8,053 research outputs found

    Coulomb effects in nucleon-deuteron polarization-transfer coefficients

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    Coulomb effects in the neutron-deuteron and proton-deuteron polarization-transfer coefficients Kyy′K_y^{y'}, Kzx′K_z^{x'}, Kyx′x′−y′y′K_y^{x'x'-y'y'} and Kyz′z′K_y^{z'z'} are studied at energies above the deuteron breakup threshold. Theoretical predictions for these observables are evaluated in the framework of the Kohn Variational Principle using correlated basis functions to expand the three-nucleon scattering wave function. The two-nucleon Argonne v18v_{18} and the three-nucleon Urbana IX potentials are considered. In the proton-deuteron case, the Coulomb interaction between the two protons is included explicitly and the results are compared to the experimental data available at Elab=10,19,22.7E_{lab}=10,19,22.7 MeV. In the neutron-deuteron case, a comparison to a recent measurement of Kyy′K_y^{y'} by Hempen {\sl et al.} at Elab=19E_{lab}=19 MeV evidences a contribution of the calculated Coulomb effects opposite to those extracted from the experiment.Comment: 7 pages, 3 figure

    Electromagnetic transitions for A=3 nuclear systems

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    Recent advances in the study of pd radiative capture in a wide range of center-of-mass energy below and above deuteron breakup threshold are presented and discussed.Comment: Invited lead talk at the 19th European Conference on Few-Body Problems in Physics, Groningen, The Netherlands, 8/23 - 8/27 2004, 5 pages, 4 figure

    Evolution in the iron abundance of the ICM

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    We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies at z>0.3z>0.3, which cover a temperature range of 3>kT>153> kT > 15 keV. Our analysis is aimed at measuring the iron abundance in the ICM out to the highest redshift probed to date. We find that the emission-weighted iron abundance measured within (0.15−0.3)Rvir(0.15-0.3) R_{vir} in clusters below 5 keV is, on average, a factor of ∼2\sim2 higher than in hotter clusters, following Z(T)≃0.88T−0.47Z⊙Z(T)\simeq 0.88 T^{-0.47} Z_\odot, which confirms the trend seen in local samples. We made use of combined spectral analysis performed over five redshift bins at 0.3>z>1.30.3> z > 1.3 to estimate the average emission weighted iron abundance. We find a constant average iron abundance ZFe≃0.25Z⊙Z_{Fe}\simeq 0.25 Z_\odot as a function of redshift, but only for clusters at z>0.5z>0.5. The emission-weighted iron abundance is significantly higher (ZFe≃0.4Z⊙Z_{Fe}\simeq0.4 Z_\odot) in the redshift range z≃0.3−0.5z\simeq0.3-0.5, approaching the value measured locally in the inner 0.15Rvir0.15 R_{vir} radii for a mix of cool-core and non cool-core clusters in the redshift range 0.1<z<0.30.1<z<0.3. The decrease in ZFeZ_{Fe} with zz can be parametrized by a power law of the form ∼(1+z)−1.25\sim(1+z)^{-1.25}. The observed evolution implies that the average iron content of the ICM at the present epoch is a factor of ∼2\sim2 larger than at z≃1.2z\simeq 1.2. We confirm that the ICM is already significantly enriched (ZFe≃0.25Z⊙Z_{Fe}\simeq0.25 Z_\odot) at a look-back time of 9 Gyr. Our data provide significant constraints on the time scales and physical processes that drive the chemical enrichment of the ICM.Comment: 4 pages, 4 figures, to appear in the Proceedings of "The Extreme Universe in the Suzaku Era", Dicember 2006, Kyoto (Japan

    Electromagnetic structure of A=2 and 3 nuclei and the nuclear current operator

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    Different models for conserved two- and three-body electromagnetic currents are constructed from two- and three-nucleon interactions, using either meson-exchange mechanisms or minimal substitution in the momentum dependence of these interactions. The connection between these two different schemes is elucidated. A number of low-energy electronuclear observables, including (i) npnp radiative capture at thermal neutron energies and deuteron photodisintegration at low energies, (ii) ndnd and pdpd radiative capture reactions, and (iii) isoscalar and isovector magnetic form factors of 3^3H and 3^3He, are calculated in order to make a comparative study of these models for the current operator. The realistic Argonne v18v_{18} two-nucleon and Urbana IX or Tucson-Melbourne three-nucleon interactions are taken as a case study. For AA=3 processes, the bound and continuum wave functions, both below and above deuteron breakup threshold, are obtained with the correlated hyperspherical-harmonics method. Three-body currents give small but significant contributions to some of the polarization observables in the 2^2H(p,γp,\gamma)3^3He process and the 2^2H(n,γn,\gamma)3^3H cross section at thermal neutron energies. It is shown that the use of a current which did not exactly satisfy current conservation with the two- and three-nucleon interactions in the Hamiltonian was responsible for some of the discrepancies reported in previous studies between the experimental and theoretical polarization observables in pdpd radiative capture.Comment: 48 pages, 25 figures, 4 tables, revtex4. Submitted to Phys. Rev.

    Chiral effective field theory predictions for muon capture on deuteron and 3He

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    The muon-capture reactions 2H(\mu^-,\nu_\mu)nn and 3He(\mu^-,\nu_\mu)3H are studied with nuclear strong-interaction potentials and charge-changing weak currents, derived in chiral effective field theory. The low-energy constants (LEC's) c_D and c_E, present in the three-nucleon potential and (c_D) axial-vector current, are constrained to reproduce the A=3 binding energies and the triton Gamow-Teller matrix element. The vector weak current is related to the isovector component of the electromagnetic current via the conserved-vector-current constraint, and the two LEC's entering the contact terms in the latter are constrained to reproduce the A=3 magnetic moments. The muon capture rates on deuteron and 3He are predicted to be 399(3) sec^{-1} and 1494 (21) sec^{-1}, respectively, where the spread accounts for the cutoff sensitivity as well as uncertainties in the LEC's and electroweak radiative corrections. By comparing the calculated and precisely measured rates on 3He, a value for the induced pseudoscalar form factor is obtained in good agreement with the chiral perturbation theory prediction.Comment: 4 pages, 2 figures, revisited version accepted for publication on Phys. Rev. Let

    Electrodisintegration of 3^3He below and above deuteron breakup threshold

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    Recent advances in the study of electrodisintegration of 3He are presented and discussed. The pair-correlated hyperspherical harmonics method is used to calculate the initial and final state wave functions, with a realistic Hamiltonian consisting of the Argonne v18 two-nucleon and Urbana IX three-nucleon interactions. The model for the nuclear current and charge operators retains one- and many-body contributions. Particular attention is made in the construction of the two-body current operators arising from the momentum-dependent part of the two-nucleon interaction. Three-body current operators are also included so that the full current operator is strictly conserved. The present model for the nuclear current operator is tested comparing theoretical predictions and experimental data of pd radiative capture cross section and spin observables.Comment: 5 pages, 5 figures, submitted to Eur. Phys. J.

    Explanation, Vindication, and the Role of Normative Theory in Legal Scholarship

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    I find Yuracko\u27s inquiry quite intriguing, but I have doubts about the central thesis. My aim, in the discussion that follows, is to press Yuracko on two fronts, one methodological, the other substantive. As concerns substance, I shall say a bit as this commentary progresses about why Yuracko would need much more support to make good on her claim that an implicit perfectionism explains decisional antidiscrimination law. My chief interest, however, lies with certain questions as to what methodology she means to follow in her efforts to develop a more complete theory of employment discrimination law. Yuracko\u27s approach is one that seems to me to be quite common in legal scholarship, at least in legal scholarship with more theoretical ambitions. For reasons that I shall offer shortly, I find the approach puzzling and problematic. I want to be clear, however, that in pressing methodological questions, my aim is not so much to pose a problem for Yuracko, who understandably adopts what seems the usual approach of scholars in her field. Rather, I mean to point out how and why, in my view, legal theorists need to pay far greater attention to methodology than they generally have. As for Yuracko, I greatly admire her efforts, in this article and elsewhere, to tackle complex questions about the theoretical unity and underpinnings of employment discrimination law. For that very reason, I want to urge her to adopt an approach better suited to exploring theoretical issues and to supporting her most central ideas

    What is the Meaning of Marriage ?

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    There is legitimacy to the conservative approach that goes beyond looking at the impact on people\u27s welfare. This article responds to Professor Arneson by offering a different framework to investigate the underlying theoretical values behind appropriate state policies regarding private relationships. Neither the prioritarian consequentialist, nor the lockean libertarian captures the other side of the marriage debate, that is, the traditional ideal. The state’s interest in fostering the conservative, traditional marriage relationships begins to emerge through the author\u27s assumptions that Kantian ethics is correct, that social stability depends on others—inviolate value, and that through love one can apprehend the value of another. Therefore, the author concludes that to the extent that same-sex marriage helps to encourage these assumptions—that is, encouraging a regard for the value of others—the state also has compelling reasons to extend the boundaries of legal marriage to encompass same sex-unions
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