The near threshold π- p →ηn reaction in an effective Lagrangian approach

The near threshold π- p →ηn reaction is studied within an effective Lagrangian approach and the isobar model. By considering the contributions from s- and u-channel nucleon pole and N*(1535) resonance, the total and differential cross-sections of the π- p →ηn reaction near threshold are calculated. Our theoretical results can fairly reproduce the current experimental data. It is also shown that while the center-of-mass energy lies in the range from the reaction threshold up to 1.65 GeV, s-channel N*(1535) resonance plays the dominant role. The effect from nucleon pole is found to be small but the interference terms between the N*(1535) resonance and the nucleon pole are significant. The contributions from t-channel processes are negligible in the present calculation.

PRODUCTION OF TWO PIONS INDUCED BY NEUTRINOS

We study the threshold production of two pions induced by neutrinos in nucleon targets. The contribution of nucleon pole, pion and contact terms is calculated using a chiral Lagrangian. The contribution of the Roper resonance, neglected in earlier studies, has also been taken into account.

Threshold pion photoproduction in the Skyrme model

Threshold photoproduction of pions on the nucleon is studied in the Skyrme model. Both the rotational and translational degrees of freedom of the Skyrmion are included in the calculations. It is found that in the case of neutral pion production there is a sizeable cancellation between the nucleon pole term and the anomalous contact term. This cancellation is a unique feature of the model and provides an important test of the validity of the model. A definitive conclusion must await a resolution of the existing controversies about the experimental data.