SN1987A - a Testing Ground for the KARMEN Anomaly

We show, that SN1987A can serve as an astrophysical laboratory for testing the viability of the assertion that a new massive neutral fermion is implied by the KARMEN data. We show that a wide range of the parameters characterizing the proposed particle is ruled out by the above constraints making this interpretation very unlikely.Comment: 12 pages, 1 eps figure embedded, to appear in Phys. Lett.

Possible Implications of Asymmetric Fermionic Dark Matter for Neutron Stars

We consider the implications of fermionic asymmetric dark matter for a "mixed neutron star" composed of ordinary baryons and dark fermions. We find examples, where for a certain range of dark fermion mass -- when it is less than that of ordinary baryons -- such systems can reach higher masses than the maximal values allowed for ordinary ("pure") neutron stars. This is shown both within a simplified, heuristic Newtonian analytic framework with non-interacting particles and via a general relativistic numerical calculation, under certain assumptions for the dark matter equation of state. Our work applies to various dark fermion models such as mirror matter models and to other models where the dark fermions have self interactions.Comment: 20 pages, 6 figure

Stationary state volume fluctuations in a granular medium

A statistical description of static granular material requires ergodic sampling of the phase space spanned by the different configurations of the particles. We periodically fluidize a column of glass beads and find that the sequence of volume fractions phi of post-fluidized states is history independent and Gaussian distributed about a stationary state. The standard deviation of phi exhibits, as a function of phi, a minimum corresponding to a maximum in the number of statistically independent regions. Measurements of the fluctuations enable us to determine the compactivity X, a temperature-like state variable introduced in the statistical theory of Edwards and Oakeshott [Physica A {\bf 157}, 1080 (1989)].Comment: published with minor change

Characterizing the Hofstadter butterfly's outline with Chern numbers

In this work, we report original properties inherent to independent particles subjected to a magnetic field by emphasizing the existence of regular structures in the energy spectrum's outline. We show that this fractal curve, the well-known Hofstadter butterfly's outline, is associated to a specific sequence of Chern numbers that correspond to the quantized transverse conductivity. Indeed the topological invariant that characterizes the fundamental energy band depicts successive stairways as the magnetic flux varies. Moreover each stairway is shown to be labeled by another Chern number which measures the charge transported under displacement of the periodic potential. We put forward the universal character of these properties by comparing the results obtained for the square and the honeycomb geometries.Comment: Accepted for publication in J. Phys. B (Jan 2009

Quantum Statistical Calculations and Symplectic Corrector Algorithms

The quantum partition function at finite temperature requires computing the trace of the imaginary time propagator. For numerical and Monte Carlo calculations, the propagator is usually split into its kinetic and potential parts. A higher order splitting will result in a higher order convergent algorithm. At imaginary time, the kinetic energy propagator is usually the diffusion Greens function. Since diffusion cannot be simulated backward in time, the splitting must maintain the positivity of all intermediate time steps. However, since the trace is invariant under similarity transformations of the propagator, one can use this freedom to "correct" the split propagator to higher order. This use of similarity transforms classically give rises to symplectic corrector algorithms. The split propagator is the symplectic kernel and the similarity transformation is the corrector. This work proves a generalization of the Sheng-Suzuki theorem: no positive time step propagators with only kinetic and potential operators can be corrected beyond second order. Second order forward propagators can have fourth order traces only with the inclusion of an additional commutator. We give detailed derivations of four forward correctable second order propagators and their minimal correctors.Comment: 9 pages, no figure, corrected typos, mostly missing right bracket

MoodBar: Increasing new user retention in Wikipedia through lightweight socialization

Socialization in online communities allows existing members to welcome and recruit newcomers, introduce them to community norms and practices, and sustain their early participation. However, socializing newcomers does not come for free: in large communities, socialization can result in a significant workload for mentors and is hard to scale. In this study we present results from an experiment that measured the effect of a lightweight socialization tool on the activity and retention of newly registered users attempting to edit for the first time Wikipedia. Wikipedia is struggling with the retention of newcomers and our results indicate that a mechanism to elicit lightweight feedback and to provide early mentoring to newcomers improves their chances of becoming long-term contributors.Comment: 9 pages, 5 figures, accepted for presentation at CSCW'1

Goodbye to Projects? Briefing Paper 1: An Overview: Projects and Principles.

YesThis briefing paper reports on research exploring ten detailed case studies of livelihoods-oriented interventions operating in Tanzania, Lesotho, South Africa and Uganda. Analysing these interventions through an audit of sustainable livelihood `principles¿ (as a proxy for best practice) revealed general lessons both about the practical opportunities and challenges for employing sustainable livelihoods approaches to the design, implementation, monitoring and evaluation of development interventions and also about the changing format of development interventions.Department for International Developmen

Goodbye to Projects? - Briefing Paper 3: The changing format of development interventions.

yesThis briefing paper reports on research exploring ten detailed case studies of livelihoods-oriented interventions operating in Tanzania, South Africa, Uganda and Lesotho. As a proxy for best practice, these interventions were analysed through an audit of sustainable livelihood `principles¿. This revealed general lessons about both the practical opportunities and challenges for employing sustainable livelihoods approaches to the design, implementation, monitoring and evaluation of development interventions and also about the changing format of development interventions.Department for International Development

Goodbye to Projects? Briefing Paper 2: The Application of the SL Principles.

YesThis briefing paper reports on research exploring ten detailed case studies of livelihoods-oriented interventions operating in Tanzania, South Africa, Uganda and Lesotho. As a proxy for best practice, these interventions were analysed through an audit of sustainable livelihood `principles¿. This revealed general lessons about both the practical opportunities and challenges for employing sustainable livelihoods approaches to the design, implementation, monitoring and evaluation of development interventions and also about the changing format of development interventions.Department for International Development