1. What if is very small ? C.S. Lam, McGill/UBC/TRIUMF Hep-ph/0104116 [PLB 507(2001)214] hep-ph/0503159 [PRD 71(2005)093001] hep-ph/0508008

2. Introduction Standard Model works extremely well, but… Standard Model works extremely well, but… 1. it assumes neutrinos to be massless (X) 2. it can accommodate but not explain fermion masses 3. it does not answer Rabi’s old question An elucidation of these questions may point us a way to how the SM should be extended An elucidation of these questions may point us a way to how the SM should be extended Horizontal symmetry Horizontal symmetry

3. Neutrino acts weird, maybe it is trying to tell us something ……. Neutrino acts weird, maybe it is trying to tell us something ……. I come into this business with a certain skepticism regarding standard ideologies, and have a penchant to looking for something really different. This is based on the notion that neutrinos are behaving very differently from the quarks and charged leptons in two respects: very large mixings, and relatively small mass splittings. (J.D. Bjorken)

4. Neutrinos masses and mixings quark Neutrino masses are tiny, mixings are very unlike quarks Is it trying to tell us something special about neutrinos?

5. neutrinos quarks

6. Neutrinos have masses, and they mix Horizontal symmetry

7. Left-Handed 2-3 Symmetry

8. 2-3 symmetry

9. 2-3 symmetry for left-handed neutrinos neutrino masses tiny: seesaw mechanism

10. Seesaw Mechanism seesaw row

11. Double Protection with Right-Handed 2-3 Symmetry seesaw suggests

12. rowcolumn even when one of the two symmetries is broken

13. Summary 2-3 symmetry for left-handed neutrinos With seesaw mechanism Double protection from right-handed 2-3 symmetry

14. Other Work on 2-3 Symmetry Mohapatra, Nasri, Yu: hep-ph/0410369, 0502026, 0505021 Mohapatra, Nasri, Yu: hep-ph/0410369, 0502026, 0505021 Grimus, Lavoura, et al: hep-ph/0305046, 0310050, 0405261, 0407112, 0408123, 0504153 Grimus, Lavoura, et al: hep-ph/0305046, 0310050, 0405261, 0407112, 0408123, 0504153 Koide et al: hep-ph/0209333, 0303256; Matsuda and Nishiura: hep-ph/0506192 Koide et al: hep-ph/0209333, 0303256; Matsuda and Nishiura: hep-ph/0506192 Datta, O’Donnell: hep-ph/0508314 Datta, O’Donnell: hep-ph/0508314

15. END