Naturally light neutrinos and unification in theories with low scale quantum gravity

Within low scale theories traditional see-saw and scalar triplet mechanisms, for neutrino mass suppression, do not work out anymore and for realistic model building some new ideas are needed. In this Letter we suggest mechanism, different from existing ones, which provides natural suppression of the neutrino masses. The mechanism is realized through extended scalars of 4-, 5- or 6-dimensional SU(2)(L) multiplets. Scenario, with fundamental mass scale M-f in alpha similar to 10(3) TeV range, requires 4-plets guaranteeing neutrino masses (<)(similar to) eV. For theories with M-f = few x 10 TeV 5-plets should be involved, while in scenarios with M-f = few TeV, 6-plets could be efficient. The considered mechanism could be successfully applied also for supersymmetric theories, building scenarios with various values of low Mf. Within considered models we also address the question of gauge coupling unification. For low scale unification, existence of compact extra dimensions tums out to be crucial. Due to additional scalar multiplets, some new examples of unification are found for both-non-SUSY and SUSY cases. Within non-SUSY scenarios introduced SU(2)L scalars take advantage and are important for successful unification. (C) 2002 Elsevier Science B.V. All rights reserved.