11,104 research outputs found

    BCS-BEC crossover in nuclear matter with the relativistic Hartree-Bogoliubov theory

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    Based on the relativistic Hartree-Bogoliubov theory, the influence of the pairing interaction strength on the di-neutron correlations and the crossover from superfluidity of neutron Cooper pairs in the 1S0^{1}S_{0} channel to Bose-Einstein condensation of di-neutron pairs is systematically investigated in the nuclear matter. The bare nucleon-nucleon interaction Bonn-B is taken in the particle-particle channel with an effective factor to simulate the medium effects and take into account the possible ambiguity of pairing force, and the effective interaction PK1 is used in the particle-hole channel. If the effective factor is larger than 1.10, a di-neutron BEC state appears in the low-density limit, and if it is smaller than 0.85, the neutron Cooper pairs are found totally in the weak coupling BCS region. The reference values of several characteristic quantities which characterize the BCS-BEC crossover are obtained respectively from the dimensionless parameter 1/(kFna)1/(k_{\rm Fn}a) with aa the scattering length and kFnk_{\rm{Fn}} the neutron Fermi momentum, the zero-momentum transfer density correlation function D(0) and the effective chemical potential Ξ½n\nu_{\rm n}.Comment: 8 pages, 4 figures, 2 tables, Accepted Thursday Jun 14, 2012 for Physical Review

    Informal payments in developing countries' public health sector

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    In China and some other developing countries' public health sectors, many patients give their doctors a payment outside the official channel before a major treatment. This secret payment has been documented as informal payment in the literature. We argue that the fundamental cause for informal payments is that patients have more information about doctors' skill than the government does. The price, set by the government, for services offered by doctors cannot fully differentiate patients' various needs. As a consequence, informal payment rises as a tool for patients to compete for the skillful doctor. We study the welfare implications of different policies that can potentially be used to regulate such payments. Patient heterogeneity plays a central role in welfare implications of different policies: when patients' willingness-to-pay differs a lot, informal payments should be allowed and when it differs little, informal payments should be banned. Also we show that selling the right to choose physicians publicly always improves social welfare.informal payments; public health sector; welfare; efficiency

    Continuous reservoir modeling updating by integrating experimental data using an ensemble Kalman Filter

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    The continuous researvoir model updating is widely used to calibrate reservoir simulation models to production data, but many challenges remain. First, few real field data are available to test the new history matching method, and most of the data sets are synthetic cases. Second, computational cost may be high when using non-Gaussian priors or nonlinear models. Third, with large complex models, the simulation runs and history matching method require huge memory allocations. This dissertation achieves a continuous reservoir model updating workflow with a meter-scale , two-phase flow experiment. Both production and seismic data are collected in the experiment. Because the data are high-frequency sequential data with noise, the EnKF method is used to efficiently integrate them. To better understand the problem, scaling analysis is done on the capillary transition zone. Two new dimensionless numbers are introduced-capillary time and capillary length. We found that for different models, if their capillary time and gravity number are equal, the capillary length would be the same. The scaling analysis results help us find a proper flow rate for the sand tank experiment. Two experiments are conducted to test the workflow and the EnKF method. In the first one, both the production and seismic data are collected and analyzed. The production data have large errors in the flow rate and they are integrated to improve reservoir models using EnKF method. The history matching results are in an acceptable range which demonstrate that even if the observation data has large error, the EnKF method still works. In the second experiment, the errors of flow rate are reduced by measuring manually with a graduated cylinder. Because the data quality are much better in the second experiment, the observations can be matched easily
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