Sterile neutrino as a bulk neutrino

If light sterile neutrinos are needed to understand the neutrino puzzles. a s is currently indicated, a major theoretical challenge is to understand wh y its mass is so small. It is a more serious problem than understanding the small mass of the familiar neutrinos, We discuss a new way to solve this p roblem by identifying the sterile neutrino as gauge neutral fermion propaga ting in the bulk of a higher-dimensional theory, with bulk size of order of a millimeter. The smallness of its mass is then a consequence of the size of the extra dimension and does not need the introduction of new symmetries . We present a realistic model for neutrino masses and mixings that impleme nts this idea. (C) 2000 Elsevier Science B.V. All rights reserved.