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 paper 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 a $\sim 10^{3}$ TeV range, requires 4-plets guaranteeing neutrino masses $\stackrel{<}{\sim}1$ eV. For theories with $M_f= few \cdot 10$ 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 $M_f$. Within considered models we also address the question of gauge coupling unification. For low scale unification, existence of compact extra dimensions turns 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.