Helicity dependence of the $ \gamma$ d $ \rightarrow$ $ \pi$ NN reactions in the $ \Delta$ -resonance region

The helicity dependence of the differential cross-section for the \( \gamma\) d \( \rightarrow\) \( \pi\) NN reactions has been measured for the first time in the \( \Delta\) -resonance region. The measurement was performed with the large-acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. The data show that the main reaction mechanisms for the \( \pi^{{\pm}}_{}\) NN channels are the quasi-free N \( \pi\) processes on one bound nucleon with nuclear dynamics playing a minor role. On the contrary, for the \( \pi^{0}_{}\) np channel nuclear mechanisms involving the reabsorption of the photoproduced \( \pi^{0}_{}\) by the np pair have to be taken into account to reproduce the experimental data.