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Relativistic constituent quark models generally describe three-quark systems with particular interactions. The corresponding invariant mass eigenvalue spectra and pertinent eigenstates should exhibit the multiplet structure anticipated for baryon resonances. Taking into account the flavour content, spin structure, and spatial distribution of the baryon wave functions together with mass relations of the eigenvalues and decay properties of the eigenstates, we can link the theoretical mass eigenstates with the experimentally measured resonances. The resulting classification of baryon resonances differs in some respects from the one suggested by the Particle Data Group. With regard to the hadronic decay widths of light and strange baryon resonances a consistent picture emerges only, if the classification includes two-star resonances.
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The description of baryon resonance decays represents a major challenge of strong interaction physics. We will report on a relativistic approach to mesonic decays of light and strange baryon resonances within constituent quark models. The calculations are performed in the point-form of relativistic quantum mechanics, specifically focussing on the strange sector. It is found that the relativistic predictions generally underestimate the experimental data. The nonrelativistic approximation of the approach leads to the decay operator of the elementary emission model. It is seen that the nonrelativistic reduction has considerable effects on the decay widths.
Constituent quark models provide a reasonable description of the baryon mass spectra. However, even in the light- and strange-flavor sectors several intriguing shortcomings remain. Especially with regard to strong decays of baryon resonances no consistent picture has so far emerged, and the existing experimental data cannot be explained in a satisfactory manner. Recently first covariant calculations with modern constituent quark models have become available for all pi, eta, and K decay modes of the low-lying light and strange baryons. They generally produced a remarkable underestimation of the experimental data for partial decay widths. We summarize the main results and discuss their impact on the classification of baryon resonances into flavor multiplets. These findings are of particular relevance for future efforts in the experimental investigation of baryon resonances.
We study the formation of baryons as composed of quarks and diquarks in hot and dense hadronic matter in a Nambu--Jona-Lasinio (NJL)--type model. We first solve the Dyson-Schwinger equation for the diquark propagator and then use this to solve the Dyson-Schwinger equation for the baryon propagator. We find that stable baryon resonances exist only in the phase of broken chiral symmetry. In the chirally symmetric phase, we do not find a pole in the baryon propagator. In the color-superconducting phase, there is a pole, but is has a large decay width. The diquark does not need to be stable in order to form a stable baryon, a feature typical for so-called Borromean states. Varying the strength of the diquark coupling constant, we also find similarities to the properties of an Efimov states.
The stopping of baryons in heavy ion collisions at beam momenta of $p_{rm lab} = 20-160A$ GeV is lacking a quantitative description within theoretical calculations. Heavy ion reactions at these energies are experimentally explored at the Super Proton Synchrotron (SPS) and the Relativistic Heavy Ion Collider (RHIC) and will be studied at future facilities such as FAIR and NICA. Since the net baryon density is determined by the amount of stopping, this is the pre-requisiste for any investigation of other observables related to structures in the QCD phase diagram such as a first-order phase transition or a critical endpoint. In this work we employ a string model for treating hadron-hadron interactions within a hadronic transport approach (SMASH, Simulating Many Accelerated Strongly-interacting Hadrons). Free parameters of the string excitation and decay are tuned to match experimental measurements in elementary proton-proton collisions, where some mismatch in the $x_F$ distribution of protons is still present. Afterwards, the model is applied to heavy ion collisions, where the experimentally observed change of the shape of the proton rapidity spectrum from a single peak structure to a double peak structure with increasing beam energy is reproduced. Heavy ion collisions provide the opportunity to study the formation process of string fragments in terms of formation times and reduced interaction cross-sections for pre-formed hadrons. A good agreement with the measured rapidity spectra of protons and pions is achieved while insights on the fragmentation process are obtained. In the future, the presented approach can be used to create event-by-event initial conditions for hybrid calculations.
We present results for kaon decay widths of baryon resonances from a relativistic study with constituent quark models. The calculations are done in the point-form of Poincare-invariant quantum mechanics with a spectator-model decay operator. We obtain covariant predictions of the Goldstone-boson-exchange and a variant of the one-gluon-exchange constituent quark models for all kaon decay widths of established baryon resonances. They are generally characterized by underestimating the available experimental data. In particular, the widths of kaon decays with increasing strangeness in the baryon turn out to be extremely small. We also consider the nonrelativistic limit, leading to the familiar elementary emission model, and demonstrate the importance of relativistic effects. It is found that the nonrelativistic approach evidently misses sensible influences from Lorentz boosts and some essential spin-coupling terms.
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