Measurement of the <em>γn</em>→K⁰Σ⁰ photoproduction at the BGOOD experiment and the relation to possible pentaquark states

Abstract

Hadrons are particles consisting of quarks bound by the strong force. Ground states and lower lying excitations can be described successfully as mesons consisting of quark-antiquark pairs and baryons consisting of three quarks. At higher excitation energies however, discrepancies between the theoretical description and the experimental observations become obvious. Multi-quark structures consisting of <em>qq<u style="text-decoration:overline;">qq</u></em> or <em>qqqq<u style="text-decoration:overline;">q</u></em> could explain the observations. These could be realized as compact colour bound structures or meson-meson or meson-baryon type systems in analogy to the binding of nucleons in a nucleus. Experimental results have confirmed tetra-quark and penta-quark objects to exist with a minimal quark content of <em>u<u style="text-decoration:overline;">u</u>c<u style="text-decoration:overline;">c</u></em> (X(3872) discovered at Belle) and <em>uudc<u style="text-decoration:overline;">c</u></em> (P_C states discovered at LHCb) respectively.<br /> Also in the <em>uds</em>-quark sector interesting observations have been made. A cusp in the cross section of the reaction &#947;p&rarr;K⁰&#931;⁺ could be explained by the same model that predicted the abovementioned penta-quark states at LHCb. The structure is described as an interference of intermediate K^*&#923; and K^*&#931; states, amplified by the presence of a vector meson-baryon dynamically generated K^*&#931; resonance, the N^*(2030). This resonance is the strange quark analogue to the P_C states in the charm quark sector. From this model a prediction is made for the reaction on the neutron &#947;n&rarr;K⁰&#931;⁰. The destructive interference leading to a cusp in &#947;p&rarr;K⁰&#931;⁺ becomes constructive and produces a peak in &#947;n&rarr;K⁰&#931; ⁰. An observation of this peak would be a smoking gun signal for meson-baryon type resonances to exist not only in the charm quark sector, but also in the strange sector.<br /> In this thesis the differential cross section of &#947;n&rarr;K⁰&#931;⁰ is measured at BGOOD at ELSA from threshold to a beam energy of 2600 MeV . A peak is observed at 1750 MeV in the angular range <nobr>0.2&lt;&#952;^K _CM&lt;0.5</nobr>. The available statistics prevent a final claim of evidence, also the structure originating from “conventional” resonances can not be strictly ruled out. However, shape and position are consistent with the model of the vector-meson baryon dynamically generated state. The enhancement being real is further supported by findings on the photoproduction of K⁺&#923;(1405). A structure in the cross section is described by a triangle singularity fed by the same N^*(2030) K^*&#931; type resonance, indicating that also the enhancement observed in the &#947;n&rarr;K⁰&#931;⁰ cross section is caused by this predicted resonance.