Experimental study of polarisation observables T, P and H in π0 and η photoproduction off quasifree nucleons with the Crystal Barrel/TAPS setup

All visible matter is made up of elementary particles and their interactions. The four fundamental interactions are gravitational, electromagnetic, weak and strong interaction. On the level of nucleons, i.e. protons and neutrons, the strong interaction acting on their constituents, the quarks and gluons, dominates. The goal of this work is a small contribution to a better understanding of this strong interaction.
Quantum chromodynamics (QCD) is the underlying gauge theory describing that interaction within the Standard Model of particle physics (SM). In contrast to the high-energy regime, the low-energy regime cannot be treated perturbatively, which makes theoretical calculations very difficult. Excitation spectra of nucleons are a vital testing ground for comparing experimental findings with theoretical predictions.
During the last two decades, much progress has been made on the theory side, e.g. lattice gauge methods, and in experiments. In particular, energy-tagged photon beams at electron accelerators have become important. They have reached a state where asymmetries measured with polarised photons and polarised targets allow for detailed partial wave analyses (PWA). However, the predicted excited states are not in agreement with experimental observations.
The present experiment was conducted at the Electron Stretcher Accelerator (ELSA) in Bonn, Germany, in 2018 and 2021 with the (almost) 4π covering Crystal Barrel/TAPS detector setup. The incident electron beam of Eγ = 3.2 GeV energy impinges on a diamond radiator, where it produces coherent bremsstrahlung photons with a linear polarisation at a coherent edge of Eγ = 1.2 GeV. The photons again impinge on a transversely polarised, deuterated butanol target. This allows the simultaneous measurement of the beam asymmetry Σ_but, target asymmetry T, recoil asymmetry P and the beam-target double polarisation observable H. Analysed are the final states π0p, π0n, ηp and ηn in exclusive reactions. The mesons are reconstructed from photons by the decay channels π0 → 2γ, η → 2γ and η → 3π0 → 6γ.
It is the first time that results for the observables T, P and H are shown for π0n and ηn, which improves the quality, contributions and understanding of partial waves in the centre of mass energy range from W = 1.5 GeV to 2.5 GeV. The reactions on the proton and the beam asymmetry of butanol show an excellent agreement with existing data and improve the quality of the world data.
Of special interest is the previously observed narrow structure in ηn around W = 1.68 GeV that has not been observed in ηp, which has led to many speculations about its origin. The results from this experiment strongly prefer an interpretation as interference of two S waves as its origin and, thus, no new resonance.