, recently measured for the first time at JLAB and at DESY., Replaced one figure, corrected reference, corrected typos

I discuss developments in the area of nucleon resonance excitations that are necessary to bring our understanding of nucleon structure in the regime of strong QCD to a qualitatively new level. They involve the collection of high quality data in various channels, a more rigorous approach in the search for "missing" quark model states, an effort to compute some critical quantities in nucleon resonance excitations from first principles, i.e. QCD, and a proposal aimed at obtaining an understanding of a fundamental quantity in nucleon structure., 9 pages, 2 figures

Status, open questions, and future prospects of the physics of excited nucleons are discussed. Emphasis is on the study of the structure of nucleons via measurements of their electromagnetic transition form factors, the search for "missing" resonances, the spin structure of the nucleon in the resonance region, and connections between the resonance and the deep-inelastic regimes., 13 pages, 7 figures, talk presented at NSTAR2000

We analyze the current status of three P-wave baryon states $N(1710){1/ 2}^+$, $N(1900){3/2}^+$, and $\Delta(1600){3/2}^+$ as given in the Review of Particles Physics (RPP). Since the evidence for a particle's existence is linked to its RPP "star" rating, we discuss its subjective present definition. We also present the accumulating evidence supporting these states and give our new "star" rating recommendations., Comment: 6 pages, 2 figures

This paper presents a comprehensive review of both the theory and experimental successes of Quantum Chromodynamics, starting with its emergence as a well defined theory in 1972-73 and following developments and results up to the present day. Topics include a review of the earliest theoretical and experimental foundations; the fundamental constants of QCD; an introductory discussion of lattice QCD, the only known method for obtaining exact predictions from QCD; methods for approximating QCD, with special focus on effective field theories; QCD under extreme conditions; measurements and predictions of meson and baryon states; a special discussion of the structure of the nucleon; techniques for study of QCD at high energy, including treatment of jets and showers; measurements at colliders; weak decays and quark mixing; and a section on the future, which discusses new experimental facilities or upgrades currently funded. The paper is intended to provide a broad background for Ph.D. students and postdocs starting their career. Some contributions include personal accounts of how the ideas or experiments were developed., Comment: Invited volume for the EJPC; 567 pages if text/figures and 4783 references occupying about 160 additional pages. arXiv abstract abridged, for the complete abstract please see the full text