Angular Momentum Transport by MHD Turbulence in Accretion Disks: Gas Pressure Dependence of the Saturation Level of the Magnetorotational Instability

The saturation level of the magnetorotational instability (MRI) is investigated using three-dimensional MHD simulations. The shearing box approximation is adopted and the vertical component of gravity is ignored, so that the evolution of the MRI is followed in a small local part of the disk. We focus on the dependence of the saturation level of the stress on the gas pressure, which is a key assumption in the standard alpha disk model. From our numerical experiments it is found that there is a weak power-law relation between the saturation level of the Maxwell stress and the gas pressure in the nonlinear regime; the higher the gas pressure, the larger the stress. Although the power-law index depends slightly on the initial field geometry, the relationship between stress and gas pressure is independent of the initial field strength, and is unaffected by Ohmic dissipation if the magnetic Reynolds number is at least 10. The relationship is the same in adiabatic calculations, where pressure increases over time, and nearly-isothermal calculations, where pressure varies little with time. Our numerical results are qualitatively consistent with an idea that the saturation level of the MRI is determined by a balance between the growth of the MRI and the dissipation of the field through reconnection. The quantitative interpretation of the pressure-stress relation, however, may require advances in the theoretical understanding of non-steady magnetic reconnection.Comment: 45 pages, 5 tables, 17 figures, accepted for publication in Ap

Loschmidt Echo and Berry phase of the quantum system coupled to the XY spin chain: Proximity to quantum phase transition

We study the Loschmidt echo (LE) of a coupled system consisting of a central spin and its surrounding environment described by a general XY spin-chain model. The quantum dynamics of the LE is shown to be remarkably influenced by the quantum criticality of the spin chain. In particular, the decaying behavior of the LE is found to be controlled by the anisotropy parameter of the spin chain. Furthermore, we show that due to the coupling to the spin chain, the ground-state Berry phase for the central spin becomes nonanalytical and its derivative with respect to the magnetic parameter $\lambda$ in spin chain diverges along the critical line $\lambda=1$, which suggests an alternative measurement of the quantum criticality of the spin chain.Comment: 15 pages, 5 figure

Measurement of Lande g factor of 5D5/2 state of BaII with a single trapped ion

We present the first terrestrial measurement of the Lande g factor of the 5D5/2 state of singly ionized barium. Measurements were performed on single Doppler-cooled 138Ba+ ions in a linear Paul trap. A frequency-stabilized fiber laser with nominal wavelength 1.762 um was scanned across the 6S1/25D5/2 transition to spectroscopically resolve transitions between Zeeman sublevels of the ground and excited states. From the relative positions of the four narrow transitions observed at several different values for the applied magnetic field, we find a value of 1.2020+/-0.0005 for g of 5D5/2.Comment: 3 figure

Prospects for Direct CP Violaton in Exclusive and Inclusive Charmless B decays

Within the Standard Model, CP rate asymmetries for $B\to K^-\pi^{+,0}$ could reach 10%. With strong final state phases, they could go up to 20--30%, even for $\bar K^0\pi^-$ mode which would have opposite sign. We can account for $K^-\pi^{+}$, $\bar K^0\pi^-$ and $\phi K$ rate data with new physics enhanced color dipole coupling and destructive interference. Asymmetries could reach 40--60% for $K\pi$ and $\phi K$ modes and are all of the same sign. We are unable to account for $K^-\pi^0$ rate. Our inclusive study supports our exclusive results.Comment: Minor changes, correct a small bug in Fig. 1(b). Version to appear in Phys. Rev. Let

Willingness to pay for treatment with highly active antiretroviral (HAART) drugs: a rural case study in Cameroon

This paper reports on the willingness of HIV/AIDS patients to pay for the most affordable triple therapy combination of antiretrovirals in a local setting in Cameroon. Questionnaires were used to evaluate willingness to pay, and patients who could still afford their medication 6 months after the survey were also investigated, to give an indication of actual ability to pay. In addition, oral interviews were carried out for clarification. In all, 84 patients out of a total of 186 were involved in the study. Results indicated that more men (39%) were willing to pay than women (22%), although more women (56%) were afflicted than men.Willingness to pay was directly dependent on cost with 69%, 22% and 9% of respondents indicating willingness to pay $1,$2 and $3 a day respectively. After 6 months of treatment, 22SAHARA-J (2004) 1(2): 107-113 Keywords: HIV/AIDS, treatment, access to antiretrovirals, drug costs, affordability. RÉSUMÉ Cette communication porte sur une bonne volonté des patients souffrants du VIH/SIDA à payer un prix assez abordable de la thérapie triple des anti-rétroviraux dans un milieu rural au Cameroun. Des questionnaires ont été utilisés afin d'évaluer cette bonne volonté à payer les médicaments. De plus, les patients qui avaient les moyens d'acheter leurs médicaments 6 mois après ces épreuves furent interviewés également. En plus de cela, les patients ont été interviewés avec le but d'obtenir des éclaircissements. Parmi les 186 patients, 84 participaient à l'étude. Les résultats de cette étude ont démontré que davantage d'hommes (39$1, $2 et$3 par jour respectivement. Au bout d'une période de 6 mois de traitement, 22% de patients étaient encore en thérapie. Une majorité de patients ont arrêté le traitement au bout de 6 mois faute de manque d'argent. En dehors du coût, le stigmate, l'incrédulité et les effets secondaires des médicaments étaient les facteurs principaux qui étaient contre la volonté de payer. Il est conseillé d'améliorer les services de consultation, de fournir de l'information, de réduire le coût de médicaments ainsi que les essais en laboratoire et l'élaboration des programmes de déstigmatisation afin d'améliorer la capacité de patients à payer pour les anti-rétroviraux. SAHARA-J (2004) 1(2): 107-113 Mots clés: le VIH/SIDA, le traitement, l'accès aux anti-rétroviraux, le coût de médicaments, avoir des moyens d'acheter