  The analysis of the nine 1-fold differential cross sections for the
$\gamma_{r,v} p \to \pi^+\pi^-p$ photo- and electroproduction reactions
obtained with the CLAS detector at Jefferson Laboratory was carried out with
the goal to establish the contributing resonances in the mass range from
1.6~GeV to 1.8~GeV. In order to describe the photo- and electroproduction data
with $Q^2$-independent resonance masses and hadronic decay widths in the $Q^2$
range below 1.5~GeV$^2$, it was found that an $N'(1720)3/2^+$ state is required
in addition to the already well-established nucleon resonances. This work
demonstrates that the combined studies of $\pi^+\pi^-p$ photo- and
electroproduction data are vital for the observation of this resonance. The
contributions from the $N'(1720)3/2^+$ state and the already established
$N(1720)3/2^+$ state with a mass of 1.745~GeV are well separated by their
different hadronic decays to the $\pi \Delta$ and $\rho p$ final states and the
different $Q^2$-evolution of their photo-/electroexcitation amplitudes. The
$N'(1720)3/2^+$ state is the first recently established baryon resonance for
which the results on the $Q^2$-evolution of the photo-/electrocouplings have
become available. These results are important for the exploration of the nature
of the ``missing'' baryon resonances.
