Yuval Grossman

We point out a dramatic new experimental signature for a class of theories with extra dimensions, where quarks and leptons are localized at slightly separated parallel “walls” whereas gauge and Higgs fields live in the bulk of the extra dimensions. The separation forbids direct local couplings between quarks and leptons, allowing for an elegant solution to the proton decay problem. We show that scattering cross sections for collisions of fermions which are separated in the extra dimensions vanish exponentially at energies high enough to probe the separation distance. This is because the separation puts a lower bound on the attainable impact parameter in the collision. We present cross sections for two body high energy scattering and estimate the power with which future colliders can probe this scenario, finding sensitivity to inverse fermion separations of order 10–70 TeV.

We study bulk fermion fields in the localized gravity model with non-factorizable metric recently proposed by Randall and Sundrum, and Gogberashvili. In addition to a tower of weak-scale Kaluza-Klein states we find a zero mode for any value of the fundamental fermion mass. If the fermion mass is larger than half the curvature of the compact dimension, the zero mode can be localized on the ``hidden'' 3-brane in the Randall-Sundrum model. Identifying this mode with a right-handed neutrino provides a new way for obtaining small Dirac neutrino masses without invoking a see-saw mechanism. Cancellation of the parity anomaly requires introducing an even number of bulk fermions. This naturally leads to a strong hierarchy of neutrino masses and generically large mixing angles.

Some rare hadronic decays of B mesons, such as B->pi K, are sensitive to isospin-violating contributions from physics beyond the Standard Model. Although commonly referred to as electroweak penguins, such contributions can often arise through tree-level exchanges of heavy particles, or through strong-interaction loop diagrams. The Wilson coefficients of the corresponding electroweak penguin operators are calculated in a large class of New Physics models, and in many cases are found not to be suppressed with respect to the QCD penguin coefficients. Several tests for these effects using observables in B->pi K decays are discussed, and nontrivial bounds on the couplings of the various New Physics models are derived.

We point out a dramatic new experimental signature for a class of theories with extra dimensions, where quarks and leptons are localized at slightly separated parallel ``walls'' whereas gauge and Higgs fields live in the bulk of the extra dimensions. The separation forbids direct local couplings between quarks and leptons, allowing for an elegant solution to the proton decay problem. We show that scattering cross sections for collisions of fermions which are separated in the extra dimensions vanish exponentially at energies high enough to probe the separation distance. This is because the separation puts a lower bound on the attainable impact parameter in the collision. We present cross sections for two body high energy scattering and estimate the power with which future colliders can probe this scenario, finding sensitivity to inverse fermion separations of order 10-70 TeV.

The experimental data on atmospheric and solar neutrinos suggest that there is near-maximal mixing between \nu_\mu and \nu_\tau but that their masses are hierarchically separated. In models of Abelian horizontal symmetries, mixing of O(1) generically implies that the corresponding masses are of the same order of magnitude. We describe two new mechanisms by which a large hierarchy between strongly mixed neutrinos can be achieved in this framework. First, a discrete Abelian symmetry can give the desired result in three ways: mass enhancement, mixing enhancement and mass suppression. Second, holomorphic zeros can give mass suppression.