Bulk Fermions in Multi-Brane Worlds

We study bulk fermion fields in various multi-brane models with localized gravity. The chiral zero mode that these models support can be identified as a right-handed sterile neutrino. In this case small neutrino Dirac masses can naturally appear due to a localization of the bulk fermion zero mode wavefunction, in an analogous way to graviton, without invoking the see-saw mechanism. The conditions and the options for localization are discussed in detail. It is shown that, considering a well motivated five dimensional mass term, the localization behaviour of this mode can resemble the graviton's at least in a region of the parameter space. As a result, the ''+-+'', ''++'' models can support, in addition to the ultralight graviton KK state, an ultralight localized and strongly coupled bulk fermion KK mode. We find that there are severe constraints on the parameter space of ''+-+'' and ''++'' models if the neutrino properties resulting from this light fermion state are to be reasonable. Furthermore, in the case that also the Bigravity scenario is realized the above special KK mode can induce too large mixing between the neutrino and the KK tower sterile modes restricting even more the allowed parameter space.Comment: 32 pages, 2 figure

Multi-Brane Worlds and modification of gravity at large scales

We discuss the implications of multi-brane constructions involving combinations of positive and negative tension brane and show how anomalously light KK states emerge when negative tension ''-'' branes are sandwiched between ''+'' branes. We present a detailed study of a ''+--+'' brane assignment which interpolates between two models that have been previously proposed in which gravity is modified at large scales due to the anomalously light states. We show that it has the peculiar characteristic that gravity changes from four dimensional (4D) to 5D at large distances and returns to 4D at even larger scales. We also consider a crystalline universe which leads to a similar structure for gravity. The problems associated with intermediate negative tension branes are discussed and a possible resolution suggested.Comment: 28 pages, 6 figures,references adde

The m->0 limit for massive graviton in dS_4 and AdS_4. How to circumvent the van Dam-Veltman-Zakharov discontinuity

We show that, by considering physics in dS_4 or AdS_4 spacetime, one can circumvent the van Dam - Veltman - Zakharov theorem which requires that the extra polarization states of a massive graviton do not decouple in the massless limit. It is shown that the smoothness of the m->0 limit is ensured if the H (``Hubble'') parameter, associated with the horizon of the dS_4 or AdS_4 space, tends to zero slower than the mass of the graviton m.Comment: 11 pages, references added, minor typos correcte

A new bigravity model with exclusively positive branes

We propose a new ``bigravity'' model with two positive tension AdS_4 branes in AdS_5 bulk and no negative tension branes. The bounce of the ``warp'' factor mimics the effect of a negative brane and thus gives rise to an anomalously light graviton KK mode. This configuration satisfies the weak energy condition and has no ghost state. In addition, the extra polarization states of the massive graviton practically decouple and thus it does not contradict to Einsteinian gravity. However, the model has certain phenomenological difficulties associated with the presence of a negative cosmological constant on the branes.Comment: 17 pages, 1 axodraw figure, comments added in the introduction, references added, minor typos correcte

Radion in Multibrane World

The radion dynamics related to the presence of moving branes with both positive or negative tensions is studied in the linearized approximation. The radion effective Lagrangian is computed for a compact system with three branes and in particular we examine the decompactification limit when one brane is sent to infinity. In the non-compact case we calculate the coupling of the gravitational modes (graviton, dilaton and radion) to matter on the branes. The character of gravity on the two branes for all possible combinations of brane tensions is also discussed. It turns out that one can have a normalizable dilaton mode even in the non-compact case. Finally, we speculate on the role of moving branes as a possible source of radion emission.Comment: 24 pages, 4 figures, comments added, version published in Nucl. Phys.

Multi-Localization in Multi-Brane Worlds

We study bulk fields in various multi-brane models with localized gravity. In the case of spin 0, 1/2 and 3/2 fields, the non-trivial background geometry itself can induce (multi-) localization of the zero modes along the extra dimension. The addition of appropriate mass terms controls the strength or/and the position of this localization and can induce (multi-) localization of spin 1 fields as well. We show that multi-brane models may also give rise to anomalously light KK modes which are also localized. The possibility of multi-localization in the context of supersymmetric brane world models in AdS_5 spacetime is also discussed.Comment: 34 pages, 5 figure

A Three three-brane Universe:New Phenomenology for the New Millennium?

We consider an extension of the Randall-Sundrum model with three parallel 3-branes in a 5-dimensional spacetime. This new construction, apart from providing a solution to the Planck hierarchy problem, has the advantage that the SM fields are confined on a positive tension brane. The study of the phenomenology of this model reveals an anomalous first KK state which is generally much lighter than the remaining tower and also much more strongly coupled to matter. Bounds on the parameter space of the model can be placed by comparison of specific processes with the SM background as well as by the latest Cavendish experiments. The model suggests a further exotic possibility if one drops the requirement of solving the hierarchy problem. In this case gravity may result from the exchange of the ordinary graviton plus an ultralight KK state and modifications of gravity may occur at both small and extremely large scales.Comment: 17 pages, 5 Postscript figures, uses axodraw.sty, corrected some typo

Multigravity in six dimensions: Generating bounces with flat positive tension branes

We present a generalization of the five dimensional multigravity models to six dimensions. The key characteristic of these constructions is that that we obtain solutions which do not have any negative tension branes while at the same time the branes are kept flat. This is due to the fact that in six dimensions the internal space is not trivial and its curvature allows bounce configurations with the above feature. These constructions give for the first time a theoretically and phenomenologically viable realization of multigravity.Comment: 27 pages, 13 figures, typos correcte