Already when the quark model was introduced in 1964, Gell-Mann suggested that hadrons might be composed of more than three quarks. This idea is experiencing a renaissance during the last decade. In 2016, the Λ(1405) hyperon has finally been assigned a KN - meaning five-quark - molecular-like structure by the Particle Data Group, based on overwhelming evidence from experiment and theory. Since 2015, LHCb unambiguously revealed the pentaquarks Pc(4312)+, Pc(4440)+ and Pc(4457)+ in the charm sector, which are located close to production thresholds of ΣcD states. The same model that predicted those pentaquarks also predicts analogous structures in the strange sector such as the N*(2030) resonance, which is thought to be a dynamically-generated K*Σ state. For the reaction γn → K0Σ0, this state is expected to magnify the constructive interference of amplitudes driven by intermediate K*Λ and K*Σ channels and thus giving rise to a resonant peak in the spectrum just below the K*Σ threshold. In the past, corresponding differential cross section data has been measured at BGOOD but no firm conclusion was possible regarding a peak around the K*Σ threshold due to limited statistics. Similar structures might occur in the charge-conjugate channel γn → K+Σ−, providing the motivation for a measurement of this reaction using the BGOOD experiment at ELSA. The differential cross section of the reaction γn → K+Σ− has been measured in an angular range of cos(ΘCM) > 0.9 at centre-of-mass energies ranging from threshold up to W = 1942 MeV. Combining this dataset with LEPS data at higher energies reveals a prominent peak located at M = 1924 MeV with a width of Γ = 72 MeV. This is in good accordance with predictions of a K+K−N bound state just below the K+K−N threshold. The LEPS data shows a further jump in the differential cross section at W = 2040 MeV, which tends to agree with the previously acquired BGOOD data of γn → K0Σ0 in search for the peak predicted analogously to the LHCb pentaquarks. These findings corroborate the possibility of the existence of dynamically-generated molecular-like states near production thresholds in the strange sector.
Johannes Groß (Tue,) studied this question.