If neutrino masses have a radiative origin, their smallness can be naturally understood even when lepton number violation occurs near the weak scale.We analyze a specific model of this type wherein the neutrino masses arise as two-loop radiative corrections. We show that the model admits the near bimaximal mixing pattern suggested by the current neutrino oscillation data. Unlike the conventional seesaw models, these two-loop models can be directly tested in lepton flavor violating decays τ → 3µ and µ → e + γ as well as at colliders by the direct observation of charged scalars needed for the mass generation. It is shown that consistency with the neutrino oscillation data requires that the leptonic rare decays should be within reach of forthcoming experiments and that the charged scalars are likely to be within reach of the LHC.

We consider the universal extra dimensions scenario of Appelquist, Cheng, and Dobrescu, in which all of the SM fields propagate into one extra compact dimension, estimated therein to be as large as ∼ (350 GeV) −1 . Tree-level KK number conservation dictates that the associated KK excitations can not be singly produced. We calculate the cross sections for the direct production of KK excitations of the gluon, g n , and two distinct towers of quarks, q • n and q • n , in proton-antiproton collisions at the Tevatron Run I and II energies in addition to proton-proton collisions at the Large Hadron Collider energy. The experimental signatures for these processes depend on the stability of the lowest-lying KK excitations of the gluons and light quarks. We find that the Tevatron Run I mass bound for KK quark and gluon final states is about 350-400 GeV, while Run II can push this up to 450-500 GeV at its initial luminosity and 500-550 GeV if the projected final luminosity is reached. The LHC can probe much further: The LHC will either discover UED KK excitations of quarks and gluons or extend the mass limit to about 3 TeV. *

We consider a minimal formulation of SO(10) Grand Unified Theory wherein all the fermion masses arise from Yukawa couplings involving one 126 and one 10 of Higgs multiplets. It has recently been recognized that such theories can explain, via the type-II seesaw mechanism, the large ν µ −ν τ mixing as a consequence of b−τ unification at the GUT scale. In this picture, however, the CKM phase δ lies preferentially in the second quadrant, in contradiction with experimental measurements. We revisit this minimal model and show that the conventional type-I seesaw mechanism generates phenomenologically viable neutrino masses and mixings, while being consistent with CKM CP violation. We also present improved fits in the type-II seesaw scenario and suggest fully consistent fits in a mixed scenario.

We reconsider the constraints on Universal Extra Dimensions (UED) models arising from precision electroweak data. We take into account the subleading contributions from new physics (expressed in terms of the X, Y . . . variables), as well as two loop corrections to the Standard Model ρ parameter. For the case of one extra dimension, we obtain a lower bound on the inverse compactification scale M = R −1 of 600 GeV (at 90% confidence level), with a Higgs mass of 115 GeV. However, in contradiction to recent claims, we find that this constraint is significantly relaxed with increasing Higgs mass, allowing for compactification scales as low as 300 GeV. LEP II data does not affect significantly these results. * On a leave of absence from: