Particle theory

Abstract:

We investigate supersymmetric SU(5) grand unified theories (GUTs) realized in 5 space time dimensions and broken down to the MSSM by SU(5)-violating boundary conditions on a S1/(Z2×Z2′) orbifold with two 3-branes. The doublet-triplet splitting problem is entirely avoided by locating the MSSM Higgs doublets on the brane on which SU(5) is not a good symmetry. An extremely simple model is then described in which the MSSM matter is also located on this SU(5)-violating brane. Although this model does not unify the MSSM matter within SU(5) multiplets, it explains gauge coupling unification. A second model with MSSM matter in the SU(5)-symmetric bulk preserves both the SU(5) explanation of fermion quantum numbers as well as gauge-coupling unification. Both models naturally avoid problematic SU(5) predictions for the Yukawa couplings of the first two generations and are consistent with proton decay constraints. We analyse the running of gauge couplings above the compactification scale in terms of a 5d effective action and derive the implications for the values of compactification scale, unification scale and of the scale at which the bulk gauge theory becomes strongly coupled. © 2001 Elsevier Science B.V.

Abstract:

A plausible explanation for the existence of additional light sterile neutrinos is that they correspond to modulini, fermionic partners of moduli, which propagate in new large dimensions. We discuss the phenomenological implications of such states and show that solar neutrino oscillation is well described by small angle MSW oscillation to the tower of Kaluza Klein states associated with the modulini. We also consider how all oscillation phenomena can be explained in a model including bulk neutrino states.

Abstract:

We study the absorption of a minimally coupled scalar in the gravitational background created by a stack of near-extremal black three-branes, and more generally by M2-, M5- and Dp-branes. The absorption probability has the form P(l)=P0(l)fl(λ), where P0(l) is the partial wave's absorption probability in the extremal case, and the thermal factor fl(λ) depends on the ratio of the frequency of the incoming wave and the Hawking temperature, λ=ω/πT. Using Langer-Olver's method, we obtain a low-temperature (λ≫1) asymptotic expansion for P(l) with coefficients determined recursively. This expansion, which turns out to be a fairly good approximation even for λ~1, accounts for all power-like finite-temperature corrections to P0(l), and we calculate a few terms explicitly. We also show that at low temperature the absorption probability contains exponentially suppressed terms, and attempt to develop an approximation scheme to calculate those. The high-temperature expansion is also considered. For the s-wave, the low-temperature gravity result is consistent with the free finite-temperature field theory calculation, while for high temperature and higher partial waves we find a disagreement. As a check of the approximation methods used, we apply them to the D1-D5-brane system, and compare results to the known exact solution. © 2001 Elsevier Science B.V.