A spaceship with a thruster - one body, one force

A spaceship with one thruster producing a constant magnitude force is analyzed for various initial conditions. This elementary problem, with one object acted upon by one force, has value as a challenge to one's physical intuition and in demonstrating the benefits and limitations of dimensional analysis. In addition, the problem can serve to introduce a student to special functions, provide a mechanical model for Fresnel integrals and the associated Cornu spiral, or be used as an example in a numerical methods course. The problem has some interesting and perhaps unexpected features.Comment: 8 pages, 12 figures. Submitted to the American Journal of Physics. After it is published, it will be found at http://scitation.aip.org/aj

Day-Night and Energy Dependence of MSW Solar Neutrinos for Maximal Mixing

It has been stated in the literature that the case of maximal mixing angle for \nu_e leads to no day-night effect for solar neutrinos and an energy independent flux suppression of 1/2. While the case of maximal mixing angle and \Delta m^2 in the MSW range does lead to suppression of the electron neutrinos reaching the earth from the sun by P_S=1/2, the situation is different for neutrinos that have passed through the earth. We make the pedagogical point that, just as with smaller mixing angles, the earth regenerates the |\nu_1> state from the predominantly |\nu_2 > state reaching the earth, leading to coherent interference effects. This regeneration can lead to a day-night effect and an energy dependence of the suppression of solar electron neutrinos, even for the case of maximal mixing. For large mixing angles, the energy dependence of the day-night asymmetry depends heavily on Delta m^2. With a sufficiently sensitive measurement of the day-night effect, this energy dependence could be used to distinguish among the large mixing angle solutions of the solar neutrino problem.Comment: JHEP style, 22 pages, 7 figures. References added, and minor rewordin

Precision Determination of Invisible-Particle Masses at the CERN LHC: II

We further develop the constrained mass variable techniques to determine the mass scale of invisible particles pair-produced at hadron colliders. We introduce the constrained mass variable M_3C which provides an event-by-event lower bound and upper bound to the mass scale given the two mass differences between the lightest three new particle states. This variable is most appropriate for short symmetric cascade decays involving two-body decays and on-shell intermediate states which end in standard-model particles and two dark-matter particles. An important feature of the constrained mass variables is that they do not rely simply on the position of the end point but use the additional information contained in events which lie far from the end point. To demonstrate our method we study the supersymmetric model SPS 1a. We select cuts to study events with two Neutralino_2 each of which decays to Neutralino_1, and two opposite-sign same-flavor (OSSF) charged leptons through an intermediate on-shell slepton. We find that with 300 fb^-1 of integrated luminosity the invisible-particle mass can be measured to M=96.4 +/- 2.4 GeV. Combining fits to the shape of the M_3C constrained mass variable distribution with the max m_ll edge fixes the mass differences to +/- 0.2 GeV.Comment: 13 pages, 10 figure

Adaptive multi-stage integrators for optimal energy conservation in molecular simulations

We introduce a new Adaptive Integration Approach (AIA) to be used in a wide range of molecular simulations. Given a simulation problem and a step size, the method automatically chooses the optimal scheme out of an available family of numerical integrators. Although we focus on two-stage splitting integrators, the idea may be used with more general families. In each instance, the system-specific integrating scheme identified by our approach is optimal in the sense that it provides the best conservation of energy for harmonic forces. The AIA method has been implemented in the BCAM-modified GROMACS software package. Numerical tests in molecular dynamics and hybrid Monte Carlo simulations of constrained and unconstrained physical systems show that the method successfully realises the fail-safe strategy. In all experiments, and for each of the criteria employed, the AIA is at least as good as, and often significantly outperforms the standard Verlet scheme, as well as fixed parameter, optimized two-stage integrators. In particular, the sampling efficiency found in simulations using the AIA is up to 5 times better than the one achieved with other tested schemes

Correlación existente entre el KT/VDI (DIALISANCIA IONICA) y el KT/V monocompartimental (KT/VM) y equilibrado (KT/VE) en pacientes en hemodiálisis: revisión sistemática de la literatura

La cuantificación de la dosis de diálisis en un elemento esencial en el manejo de los pacientes en hemodiálisis. La dosis de diálisis es un buen marcador de diálisis adecuada y se ha correlacionado con la supervivencia global de los pacientes. El cálculo del aclaramiento fraccional de urea (Kt/V) es el procedimiento más utilizado para cuantificar la dosis de diálisis. En el enfermo tratado con hemodiálisis, el Kt/V se determina habitualmente a partir de las concentraciones de urea al inicio y al final de la diálisis, mediante diversas fórmulas que reciben el nombre genérico de métodos simplificados. La determinación de la dosis de diálisis de manera convencional requiere la toma de múltiples muestras, constituyendo una limitante del método. El monitor de conductividad es un dispositivo que funciona en el circuito del baño de diálisis y que permite calcular la dialisancia iónica del dializador durante la sesión de hemodiálisis, sin necesidad de toma de muestras sanguíneas. El objetivo del presente estudio consiste en realizar una revisión sistemática de los estudios que comparan y correlacionan los resultados del Kt/V obtenido mediante dialisancia iónica con el Kt/V obtenido con las fórmulas simplificadas según el modelo monocompartimental de distribución de la urea. Se realizó una búsqueda computarizada de las bases de datos electrónicas PUB MED, EMBASE, Cochrane Library, entre los años de 1995 y marzo de 2010. Se incluyeron estudios de cohorte prospectivo, donde se efectuaba la correlación entre la efectividad de las dosis de diálisis medida mediante dialisancia iónica y los modelos monocompartimentales, principalmente Daugirdas 1993. Se valoró la calidad de los artículos utilizando la Newcastle- Otawa Scale. Se seleccionaron para el análisis 22 estudios que cumplieron los criterios de inclusión propuest

The Precision Determination of Invisible-Particle Masses at the LHC

We develop techniques to determine the mass scale of invisible particles pair-produced at hadron colliders. We employ the constrained mass variable m_2C, which provides an event-by-event lower-bound to the mass scale given a mass difference. We complement this variable with a new variable m_2C,UB which provides an additional upper bound to the mass scale, and demonstrate its utility with a realistic case study of a supersymmetry model. These variables together effectively quantify the `kink' in the function Max m_T2 which has been proposed as a mass-determination technique for collider-produced dark matter. An important advantage of the m_2C method is that it does not rely simply on the position at the endpoint, but it uses the additional information contained in events which lie far from the endpoint. We found the mass by comparing the HERWIG generated m_2C distribution to ideal distributions for different masses. We find that for the case studied, with 100 fb^-1 of integrated luminosity (about 400 signal events), the invisible particle's mass can be measured to a precision of 4.1 GeV. We conclude that this technique's precision and accuracy is as good as, if not better than, the best known techniques for invisible-particle mass-determination at hadron colliders.Comment: 20 pages, 11 figures, minor correction

Quasi-degenerate neutrinos and tri-bi-maximal mixing

We consider how, for quasi-degenerate neutrinos with tri-bi-maximal mixing at a high-energy scale, the mixing angles are affected by radiative running from high to low-energy scales in a supersymmetric theory. The limits on the high-energy scale that follow from consistency with the observed mixing are determined. We construct a model in which a non-Abelian discrete family symmetry leads both to a quasi-degenerate neutrino mass spectrum and to near tri-bi-maximal mixing.Comment: 8 pages, 3 figure