On Neutrino Masses and Mixings from Extra Dimensions

In the framework of a Kaluza-Klein type theory with the Standard Model fields localized on a 4-dimensional section while gravity propagates in a full $4+\delta$-dimensional space-time, we examine a mechanism of naturally small neutrino mass generation through couplings of Standard Model singlet fermion(s) living also in the full space-time. A numerical studies is performed on the charged current universality constraint from the ratio of pion decay partial widths. The bounds obtained on the fundamental mass scale is stringent. In the case of two extra dimensions, we obtain a bound of order $1600 {TeV}$.