Study of the helicity dependence of double pion photoproduction on the proton

In this PhD thesis the helicity dependence of double pion photoproduction on the proton is studied. The subject is approached experimentally as well as theoretically. The main physical motivations are the study of the nucleon resonances and the mechanisms responsible for the observed double pion photoproduction cross sections. The emphasis is on the γp → pπ+π− and the γp → nππ processes. The obtained results are the first of their kind. The experimental part fits in with the Gerasimov-Drell-Hearn (GDH) experiment carried out at the accelerator facility MAMI in Mainz, Germany. This experiment allows to access doubly polarised observables for the pion photoproduction channels open at these energies. The data analysis for the pπ+π− and nππ photoproduction reactions on the proton is performed in two steps: the development of the analysis methods for unpolarised calibration data and the application of these methods to the doubly polarised data from the GDH experiment. The analysis methods for both reaction channels are well optimised in terms of statistical and systematical errors and the results for the unpolarised total cross sections are successfully verified with existing data. The total helicity cross section difference σ3/2 − σ1/2 and the separate helicity cross sections σ1/2 and σ3/2 are obtained for both reactions. For pπ+π−, the σ3/2 cross section is positive and exhibits a broad peak around 600–650 MeV, whereas σ1/2 is smaller and behaves more smoothly. The σ3/2 for the nππ has a strong resonant peak at about 750 MeV while σ1/2 is again smoother. Within the measured energy range, from threshold up to 800 MeV, the contributions of the pπ+π− and nππ reactions to the GDH sum rule are −(25.4 ± 1 ± 1.5) μb (statistical and systematical error) and −(11.3 ± 0.7 ± 0.7) μb, respectively. Their contributions to the forward spin polarisability are−(0.070 ± 0.004± 0.004) 10−4 fm and−(0.025 ± 0.003 ± 0.002) 10−4 fm. In the theoretical part, a Regge Plus Resonances (RPR) model for double pion photoproduction is presented. Regge trajectory exchange provides well behaved cross sections at higher energies, i.e. the cross sections decrease with a power of the energy. In the resonance region, resonant mechanisms are added to overcome the fact that the Regge description yields only an average over all resonances (duality). The description is unitary and gauge invariant. By comparing the model predictions with the data it was found that the pπ+π− process is dominated by the non-resonant π∆ production via π exchange. Extra contributions stem from the non-resonant ππN production via π exchange, the D13-resonant π∆ and ρN production and the non-resonant ρN production via ρ exchange. These mechanisms qualitatively describe the observed features in the measured cross sections. For the nππ reaction, the non-resonant π∆ production via π exchange is not as predominant and the other mechanisms are relatively more important, in particular the ones involving an intermediate ρ. The contribution of double pion production to the proton GDH sum rule and its forward spin polarisability is predicted to be about 25% and 12% of the total value, respectively. These predictions represent a calculation including non-resonant π∆ production and D13-resonant π∆ and ρN production.