Effects Of Wave Reflections In Pulsatile Flow Through Vascular Networks

The subject of this thesis is wave propagation in fluid-filled distensible tubes, with emphasis on the effects of wave reflections in pulsatile flow through vascular networks.;A computing scheme for wave propagation in vascular trees with branching junctions is developed. Due to the large number of junctions usually present in vascular trees, only one-dimensional theory is practical. The propagating waves consist of transmitted and reflected components. A generalized model of branching trees is considered. An iterative formula for pressure in each vessel is derived, which leads to explicit expressions for pressure and flow distributions in the entire tree. Pressure distributions in different tree structures are then calculated and flow features are examined.;Analytical expressions for reflection coefficients at a converging junction are derived, and the distributions of pressure and flow in a closed bypass loop with two branching vessels are obtained. Distributions of pressure amplitude in two loops simulating a coronary bypass and an abdominal bypass are then calculated using these solutions, and flow features are examined. A converging junction with multiple inflow vessels is also considered, and general governing equations which can be solved for the reflection coefficients in each of the inflow vessels are derived.;A bypass loop consisting of three branching vessels are also considered. Reflection coefficients at the junctions are obtained by transforming the loop into a tree. Analytical expressions for the distributions of pressure and flow are then derived. Again, pressure amplitude in a loop simulating a coronary bypass is calculated and wave characteristics are examined.;The main feature of our approach is that the solutions are obtained in terms of elementary functions and give transmitted and reflected wave components explicitly, hence both global and local flow properties can be studied for waves propagating in complex vascular networks. Results obtained in this thesis can be used not only to investigate wave characteristics in actual blood vessel systems, but also to lay down the necessary ground for more sophisticated real-time simulation of pulse wave propagation, which can eventually provide a useful tool for clinical analysis and diagnosis of cardiovascular diseases relating to blood flow problems

Study of the quasi-two-body decays B^{0}_{s} \rightarrow \psi(3770)(\psi(3686))\pi^+\pi^- with perturbative QCD approach

In this note, we study the contributions from the S-wave resonances, f_{0}(980) and f_{0}(1500), to the B^{0}_{s}\rightarrow \psi(3770)\pi^ {+}\pi^{-} decay by introducing the S-wave \pi\pi distribution amplitudes within the framework of the perturbative QCD approach. Both resonant and nonresonant contributions are contained in the scalar form factor in the S-wave distribution amplitude \Phi^S_{\pi\pi}. Since the vector charmonium meson \psi(3770) is a S-D wave mixed state, we calculated the branching ratios of S-wave and D-wave respectively, and the results indicate that f_{0}(980) is the main contribution of the considered decay, and the branching ratio of the \psi(2S) mode is in good agreement with the experimental data. We also take the S-D mixed effect into the B^{0}_{s}\rightarrow \psi(3686)\pi^ {+}\pi^{-} decay. Our calculations show that the branching ratio of B^{0}_{s}\rightarrow \psi(3770)(\psi(3686))\pi^ {+}\pi^{-} can be at the order of 10^{-5}, which can be tested by the running LHC-b experiments.Comment: 10 pages, 3 figure

Vacuum stability in stau-neutralino coannihilation in MSSM

The stau-neutralino coannihilation provides a feasible way to accommodate the observed cosmological dark matter (DM) relic density in the minimal supersymmetric standard model (MSSM). In such a coannihilation mechanism the stau mass usually has an upper bound since its annihilation rate becomes small with the increase of DM mass. Inspired by this observation, we examine the upper limit of stau mass in the parameter space with a large mixing of staus. We find that the stau pair may dominantly annihilate into dibosons and hence the upper bound on the stau mass ($\sim400$ GeV) obtained from the $f\bar{f}$ final states can be relaxed. Imposing the DM relic density constraint and requiring a long lifetime of the present vacuum, we find that the lighter stau mass can be as heavy as about 1.4 TeV for the stau maximum mixing. However, if requiring the present vacuum to survive during the thermal history of the universe, this mass limit will reduce to about 0.9 TeV. We also discuss the complementarity of vacuum stability and direct detections in probing this stau coannihilation scenario.Comment: 12 pages, 6 figure