Bulk fermions in multi-brane worlds

We study bulk fermion fields in various multi-brane models with localizedgravity. The chiral zero mode that these models support can be identified as aright-handed sterile neutrino. In this case small neutrino Dirac masses cannaturally appear due to a localization of the bulk fermion zero modewavefunction, in an analogous way to graviton, without invoking the see-sawmechanism. The conditions and the options for localization are discussed indetail. It is shown that, considering a well motivated five dimensional massterm, the localization behaviour of this mode can resemble the graviton's atleast in a region of the parameter space. As a result, the ''+-+'', ''++''models can support, in addition to the ultralight graviton KK state, anultralight localized and strongly coupled bulk fermion KK mode. We find thatthere are severe constraints on the parameter space of ''+-+'' and ''++''models if the neutrino properties resulting from this light fermion state areto be reasonable. Furthermore, in the case that also the Bigravity scenario isrealized the above special KK mode can induce too large mixing between theneutrino and the KK tower sterile modes restricting even more the allowedparameter space.Comment: 32 pages, 2 figure