Minimal Supergravity with m_0^2 < 0

We extend the parameter space of minimal supergravity to negative values of m_0^2, the universal scalar mass parameter defined at the grand unified scale. After evolving to the weak scale, all scalars can be non-tachyonic with masses consistent with collider constraints. This region of parameter space is typically considered excluded by searches for charged dark matter, since the lightest standard model superpartner is a charged slepton. However, if the gravitino is the lightest supersymmetric particle, the charged slepton decays, and this region is allowed. This region provides qualitatively new possibilities for minimal supergravity, including spectra with light sleptons and very heavy squarks, and models in which the lightest slepton is the selectron. We show that the m_0^2 < 0 region is consistent with low energy precision data and discuss its implications for particle colliders. These models may provide signals of supersymmetry in even the first year of operation at the Large Hadron Collider.Comment: 16 page

Clonal expansion under the microscope: studying lymphocyte activation and differentiation using live-cell imaging

Clonal expansion of lymphocytes is a hallmark of vertebrate adaptive immunity. A small number of precursor cells that recognize a specific antigen proliferate into expanded clones, differentiate and acquire various effector and memory phenotypes, which promote effective immune responses. Recent studies establish a large degree of heterogeneity in the level of expansion and in cell state between and within expanding clones. Studying these processes in vivo, while providing insightful information on the level of heterogeneity, is challenging due to the complex microenvironment and the inability to continuously track individual cells over extended periods of time. Live cell imaging of ex vivo cultures within micro fabricated arrays provides an attractive methodology for studying clonal expansion. These experiments facilitate continuous acquisition of a large number of parameters on cell number, proliferation, death and differentiation state, with single-cell resolution on thousands of expanding clones that grow within controlled environments. Such data can reveal stochastic and instructive mechanisms that contribute to observed heterogeneity and elucidate the sequential order of differentiation events. Intercellular interactions can also be studied within these arrays by following responses of a controlled number of interacting cells, all trapped within the same microwell. Here we describe implementations of live-cell imaging within microwell arrays for studies of lymphocyte clonal expansion, portray insights already gained from these experiments and outline directions for future research. These tools, together with in vivo experiments tracking single-cell responses, will expand our understanding of adaptive immunity and the ways by which it can be manipulated

Effects of supersymmetric grand unification scale physics on Γ(b→sγ)\Gamma \left( b\to s\gamma\right)

Although calculations of the $b\rightarrow s\gamma$ rate in supersymmetric grand unified models have always either ignored the gluino mediated contribution or found it to be negligible, we show that taking universal supersymmetry breaking masses at the Planck scale, rather than at the gauge unification scale as is customary, leads to the gluino contribution being more significant and in fact sometimes even larger than the chargino mediated contributions when $\mu >0$ and $\tan{\beta}$ is of order 1. The impact is greatest felt when the gluinos are relatively light. Taking the universal boundary condition at the Planck scale also has an effect on the chargino contribution by increasing the effect of the wino and higgsino-wino mediated decays. The neutralino mediated contribution is found to be enhanced, but nevertheless it remains relatively insignificant.Comment: Title changed, final version as accepted for PRD, 12 pages, 6 Figures (Figs.2-6 included, uuencoded, epsf.tex

Precision Measurements at The Higgs Resonance: A Probe of Radiative Fermion Masses

The possibility of radiative generation of fermion masses from soft supersymmetry breaking chiral flavor violation is explored. Consistent models are identified and classified. Phenomenological implications for electric dipole moments and magnetic moments, as well as collider probes -- in particular those relevant at the Higgs resonance -- are discussed. It is shown that partial widths $\Gamma_{h^{0} \to ff}$ are enhanced compared with the minimal supersymmetric standard model.Comment: 7 pages, 1 figure, aipproc.sty (included). Talk presented by N. Polonsky at the Workshop on Physics at the First Muon Collider and the Front End of a Muon Collider, Fermilab, November 6-9, 199

The strong coupling, unification, and recent data

The prediction of the strong coupling assuming (supersymmetric) coupling constant unification is reexamined. We find, using the new electroweak data, $\alpha_{s}(M_{Z}) \approx 0.129 \pm 0.010$. The implications of the large $\alpha_{s}$ value are discussed. The role played by the $Z$ beauty width is stressed. It is also emphasized that high-energy (but not low-energy) corrections could significantly diminish the prediction. However, unless higher-dimension operators are assumed to be suppressed, at present one cannot place strong constraints on the super-heavy spectrum. Non-leading electroweak threshold corrections are also discussed.Comment: 12 pages, LaTex + RevTex, uuencoded postscript file (including 13 figures) is attached. Also available at ftp://dept.physics.upenn.edu/pub/Ni