This thesis presents measurements of spin-density matrix elements in unpolarised π+π − photoproduction on a proton target. The dominant resonance contribution to the dipion system is the ρ(770) meson. Due to the large production cross section for this resonance, a valid comparison can be made between the obtained final results and ρ(770) spin-density matrix elements measured previously with other experiments. The measurement was performed over the 3.0 – 11.6 GeV beam energy regime, which is a more extensive energy range than has ever been studied previously for the ρ(770). Results were obtained by analysing data from the GlueX experiment based at Jefferson Lab.
Extended maximum likelihood fits were applied to extract three spin-density matrix elements using Markov chain Monte Carlo based parameter estimations. This was performed using various binning configurations to probe the energy, mass, and four-momentum transfer dependence of the determined physics observables. Spin-density matrix elements are shown to be consistent with the model of s-channel helicity conservation at low −t. The effects of pomeron and f2 exchanges are clearly visible in the energy dependence of the measured observables. These observations provide valuable insights into the relative strengths of both processes as a function of the photon energy, and may enable theorists to disentangle the f2/P coupling ratio. Spin-density matrix elements are seen to be highly dependent on the reconstructed resonance mass. This observation is likely to be a result of non-resonant S-wave background processes, and emphasises the need for a more detailed model of the π +π − angular distribution that considers all of the competing angular momentum components that contribute to the measured final state.
The statistical precision of measurements performed for this thesis surpass what was achievable in previous studies of the ρ(770) by several orders of magnitude. Studies of the energy and four-momentum transfer dependence, and insights into the effects of non ρ(770) background contributions provide valuable input for production models. This will help inform the choice of wave-sets used for partial wave analyses, supporting GlueX in its search for exotic hybrid meson states
