I read in a paper that delta_I = 1/2 rule of the strangeness-changing weak current implies the ratio of cross section of proton->Sigma0 process to neutron->Sigma- process to be 1/2. 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 the change in the isospin of initial and final particle which has to be 1/2 for these processes? Will be great if somebody could explain. Thank you.