Spectroscopy of Λ hypernuclei

The composite nature of baryons manifests itself in the existence of a rich spectrum of excited states, in particular in the important mass region 1–2 GeV for the light-flavoured baryons. The properties of these resonances can be identified by systematic investigations using electromagnetic and strong probes, primarily with beams of electrons, photons, and pions. After decades of research, the fundamental degrees of freedom underlying the baryon excitation spectrum are still poorly understood. The search for hitherto undiscovered but predicted resonances continues at many laboratories around the world. Recent results from photo- and electroproduction experiments provide intriguing indications for new states and shed light on the structure of some of the known nucleon excitations. The continuing study of available data sets with consideration of new observables and improved analysis tools have also called into question some of the earlier findings in baryon spectroscopy. Other breakthrough measurements have been performed in the heavy-baryon sector, which has seen a fruitful period in recent years, in particular at the B factories and the Tevatron. First results from the large hadron collider indicate rapid progress in the field of bottom baryons. In this review, we discuss the recent experimental progress and give an overview of theoretical approaches.

https://iopscience.iop.org/article/10.1088/0034-4885/76/7/076301/pdf

Progress towards understanding baryon resonances.

The reactions gamma p --> K+ Lambda and gamma p --> K+ Sigma0 were measured in the energy range from threshold up to a photon energy of 2.6 GeV. The data were taken with the SAPHIR detector at the electron stretcher facility, ELSA. Results on cross sections and hyperon polarizations are presented as a function of kaon production angle and photon energy. The total cross section for Lambda production rises steeply with energy close to threshold, whereas the Sigma0 cross section rises slowly to a maximum at about E_gamma = 1.45 GeV. Cross sections together with their angular decompositions into Legendre polynomials suggest contributions from resonance production for both reactions. In general, the induced polarization of Lambda has negative values in the kaon forward direction and positive values in the backward direction. The magnitude varies with energy. The polarization of Sigma0 follows a similar angular and energy dependence as that of Lambda, but with opposite sign.

Measurement of gamma p --> K+ Lambda and gamma p --> K+ Sigma0 at photon energies up to 2.6 GeV

Elastic πN scattering and the world data of the family of reactions π
 −
 p → ηn, K
 0
 Λ, K
 0
 Σ
 0, K
 +
 Σ
 −, and π
 +
 p → K
 +
 Σ
 + are described simultaneously in an analytic, unitary, coupled-channel approach. SU(3) flavor symmetry is used to relate both the t - and the u - channel exchanges that drive the meson-baryon interaction in the different channels. Angular distributions, polarizations, and spin-rotation parameters are compared with available experimental data. Partial-wave amplitudes are determined and the resonance content is extracted from the analytic continuation, including resonance positions and branching ratios, and possible sources of uncertainties are discussed. The results provide the final-state interactions for the ongoing analysis of photo- and electroproduction data.

Coupled-channel dynamics in the reactions πN → πN, ηN, KΛ, KΣ

The reactions $\gamma p \rightarrow K^ + \Lambda$
 and $\gamma p \rightarrow K^ + \Sigma^0$
 were measured in the energy range from threshold up to a photon energy of 26 GeV The data were taken with the SAPHIR detector at the electron stretcher facility, ELSA Results on cross-sections and hyperon polarizations are presented as a function of the kaon production angle and the photon energy The total cross-section for $\Lambda$
 production rises steeply with energy close to threshold, whereas the $\Sigma^0$
 cross-section rises slowly to a maximum at about $E_\gamma = 145$
 GeV Cross-sections together with their angular decompositions into Legendre polynomials suggest contributions from resonance production for both reactions In general, the induced polarization of $\Lambda$
 has negative values in the kaon forward direction and positive values in the backward direction The magnitude varies with energy The polarization of $\Sigma^0$
 follows a similar angular and energy dependence as that of $\Lambda$
 , but with opposite sign

Measurement of gamma p ---> K+ Lambda and gamma p ---> K+ Sigma0 at photon energies up to 2.6-GeV

Measurement of γp→K+Λ and γp→K+Σ0 at photon energies up to 2 GeV

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Measurement of γp→K+Λ and γp→K+Σ0 at photon energies up to 2 GeV