Light Neutrinos without Heavy Mass Scales: A Higher-Dimensional Seesaw Mechanism

Recent theoretical developments have shown that extra spacetime 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 via the seesaw mechanism. In this short note, we make two observations. First, we show that a higher-dimensional analogue of the seesaw mechanism may be capable of generating naturally light neutrino masses without the introduction of a heavy mass scale. In particular, we show that mixing between the left-handed neutrino and an entire Kaluza-Klein tower of right-handed neutrinos produces a naturally suppressed neutrino mass regardless of the intrinsic energy scale of the right-handed neutrinos. Second, we consider whether neutrino masses are actually required in order to explain neutrino oscillations, and show that our higher-dimensional seesaw mechanism may be able to permit neutrino oscillations even if the neutrino masses vanish. Essentially, neutrino oscillations can be induced indirectly via the masses of the Kaluza-Klein states. Thus, within the context of a higher-dimensional seesaw mechanism, we propose that neutrino oscillations can be explained without neutrino masses and without heavy mass scales.