40,039 research outputs found

    Collisional-dissociative recombination of electrons with molecular ions

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    Recombination rate calculation of electrons in plasma having ions of molecules with both repulsive and bound neutral state

    Naive time-reversal odd phenomena in semi-inclusive deep-inelastic scattering from light-cone constituent quark models

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    We present results for leading-twist azimuthal asymmetries in semi-inclusive lepton-nucleon deep-inelastic scattering due to naively time-reversal odd transverse-momentum dependent parton distribution functions from the light-cone constituent quark model. We carefully discuss the range of applicability of the model, especially with regard to positivity constraints and evolution effects. We find good agreement with available experimental data from COMPASS and HERMES, and present predictions to be tested in forthcoming experiments at Jefferson Lab.Comment: 10 pages, 7 figures, discussion of evolution effects extended, to appear in Phys.Rev.

    Two-Loop Virtual Corrections to Drell-Yan Production at order alpha_s alpha^3

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    The Drell-Yan mechanism for the production of lepton pairs is one of the most basic processes for physics studies at hadron colliders. It is therefore important to have accurate theoretical predictions. In this work we compute the two-loop virtual mixed QCD x QED corrections to Drell-Yan production. We evaluate the Feynman diagrams by decomposing the amplitudes into a set of known master integrals and their coefficients, which allows us to derive an analytical result. We also perform a detailed study of the ultraviolet and infrared structure of the two-loop amplitude and the corresponding poles in epsilon.Comment: 20 pages, 3 figure

    The Stability of an Isotropic Cosmological Singularity in Higher-Order Gravity

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    We study the stability of the isotropic vacuum Friedmann universe in gravity theories with higher-order curvature terms of the form (RabRab)n(R_{ab}R^{ab})^{n} added to the Einstein-Hilbert Lagrangian of general relativity on approach to an initial cosmological singularity. Earlier, we had shown that, when % n=1, a special isotropic vacuum solution exists which behaves like the radiation-dominated Friedmann universe and is stable to anisotropic and small inhomogeneous perturbations of scalar, vector and tensor type. This is completely different to the situation that holds in general relativity, where an isotropic initial cosmological singularity is unstable in vacuum and under a wide range of non-vacuum conditions. We show that when n1n\neq 1, although a special isotropic vacuum solution found by Clifton and Barrow always exists, it is no longer stable when the initial singularity is approached. We find the particular stability conditions under the influence of tensor, vector, and scalar perturbations for general nn for both solution branches. On approach to the initial singularity, the isotropic vacuum solution with scale factor a(t)=tP/3a(t)=t^{P_{-}/3} is found to be stable to tensor perturbations for 0.5<n<1.13090.5<n< 1.1309 and stable to vector perturbations for 0.861425<n10.861425 < n \leq 1, but is unstable as t0t \to 0 otherwise. The solution with scale factor a(t)=tP+/3a(t)=t^{P_{+}/3} is not relevant to the case of an initial singularity for n>1n>1 and is unstable as t0t \to 0 for all nn for each type of perturbation.Comment: 25 page

    Renormalization of the Vector Current in QED

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    It is commonly asserted that the electromagnetic current is conserved and therefore is not renormalized. Within QED we show (a) that this statement is false, (b) how to obtain the renormalization of the current to all orders of perturbation theory, and (c) how to correctly define an electron number operator. The current mixes with the four-divergence of the electromagnetic field-strength tensor. The true electron number operator is the integral of the time component of the electron number density, but only when the current differs from the MSbar-renormalized current by a definite finite renormalization. This happens in such a way that Gauss's law holds: the charge operator is the surface integral of the electric field at infinity. The theorem extends naturally to any gauge theory.Comment: 9 pages. Corresponds to published version (Phys. Rev. D), including appendix about Weeks's parado

    Bending vibrational data accuracy study

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    Computer program for predicting structural bending vibrational dat

    Relative distributions of W's and Z's at low transverse momenta

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    Despite large uncertainties in the W±W^\pm and Z0Z^0 transverse momentum (qTq_T) distributions for q_T\lsim 10 GeV, the ratio of the distributions varys little. The uncertainty in the ratio of WW to ZZ qTq_T distributions is on the order of a few percent, independent of the details of the nonperturbative parameterization.Comment: 13 pages in revtex, 5 postscript figures available upon request, UIOWA-94-0

    A concept for a fuel efficient flight planning aid for general aviation

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    A core equation for estimation of fuel burn from path profile data was developed. This equation was used as a necessary ingredient in a dynamic program to define a fuel efficient flight path. The resultant algorithm is oriented toward use by general aviation. The pilot provides a description of the desired ground track, standard aircraft parameters, and weather at selected waypoints. The algorithm then derives the fuel efficient altitudes and velocities at the waypoints

    Cosmological models with flat spatial geometry

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    The imposition of symmetries or special geometric properties on submanifolds is less restrictive than to impose them in the full space-time. Starting from this idea, in this paper we study irrotational dust cosmological models in which the geometry of the hypersurfaces generated by the fluid velocity is flat, which supposes a relaxation of the restrictions imposed by the Cosmological Principle. The method of study combines covariant and tetrad methods that exploits the geometrical and physical properties of these models. This procedure will allow us to determine all the space-times within this class as well as to study their properties. Some important consequences and applications of this study are also discussed.Comment: 12 pages, LaTeX2e, IOP style. To appear in Classical and Quantum Gravit
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