String Unification and Leptophobic Z′Z' in Flipped SU(5)

We summarize recent developments in the prediction for $\alpha_s(M_Z)$, self-consistent string unification and the dynamical determination of mass scales, and leptophobic $Z'$ gauge bosons in the context of stringy flipped SU(5). [To appear in Proceedings of Fourth International Conference on Supersymmetry (SUSY96), University of Maryland (May 1996).]Comment: 5 pages, LaTeX (uses espcrc2.sty), 5 figures (included

Vanishing Str M^2 in the presence of anomalous U_A(1)

We show that the presence of an anomalous $\rm U_A(1)$ factor in the gauge group of string-derived models may have the new and important phenomenological consequence of allowing the vanishing of ${\rm Str}\,{\cal M}^2$ in the ``shifted" vacuum, that results in the process of cancelling the anomalous $\rm U_A(1)$. The feasibility of this effect seems to be enhanced by a vanishing vacuum energy, and by a ``small" value of ${\rm Str}\,{\cal M}^2$ in the original vacuum. In the class of free-fermionic models with vanishing vacuum energy that we focus on, a necessary condition for this mechanism to be effective is that ${\rm Str}\,{\cal M}^2>0$ in the original vacuum. A vanishing ${\rm Str}\,{\cal M}^2$ ameliorates the cosmological constant problem and is a necessary element in the stability of the no-scale mechanism.Comment: 7 pages, no figures, LaTe

Light dynamics in glass-vanadium dioxide nanocomposite waveguides with thermal nonlinearity

We address the propagation of laser beams in Si02-VO2 nanocomposite waveguides with thermo-optical nonlinearity. We show that the large modifications of the absorption coefficient as well as notable changes of refractive index of VO2 nanoparticles embedded into the SiO2 host media that accompany the semiconductor-to-metal phase transition may lead to optical limiting in the near-infrared wave range.Comment: 13 pages, 3 figures, to appear in Optics Letter

Direct detection of dark matter in SU(5)xU(1) supergravity

We compute the scattering rates for the lightest neutralino $\chi^0_1$ in the forthcoming germanium $(^{73}{\rm Ge}+^{76}{\rm Ge})$ detector and a proposed lead detector $(^{207}{\rm Pb})$, within the framework of $SU(5)\times U(1)$ supergravity. We find that in only a small portion (\lsim10\%) of the parameter spaces of this class of models, are the rates in the germanium detector above the expected initial experimental sensitivity of 0.1 events/kg/day. However, a much larger portion (\lsim40\%) of the parameter spaces could be probed with an improved background rejection capability (0.01 events/kg/day) and/or a more sensitive detector $(^{207}{\rm Pb})$.Comment: 10 pages, Latex, 4 figures included (uuencoded