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

Ghosal, Debdeep. Measurement of the double polarization observable E for the photoproduction of $\pi^{0} \pi^{+/-}$ pairs off deuteron and dButanol targets. 2021, Doctoral Thesis, University of Basel, Faculty of Science.

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Official URL: https://edoc.unibas.ch/87906/

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.