The type-I seesaw mechanism combines a Dirac neutrino mass with a much larger right-handed-neutrino Majorana mass term . Diagonalizing the mass matrix gives a light eigenvalue of order and a heavy eigenvalue of order .
The Weinberg operator is the dimension-five Standard Model operator . After the Higgs field condenses, it gives a left-handed neutrino Majorana mass of order .
The type-II seesaw mechanism couples two lepton doublets to an electroweak scalar triplet. A small neutral-triplet vacuum expectation value directly generates a left-handed neutrino Majorana mass term.
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The Seesaw mechanism is a theoretical framework in particle physics that explains the small masses of neutrinos, which are fundamental particles involved in weak interactions. This mechanism is an extension of the Standard Model of particle physics and relates to the concept of lepton mixing and mass generation. In the Seesaw mechanism, it is proposed that there exist heavy neutrinos (sometimes called right-handed or sterile neutrinos) along with the known light neutrinos (the left-handed neutrinos).