sterile neutrino and neutrino mass in Seesaw framework

Neutrino masses and right-handed neutrino always was a blind spot for Standard Model of particle physics. Discovering neutrino oscillations and therefore nonzero neutrino mass predictions opened the door for physics beyond SM. One of the most efficient and natural method for generating light neutrino masses along with sterile right-handed neutrinos is “Seesaw” mechanism (first introduced by Gell-Mann, Ramond, Slansky). Here we discuss most generic form of seesaw model constrained by four assumptions and with neutrino oscillation data entries, which is most fundamental but not ideal for phenomenology standpoint, therefore we briefly discuss few other alternative types of seesaw mechanism.