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Exclusive $pi^+pi^-p$ photo- and electroproduction data from CLAS have considerably extended the information on the spectrum and structure of nucleon resonances. The data from the $pi^+pi^-p$ and $Npi$ channels have provided results on the electrocouplings of most resonances in the mass region up to 1.8 GeV and at photon virtualities up to 5.0 GeV$^2$. The recent CLAS data on $pi^+pi^-p$ photoproduction have improved knowledge on the photocouplings of nucleon resonances in the mass range of 1.6 GeV $<$ $M_{N^*}$ $<$ 2.0 GeV and on their decays to the $pi Delta$ and $rho p$ final hadron states. For the first time, the electrocouplings of the $N(1440)1/2^+$ and $N(1520)3/2^-$ excited states have become available from $pi^+pi^-p$ data at 2.0 GeV$^2$ $<$ $Q^2$ $<$ 5.0 GeV$^2$. Analyses of the combined $pi^+pi^-p$ photo- and electroproduction data have revealed evidence for the candidate-state $N(1720)3/2^+$. The new results on the nucleon resonance spectrum, electroexcitation amplitudes from analysis of the CLAS $pi^+pi^-p$ photo- and electroproduction data, and their impact on the exploration of strong QCD are presented.
We report on the first measurement of cross sections for exclusive deeply virtual pion electroproduction off the proton, $e p to e^prime n pi^+$, above the resonance region at backward pion center-of-mass angles. The $varphi^*_{pi}$-dependent cross s
A theory of two-pion photo- and electroproduction off the nucleon is derived considering all explicit three-body mechanisms of the interacting $pipi N$ system. The full three-body dynamics of the interacting $pipi N$ system is accounted for by the Fa
We present a calculation of pion photo- and electroproduction in manifestly Lorentz-invariant baryon chiral perturbation theory up to and including order q^4. We fix the low-energy constants by fitting experimental data in all available reaction chan
We discuss the extended on-mass-shell scheme for manifestly Lorentz-invariant baryon chiral perturbation theory. We present a calculation of pion photo- and electroproduction up to and including order $q^4$. The low-energy constants have been fixed b
Studies of the nucleon resonance electroexcitation amplitudes in a wide range of photon virtualities offer unique information on many facets of strong QCD behind the generation of all prominent excited nucleon states. Advances in the evaluation of re