The impact of $γN$ and $γ^* N$ interactions on our understanding of nucleon excitations

TL;DR

This review synthesizes advances in nucleon and Δ resonance spectroscopy from γN and γ*N interactions, spanning experimental data, partial-wave analyses, and state-of-the-art theory. It juxtaposes quark-model classifications with dynamical meson-baryon effects, highlighting how electroproduction reveals a dual picture: a short-distance three-quark core that dominates at high $Q^2$ and a peripheral meson-baryon cloud shaping low-$Q^2$ behavior. Key findings include the Roper N(1440)1/2+ as a three-quark core with strong meson-cloud dressing, evidence for a rich high-mass spectrum with many states beyond naive quark-model expectations, and the Lambda(1405) as a dynamically generated, two-pole structure with significant MB components. The review also discusses how lattice QCD, functional methods, and AdS/QCD-inspired holography contribute to a coherent, QCD-grounded understanding of baryon structure, while acknowledging open questions about hybrids, pentaquarks, and the precise balance between quark-core and MB dynamics across resonances. The synthesis underscores the necessity of interdisciplinary approaches and future high-$Q^2$ electroproduction and lattice studies to unravel the origins and nature of excited baryons and their transitions, with implications for our understanding of confinement and hadron mass generation.

Abstract

We review recent progress in our understanding of the nucleon excitation spectrum. Thanks to dedicated efforts at facilities such as ELSA, MAMI and Jefferson Lab, several new nucleon resonances have been discovered, and evidence for previously elusive states has been significantly improved. Numerous decay channels have been observed for the first time, and resonance properties are being extracted from these data by several groups through coupled-channel analyses of varying complexity. Electroproduction experiments have provided further insights into the internal structure of light baryon resonances -- for example, the long-debated Roper resonance $N(1440)$ is observed as a three-quark state with a significant meson-cloud component.While the non-relativistic quark model remains a valuable tool for organizing the spectrum of nucleon and $Δ$ resonances, a variety of theoretical frameworks have emerged to offer deeper understanding, including phenomenological quark models, holographic QCD, functional methods, effective field theories, and lattice QCD. We examine the interplay between these approaches, highlight their respective strengths and explore how they complement each other in shaping our knowledge of light baryon resonances. We address several open questions in baryon spectroscopy, including the nature of the enigmatic $Λ(1405)$, ongoing searches for exotic states such as hybrid baryons and pentaquarks, and the dichotomy between microscopic descriptions of baryons in terms of quarks and gluons versus effective hadronic descriptions based on meson-baryon dynamics.

Figures (65)

• Figure 1.1: (color online) The strong interaction coupling constant, covering the non-perturbative to perturbative regions. References to the data can be found in Deur:2022msf.
• Figure 2.1: Comparison of PWA results on pole positions ($M$ and $\Gamma$) of $N^*$ (left) and $\Delta^*$ (right) resonances, including JüBo Ronchen:2022hqk, BnGa CLAS:2024iirSarantsev:2025lik, Kent Hunt:2018wqz, SAID Arndt:2006bf, and the RPP 2024 ParticleDataGroup:2024cfk. Graphics credit: Ulrike Thoma.
• Figure 2.1: $\chi^2$ per data point for the consistency of the results on the pole positions of 4* resonances from different PWAs with the RPP mean values within the estimated ranges. BnGa = Bonn-Gatchina, JüBo = Jülich-Bonn, Gi = Gieß en, CM = Carnegie-Mellon, PiA = Pittsburgh-Argonne, KH = Karlsruhe-Helsinki. In most analyses, $N_{\rm data}=28$ or 30.
• Figure 2.2: (color online) Light and strange baryon spectrum from the PDG ParticleDataGroup:2024cfk. The columns correspond to different $J^P$ and the colors to isospin and hypercharge. The different font weights represent four-, three-, two- and one-star resonances.
• Figure 2.3: (Color online) Modeling the transition of the universe from the quark-gluon phase to the hadron phase during the first microseconds after the Big Bang. Left: Ratio of strangeness to baryon chemical potential near the critical point in the transition. The shaded area shows the projections from Lattice QCD. The dashed lines represent the hadron resonance gas model projections with all known hadrons as cataloged by the PDG in the RPP editions in 2012 (red dashed-dotted) and 2016 (green dashed-dotted), including $^{***}$ and $^{****}$ states. Right: The $N^*$ and $\Delta^*$ resonances are the additional 2016 states with only $^{*}$ and $^{**}$ states, which are unconfirmed. If a number of those could be confirmed experimentally they could have a significant impact on a more complete understanding of this fundamental hadron phase transition, as the black dashed line indicates. The graphics is adapted from Chatterjee:2017yhp.