document: Certainly therefore the see-saw mechanism is an attractive explanation of why the light neutrino masses are so small. However, it is not without its faults. In particular, there is a tension between the strongly hierarchical nature of the observed Yukawa couplings in the quark and charged lepton sectors, and the essentially hierarchy-free masses implied by the Δm2's. Moreover, both the θ12 and θ23 mixing angles are large while the angle θ13 is small which is in sharp contrast with the corresponding mixings in the quark sector which are all small. These problems can be solved in specific models, for example, the Δm2 values can be fitted by taking the spectrum of rhd neutrino masses to be hierarchical in such a way as to almost compensate for the hierarchical neutrino Yukawa couplings. But this has the price of introducing a wide range of rhd neutrino masses MR∼1010–1015 which then require explanation.
doc_bio_tags: O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O B I O O B O B I I O O O O O O O O O O O O B I I I I O O O B I I O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O B I I I O O O O O O O O O O O B I I B O O O O
extractive_keyphrases: charged lepton sectors hierarchical neutrino yukawa couplings light neutrino masses mr∼1010–1015 quark rhd neutrino masses the angle θ13 yukawa couplings θ12 and θ23 mixing angles
abstractive_keyphrases: hierarchy-free masses implied by the δm2's quark sector see-saw mechanism yukawa couplings in the quark and charged lepton sectors δm2 values can be fitted by taking the spectrum of rhd neutrino masses