  A new mechanism for generating neutrino masses without a high-energy mass
scale is proposed. The mechanism needs a fundamental mass scale M in the
100-1000 TeV region and a minimal field content beyond the Standard Model one
containing a pair of fermion singlets and a pair of weak doublet fermions for
each neutrino mass, all of them with a mass of order M. The neutrino mass
appears by a multiple seesaw-type tree-level diagram. We provide an explicit
model based on supersymmetry and an abelian symmetry which provides the
required fermion mass matrix. The mechanism is natural in the context of string
theories with a low fundamental scale. Within an explicit example where the
abelian symmetry is also responsible for the generation of fermion masses and
mixings, we give a hint relating the fermion mass matrices and the weak mixing
angle. By assuming the weak-strong couplings unification, one naturally finds
$\sin^2 \theta_w = 1/4$ at the fundamental scale.

High Energy Physics - Phenomenology