Micrometer Gravitinos and the Cosmological Constant

We compute the 4--dimensional cosmological constant in string compactifications in which the Standard Model fields live on a non-supersymmetric brane inside a supersymmetric bulk. The cosmological constant receives contributions only from the vacuum energy of the bulk supergravity fields, but not from the vacuum energy of the brane fields. The latter is absorbed in a warp factor. Supersymmetry breaking on the brane at the TeV scale implies supersymmetry breaking in the bulk at the micrometer scale. This produces a tiny cosmological constant that agrees with experiment within a few orders of magnitude. Our argument predicts superpartners of the graviton with mass of order $10^{-3}$ eV. They should be observable in short-distance tests of Einstein Gravity.Comment: estimate in section 4 refined; scale of supersymmetry breaking changes to 2-6 Te

Jockeying for Position: Strategic High School Choice Under Texas' Top Ten Percent Plan

Beginning in 1998, all students in the state of Texas who graduated in the top ten percent of their high school classes were guaranteed admission to any in-state public higher education institution, including the flagships. While the goal of this policy is to improve college access for disadvantaged and minority students, the use of a school-specific standard to determine eligibility could have unintended consequences. Students may increase their chances of being in the top ten percent by choosing a high school with lower-achieving peers. Our analysis of students’ school transitions between 8th and 10th grade three years before and after the policy change reveals that this incentive influences enrollment choices in the anticipated direction. Among the subset of students with both motive and opportunity for strategic high school choice, as many as 25 percent enroll in a different high school to improve the chances of being in the top ten percent. Strategic students tend to choose the neighborhood high school in lieu of more competitive magnet schools and, regardless of own race, typically displace minority students from the top ten percent pool. The net effect of strategic behavior is to slightly decrease minority students’ representation in the pool.

Reflection symmetry breaking scenarios with minimal gauge form coupling in brane world cosmology

This article synthesises and extends recent work on the cosmological consequences of dropping the usual Z_2 reflection symmetry postulate in brane world scenarios. It is observed that for a cosmological model of homogeneous isotropic type, the relevant generalised Birkhoff theorem establishing staticity of the external vacuum in the maximally symmetric ``bulk'' outside a freely moving world brane will remain valid for the case of motion that is forced by minimal (generalised Wess Zumino type) coupling to an external antisymmetric gauge field provided its kinetic action contribution has the usual homogeneous quadratic form. This means that the geometry on each side of the brane worldsheet will still be of the generalised Schwarzschild anti de Sitter type. The usual first integrated Friedmann equation for the Hubble expansion rate can thereby be straightforwardly generalised by inclusion of new terms involving 2 extra parameters respectively measuring the strength of the gauge coupling and the degree of deviation from reflection symmetry. Some conceivable phenomenological implications are briefly outlined, and corresponding limitations are derived for possible values of relevant parameters.Comment: 13 pages latex (revised to rectify a couple of sign transcription errors

Singlet fermions on curved extra dimension tori

A model with two curved compact extra dimensions is introduced. The model is based on a four-brane immersed in a six dimensional space, where the extra dimensions are compact but not flat. They have topology of torus. The form of the metric in the empty bulk is studied and the gauge singlet fermion structure showed to be very simple. It contain only one massless low energy mode which couples to brane matter, while the massive modes are not related to the volume of torus while the Planck mass is related to the volume of extra dimensions. In the model "our" brane is situated on a nearly singular line of the torus.Comment: 11 pages. Some references adde