Measurement of the double polarization observable E for the photoproduction of $\pi^{0} \pi^{+/-}$ pairs off deuteron and dButanol targets

The binding of nucleons to nuclei and the formation of quarks to nucleons or other hadrons, is governed by the strong interaction. Quantum Chromodynamics (QCD), is well established as a theory of strong interaction and has a characteristic property that the coupling constant decreases as a function of the momentum transfer (energy). In high-energy regime, quarks and gluons behave as free particles and interact only weakly; here the coupling constant is small. Thus perturbative QCD at high energies can deal with the interations of quarks and gluons. However, at lower energies, the quarks interact strongly and virtual gluons can produce gluon-gluon pairs. Due to the large coupling constant here, perturbative calculations of QCD are meaningless and fail to explain the confinement. To verify QCD models at low energies, study of nucleon excitation spectrum is of particular interest. However, over the past years comparison of the model predictions with the experimentally observed states have shown a large discrepancy in number and ordering of the levels. Many more states have been predicted than they have been actually experimentally observed. This issue is known as the problem of missing resonances. This mismatch can either originate from the effective degrees of freedom of the models or due to the experimental bias.
In the beginning of hadron spectroscopy, results were obtained from pion-nucleon scattering experiments mostly. However, since the intermediate nucleon resonance depends on the production mechanism, mostly resonances which couple to the $\pi N$ have been observed so far. To compensate this, in the last decades those results have been supplemented with data of unpolarized cross sections obtained from photoproduction of mesons at various acceleration facilities. Nevertheless, the problem of missing resonances persists because of the many broadly overlapping resonances. Therefore, current experiments focus more on the measurement of single and double polarization observables, which may improve the situation. Observables are actually model-independent and sensitive to interference terms that can enhance the weakly contributing suppressed resonances.
Photoproduction of $\pi^0\pi^{\pm}$-pairs from quasi-free nucleon bound in the deuterons has been investigated in view of the helicity dependence of these two reactions. While measurements with a liquid deuterium target were used to extract the unpolarized cross sections, along with that a deuterated longitudinally polarized solid butanol target together with a circularly polarized photon beam helped in calculating the observable E. With these results the spin-dependent cross sections $\sigma_{1/2}$ and $\sigma_{3/2}$ corresponding to the anti-parallel and parallel spin configurations of the photon beam and target nucleon have been derived. The measurements were done at the Mainz MAMI accelerator facility with tagged photon beams produced by bremsstrahlung process for, a longitudinally polarized 1.5 GeV electron beam. The reaction products from the three different target types (deuteron, dbutanol and carbon) were detected with an almost 4$\pi$ solid angle covering calorimeter comprising of Crystal Ball and TAPS detectors along with the charged particle identifier plastic scintillators. The analysis results are sensitive to sequential decays of nucleon resonances via intermediate states and also to the decay of nucleon resonances by emission of charged $\rho$ mesons. Furthermore, the results with unpolarized target have been compared to the available previous data sets while the polarized results have been compared with the recent MAID model prediction.