This volume presents three papers by Bruno Pontecorvo translated from the original Russian edition. The physics results discussed in the papers have been obtained by Pontecorvo in the years 1955-56 at the synchrocyclotron of Dubna, in USSR, a proton accelerator that, at that time, was the most powerful particle-machine in the world. Thanks to the large energy available, important results have been obtained using secondary beams of charged pions interacting with hydrogen and deuterium. A detailed analysis of the data collected is presented in terms of the theoretical models available at that time, based on "Partial Waves Analysis" of the interaction cross sections. Moreover, a reanalysis of the negative-pion- proton cross-sections presented in the introduction, proves the existence of the ?0-baryon particle, a member of the baryonic decuplet of the SU(3) symmetry group. The ?-baryon family (?++, ?+, ?0, ?-) represented an important milestone for the following developments in particle ph.