This paper reports the first results for how virtual photons excite several nucleon resonances in the so‑called third resonance region. The team measured π+π−p electroproduction with the CLAS detector at Jefferson Lab and analyzed the data for invariant mass W between 1.56 and 1.76 GeV and photon virtuality Q^2 (the virtual photon four‑momentum squared) between 2.0 and 5.0 GeV^2. From those data they extracted the γ_v p N* electrocouplings, which quantify the strength of the interaction that turns a proton into an excited nucleon state (an N*). For the first time electrocouplings are available at these Q^2 for resonances that mostly decay into two pions and a nucleon, and contributions from a previously reported new state, N′(1720) 3/2+, were seen for Q^2 below 5.0 GeV^2.

The researchers used detailed differential cross sections measured in many kinematic bins. Cross sections tell how often the final particles appear with given momenta and angles. They fit those cross sections with the Jefferson Lab–Moscow State University (JM) reaction model to separate resonant contributions from non‑resonant background. The analysis produced consistent electrocouplings for the N(1675) 5/2− and N(1680) 5/2+ when these resonances were studied independently in single‑pion (πN) and two‑pion (π+π−p) final states. That agreement supports the model’s ability to pull out resonance information from different reaction channels.

At a basic level, electrocouplings describe how a proton absorbs a virtual photon and becomes an excited state. The quantity Q^2 sets the probe’s resolution: larger Q^2 sees smaller internal distances. Theory and previous work indicate that above Q^2 ≈ 2 GeV^2 the internal structure probed is dominated by a three‑quark core rather than by surrounding meson‑baryon “cloud.” The new results therefore give experimental access to that more compact part of the resonance structure. Practically, the fits used five‑fold differential cross sections (five independent kinematic variables for the three final particles) and employed an improved method to account for regions where the detector had low acceptance.