<em>K</em>⁺<em>Σ</em>⁻ Photoproduction at Forward Angles at the BGOOD Experiment

Abstract

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 <em>Λ</em>(1405) hyperon has finally been assigned a <em><SPAN STYLE="text-decoration:overline">K</SPAN>N</em> - 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 <em>P_c</em>(4312)⁺, <em>P_c</em>(4440)⁺ and <em>P_c</em>(4457)⁺ in the charm sector, which are located close to production thresholds of <em>Σ_c<SPAN STYLE="text-decoration:overline">D</SPAN></em> states. The same model that predicted those pentaquarks also predicts analogous structures in the strange sector such as the <em>N</em>^*(2030) resonance, which is thought to be a dynamically-generated <em>K</em>^*<em>Σ</em> state. For the reaction <em>γn</em> → <em>K</em>⁰<em>Σ</em>⁰, this state is expected to magnify the constructive interference of amplitudes driven by intermediate <em>K</em>^*<em>Λ</em> and <em>K</em>^*<em>Σ</em> channels and thus giving rise to a resonant peak in the spectrum just below the <em>K</em>^*<em>Σ</em> 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 <em>K</em>^*<em>Σ</em> threshold due to limited statistics. Similar structures might occur in the charge-conjugate channel <em>γn</em> → <em>K</em>⁺<em>Σ</em>⁻, providing the motivation for a measurement of this reaction using the BGOOD experiment at ELSA.<br /> The differential cross section of the reaction <em>γn</em> → <em>K</em>⁺<em>Σ</em>⁻ has been measured in an angular range of cos(<em>Θ</em>_CM) > 0.9 at centre-of-mass energies ranging from threshold up to <em>W</em> = 1942 MeV. Combining this dataset with LEPS data at higher energies reveals a prominent peak located at <em>M</em> = 1924 MeV with a width of <em>Γ</em> = 72 MeV. This is in good accordance with predictions of a <em>K</em>⁺<em>K</em>⁻<em>N</em> bound state just below the <em>K</em>⁺<em>K</em>⁻<em>N</em> threshold. The LEPS data shows a further jump in the differential cross section at <em>W</em> = 2040 MeV, which tends to agree with the previously acquired BGOOD data of <em>γn</em> → <em>K</em>⁰<em>Σ</em>⁰ 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.