Light neutrinos without heavy mass scales: a higher-dimensional seesaw mechanism

Recent theoretical developments have shown that extra space-time dimensions can lower the fundamental GUT, Planck, and string scales. However, recent evidence for neutrino oscillations suggests the existence of light non-zero neutrino masses, which in turn suggests the need for a heavy mass scale vi a the seesaw mechanism. In this paper, we make several observations in this regard. First, we point out that allowing the right-handed neutrino to exp erience extra space-time dimensions naturally permits the left-handed neutr ino mass to be power-law suppressed relative to the masses of the other fer mions. This occurs due to the power-law running of the neutrino Yukawa coup lings, and therefore does not require a heavy scale for the right-handed ne utrino. Second, we show that a higher-dimensional analogue of the seesaw me chanism may also be capable of generating naturally light neutrino masses w ithout the introduction of a heavy mass scale. Third, we show that such a h igher-dimensional seesaw mechanism may even be able to explain neutrino osc illations without neutrino masses, with oscillations induced indirectly via the masses of the Kaluza-Klein states. Fourth, we point out that even when higher-dimensional right-handed neutrinos are given a bare Majorana mass, the higher-dimensional seesaw mechanism surprisingly replaces this mass sca le with the radius scale of the extra dimensions. Finally, we also discuss a possible new mechanism for inducing lepton-number violation by shifting t he positions of D-branes in Type I string theory. (C) 1999 Published by Els evier Science B.V. All rights reserved.