21,472 research outputs found

    Euler's fluid equations: Optimal Control vs Optimization

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    An optimization method used in image-processing (metamorphosis) is found to imply Euler's equations for incompressible flow of an inviscid fluid, without requiring that the Lagrangian particle labels exactly follow the flow lines of the Eulerian velocity vector field. Thus, an optimal control problem and an optimization problem for incompressible ideal fluid flow both yield the \emph {same} Euler fluid equations, although their Lagrangian parcel dynamics are \emph{different}. This is a result of the \emph{gauge freedom} in the definition of the fluid pressure for an incompressible flow, in combination with the symmetry of fluid dynamics under relabeling of their Lagrangian coordinates. Similar ideas are also illustrated for SO(N) rigid body motion.Comment: 12 page

    A subset solution to the sign problem in random matrix simulations

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    We present a solution to the sign problem in dynamical random matrix simulations of a two-matrix model at nonzero chemical potential. The sign problem, caused by the complex fermion determinants, is solved by gathering the matrices into subsets, whose sums of determinants are real and positive even though their cardinality only grows linearly with the matrix size. A detailed proof of this positivity theorem is given for an arbitrary number of fermion flavors. We performed importance sampling Monte Carlo simulations to compute the chiral condensate and the quark number density for varying chemical potential and volume. The statistical errors on the results only show a mild dependence on the matrix size and chemical potential, which confirms the absence of sign problem in the subset method. This strongly contrasts with the exponential growth of the statistical error in standard reweighting methods, which was also analyzed quantitatively using the subset method. Finally, we show how the method elegantly resolves the Silver Blaze puzzle in the microscopic limit of the matrix model, where it is equivalent to QCD.Comment: 18 pages, 11 figures, as published in Phys. Rev. D; added references; in Sec. VB: added discussion of model satisfying the Silver Blaze for all N (proof in Appendix E

    An Optimal Control Formulation for Inviscid Incompressible Ideal Fluid Flow

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    In this paper we consider the Hamiltonian formulation of the equations of incompressible ideal fluid flow from the point of view of optimal control theory. The equations are compared to the finite symmetric rigid body equations analyzed earlier by the authors. We discuss various aspects of the Hamiltonian structure of the Euler equations and show in particular that the optimal control approach leads to a standard formulation of the Euler equations -- the so-called impulse equations in their Lagrangian form. We discuss various other aspects of the Euler equations from a pedagogical point of view. We show that the Hamiltonian in the maximum principle is given by the pairing of the Eulerian impulse density with the velocity. We provide a comparative discussion of the flow equations in their Eulerian and Lagrangian form and describe how these forms occur naturally in the context of optimal control. We demonstrate that the extremal equations corresponding to the optimal control problem for the flow have a natural canonical symplectic structure.Comment: 6 pages, no figures. To appear in Proceedings of the 39th IEEEE Conference on Decision and Contro

    Charm spectroscopy in DELPHI

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    The production of charmed particles has been studied using 3.5 milllion hadronic Z decays collected by the DELPHI collaboration at LEP between 1992 and 1995. Large samples of D meson decays have been exclusively reconstructed, allowing to look for Dpi^*pi and Dpipi^*pipi final states. The production fractions of the narrow D10(2420)_1^0(2420) and D20(2460)_2^{*0}(2460) orbital states are measured in c and b quark jets separately. Evidence for a radial state D(2637)^{*'}(2637) is presented in the D+pi+pi^{*+}pi^+pi^- decay mode. %DB Interesting perspectives to look for the wide orbital states in semileptonic B %DB decays are discussed

    Beauty and charm physics at LEP

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    Recent results in charm and beauty physics at LEP are reported. They allow refined tests of strong and electroweak interactions. The importance of measuring as accurately as possible the apex of the unitarity is emphasized