248 research outputs found

    Simulating the Electroweak Phase Transition in the SU(2) Higgs Model

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    Numerical simulations are performed to study the finite temperature phase transition in the SU(2) Higgs model on the lattice. In the presently investigated range of the Higgs boson mass, below 50 GeV, the phase transition turns out to be of first order and its strength is rapidly decreasing with increasing Higgs boson mass. In order to control the systematic errors, we also perform studies of scaling violations and of finite volume effects.Comment: 46 pages with 16 figures, DESY-94-15

    Defect fugacity, Spinwave Stiffness and T_c of the 2-d Planar Rotor Model

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    We obtain precise values for the fugacities of vortices in the 2-d planar rotor model from Monte Carlo simulations in the sector with {\em no} vortices. The bare spinwave stiffness is also calculated and shown to have significant anharmonicity. Using these as inputs in the KT recursion relations, we predict the temperature T_c = 0.925, using linearised equations, and Tc=0.899±>.005T_c = 0.899 \pm >.005 using next higher order corrections, at which vortex unbinding commences in the unconstrained system. The latter value, being in excellent agreement with all recent determinations of T_c, demonstrates that our method 1) constitutes a stringent measure of the relevance of higher order terms in KT theory and 2) can be used to obtain transition temperatures in similar systems with modest computational effort.Comment: 7 pages, 4 figure

    CO2-related vasoconstriction superimposed on ischemic medullary brain autonomic nuclei may contribute to sudden death

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    Introduction: In 2015, a multinational randomized controlled phase IV clinical trial of adaptive servoventilation for the management of heart failure with central sleep apnea was halted in progress because more patients in the study group were dying than in the control group. One year later, another large clinical trial reported results on the effectiveness of continuous positive airway pressure (CPAP) in preventing sudden death and other cardiovascular events such as heart attack and stroke in patients with preexisting vascular disease as well as obstructive sleep apnea. Background: Sudden unexpected death has been associated with many types of small and nonmalignant medullary brain lesions, like demyelination plaques \u2014 largely asymptomatic until they caused sudden death. Many such medullary lesions, typically without hemorrhage or mass effect, have in themselves been previously considered relatively harmless \u2014 in cases where they have been known to be present. Discussion: Why did not the improved pulmonary ventilation and subsequently improved gas exchange provided during the CPAP and servoventilation clinical trials help to resolve any ischemic lesions that may have been present both in the heart and in the medulla, thereby tending to normalize interactions between the vagal neural structures and the heart? CO2 is a potent dilator of brain vasculature, thereby increasing blood flow to the brain. When ventilation is increased, even if only to improve it back toward normal from a depressed steady-state level, the alveolar partial pressure of carbon dioxide is decreased, likely resulting in a converse relative vasoconstriction in the brain, thereby reducing blood flow in the brain, especially in watershed areas like the solitary tract nucleus. In normal physiology, this is demonstrated impressively by the ability of hyperventilation to induce loss of consciousness. Conclusions: The findings of several clinical trials recently reported, taken together with neuropathology case studies reported elsewhere, suggest that additional research is warranted in regard to the mechanisms by which focal medullary autonomic brain ischemia may be related to sudden death in general medical illnesses \u2014 and how it may additionally be influenced by changes in arterial CO2 levels

    Numerical tests of the electroweak phase transition and thermodynamics of the electroweak plasma

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    The finite temperature phase transition in the SU(2) Higgs model at a Higgs boson mass MH34M_H \simeq 34 GeV is studied in numerical simulations on four-dimensional lattices with time-like extensions up to Lt=5L_t=5. The effects of the finite volume and finite lattice spacing on masses and couplings are studied in detail. The errors due to uncertainties in the critical hopping parameter are estimated. The thermodynamics of the electroweak plasma near the phase transition is investigated by determining the relation between energy density and pressure.Comment: latex2e, 32 pages, 11 figures with epsfig; A few comments and a new table are adde

    Electroweak phase transition by four dimensional simulations

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    The finite temperature phase transition in the SU(2)-Higgs model at a Higgs boson mass MH35M_H \simeq 35 GeV is studied in numerical simulations on four dimensional lattices with time-like extensions up to Lt=5L_t=5. Tc/MHT_c/M_H is extrapolated to the continuum limit and a comparison with the perturbative prediction is made. A one-loop calculation to the coupling anisotropies of the SU(2)-Higgs model on lattices with asymmetric lattice spacings is presented. Our numerical simulations show that the above perturbative result is applicable in the phenomenologically interesting parameter region.Comment: 3 pages, Latex, 3 figures, Talk presented at LATTICE96(electroweak) by Z. Fodo

    Identification of the critical temperature from non-equilibrium time-dependent quantities

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    We present a new procedure able to identify and measure the critical temperature. This method is based on the divergence of the relaxation time approaching the critical point in quenches from infinite temperature. We introduce a dimensionless quantity that turns out to be time-independent at the critical temperature. The procedure does not need equilibration and allows for a relatively fast identification of the critical temperature. The method is first tested in the ferromagnetic Ising model and then applied to the one-dimensional Ising spin glass with power-law interactions. Here we always find a finite critical temperature also in presence of a uniform external field, in agreement with the mean-field picture for the low temperature phase of spin glasses.Comment: 6 pages, 10 figure

    Critical aging of a ferromagnetic system from a completely ordered state

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    We adapt the non-linear σ\sigma model to study the nonequilibrium critical dynamics of O(n) symmetric ferromagnetic system. Using the renormalization group analysis in d=2+ϵd=2+\epsilon dimensions we investigate the pure relaxation of the system starting from a completely ordered state. We find that the average magnetization obeys the long-time scaling behavior almost immediately after the system starts to evolve while the correlation and response functions demonstrate scaling behavior which is typical for aging phenomena. The corresponding fluctuation-dissipation ratio is computed to first order in ϵ\epsilon and the relation between transverse and longitudinal fluctuations is discussed.Comment: 5 pages, revtex

    Rejection-free Geometric Cluster Algorithm for Complex Fluids

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    We present a novel, generally applicable Monte Carlo algorithm for the simulation of fluid systems. Geometric transformations are used to identify clusters of particles in such a manner that every cluster move is accepted, irrespective of the nature of the pair interactions. The rejection-free and non-local nature of the algorithm make it particularly suitable for the efficient simulation of complex fluids with components of widely varying size, such as colloidal mixtures. Compared to conventional simulation algorithms, typical efficiency improvements amount to several orders of magnitude

    Monte Carlo Simulations of Short-time Critical Dynamics with a Conserved Quantity

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    With Monte Carlo simulations, we investigate short-time critical dynamics of the three-dimensional anti-ferromagnetic Ising model with a globally conserved magnetization msm_s (not the order parameter). From the power law behavior of the staggered magnetization (the order parameter), its second moment and the auto-correlation, we determine all static and dynamic critical exponents as well as the critical temperature. The universality class of ms=0m_s=0 is the same as that without a conserved quantity, but the universality class of non-zero msm_s is different.Comment: to appear in Phys. Rev.
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