I read in a paper http://nngroup.physics.sunysb.edu/~nngroup/misc/Documents/NeutrinoReactionsAtAcceleratorEnergies.pdf that delta_I = 1/2 rule of the strangeness-changing weak current implies the ratio of cross section of proton->Ʃ0 process to neutron->Ʃ- process to be 1/2. The delta_I = 1/2 rule is mentioned just before equation (3.40) on page 317. I do not understand what is this selection rule, and how does it effect the cross section ratio of the 2 processes? Is delta_I = 1/2 rule is that the change in the isospin of initial and final particle which has to be 1/2 for these processes? These are the cabbibo suppressed 'charged current processes', where when a neutrino interacts with a nucleon, converts to a lepton producing a W-, which converts a u-quark in the nucleon to a s-quark. So this is a strangness changing process (delta_S = 1) where a 'proton converts to a Ʃ0' or a 'neutron converts to a Ʃ-', both processes are written in equation (3.39) and their cross section ratio is written to be 1/2 accodording to delta_I = 1/2 rule in equation (3.40) which I want to understand. Thanks much for the help!