Steven J. Oliver

Explicit supersymmetry breaking on boundaries of warped extra dimensions

Explicit supersymmetry breaking is studied in higher dimensional theories by having boundaries respect only a subgroup of the bulk symmetry. If the boundary symmetry is the maximal subgroup allowed by the boundary conditions imposed on the fields, then the symmetry can be consistently gauged; otherwise gauging leads to an inconsistent theory. In a warped fifth dimension, an explicit breaking of all bulk supersymmetries by the boundaries is found to be inconsistent with gauging; unlike the case of flat 5D, complete supersymmetry breaking by boundary conditions is not consistent with supergravity. Despite this result, the low energy effective theory resulting from boundary supersymmetry breaking becomes consistent in the limit where gravity decouples, and such models are explored in the hope that some way of successfully incorporating gravity can be found. A warped constrained standard model leads to a theory with one Higgs boson with mass expected close to the experimental limit. A unified theory in a warped fifth dimension is studied with boundary breaking of both SU(5) gauge symmetry and supersymmetry. The usual supersymmetric prediction for gauge coupling unification holds even though the TeV spectrum is quite unlike the MSSM. Such a theory may unify matter and Higgs in the same SU(5) hypermultiplet.

Dark Energy and Right-Handed Neutrinos

We explore the possibility that a CP violating phase of the neutrino mass matrix is promoted to a pseudo-Goldstone-boson field and is identified as the quintessence field for Dark Energy. By requiring that the quintessence potential be calculable from a Lagrangian, and that the extreme flatness of the potential be stable under radiative corrections, we are led to an essentially unique model. Lepton number is violated only by Majorana masses of light right-handed neutrinos, comparable to the Dirac masses that mix right- with left-handed neutrinos. We outline the rich and constrained neutrino phenomenology that results from this proposal.

Late time neutrino masses, the lsnd experiment, and the cosmic microwave background

Models with low-scale breaking of global symmetries in the neutrino sector provide an alternative to the seesaw mechanism for understanding why neutrinos are light. Such models can easily incorporate light sterile neutrinos required by the Liquid Scintillator Neutrino Detector experiment. Furthermore, the constraints on the sterile neutrino properties from nucleosynthesis and large-scale structure can be removed due to the nonconventional cosmological evolution of neutrino masses and densities. We present explicit, fully realistic supersymmetric models, and discuss the characteristic signatures predicted in the angular distributions of the cosmic microwave background.

Why Are Neutrinos Light? -- An Alternative

We review the recent proposal that neutrinos are light because their masses are proportional to a low scale, f, of lepton flavor symmetry breaking. This mechanism is testable because the resulting pseudo-Goldstone bosons, of mass m_G, couple strongly with the neutrinos, affecting the acoustic oscillations during the eV era of the early universe that generate the peaks in the CMB radiation. Characteristic signals result over a very wide range of (f, m_G) because of a change in the total relativistic energy density and because the neutrinos scatter rather than free-stream. Thermodynamics allows a precise calculation of the signal, so that observations would not only confirm the late-time neutrino mass mechanism, but could also determine whether the neutrino spectrum is degenerate, inverted or hierarchical and whether the neutrinos are Dirac or Majorana. The flavor symmetries could also give light sterile states. If the masses of the sterile neutrinos turn on after the MeV era, the LSND oscillations can be explained without upsetting big bang nucleosynthesis, and, since the sterile states decay to lighter neutrinos and pseudo-Goldstones, without giving too much hot dark matter.

Cooling atoms in far-detuned optical lattices

Summary form only given. Our approach to high lattice occupancy requires a way to cool atoms trapped in a far off-resonance lattice (FORL) and an adiabatic compression sequence to increase the spatial density. For the laser cooling to work, the FORL polarizations must be set so that all magnetic sublevels see the same potential due to the FORL. With this condition satisfied, polarization gradient cooling with independent light works at least as well in the FORL at very high atomic density as it does in free space at low density. The 3D FORL is made from three orthogonal standing waves. After cooling Cs atoms in the 3D FORL, we shut off the horizontal lattice beams adiabatically, so that a 1D FORL trap remains. The atoms are left stacked in pancake-shaped distributions, confined to 50 nm in the vertical direction and 0.4 mm horizontally. The trap depth is 200 /spl mu/K, but the atoms have less than 1 /spl mu/K initial kinetic energy, so they are all near the top of their trajectories in the transverse, Gaussian-shaped potential. The atoms collapse toward the center of the trap. At the moment of peak density we turn the horizontal lattice beams back on adiabatically, trapping 84% of the atoms at lattice sites. We finally laser cool the atoms again in the 3D FORL. The final laser cooling causes collisional loss from lattice sites with more than one atom. Ultimately, 44% of the sites have a single atom cooled to near its vibrational ground state. A theoretical model of site occupation based on Poisson statistics agrees well with our experimental results.