9,117 research outputs found

    Influence of the Dirac sea on proton electromagnetic knockout

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    We use the relativistic distorted-wave impulse approximation (RDWIA) to study the effects of negative-energy components of Dirac wave functions on the left-right asymmetry for (e,e'p) reactions on 16-O with 0.2 < Q^2 < 0.8 and 12-C with 0.6 < Q^2 < 1.8 (GeV/c)^2. Spinor distortion is more important for the bound state than for the ejectile and the net effect decreases with Q^2. Spinor distortion breaks Godon equivalence and the data favor the CC2 operator with intermediate coupling to the sea. The left-right asymmetry for Q^2 < 1.2 (GeV/c)^2 is described well by RDWIA calcuations, but at Q^2 = 1.8 (GeV/c)^2 the observed variation with missing momentum is flatter than predicted.Comment: 12 pages, 9 figures, to be submitted to PR

    RDWIA analysis of 12C(e,e'p) for Q^2 < 2 (GeV/c)^2

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    We analyze data for 12C(e,e'p) with Q^2 < 2 (GeV/c)^2 using the relativistic distorted-wave impulse approximation (RDWIA) based upon Dirac-Hartree wave functions. The 1p normalization extracted from data for Q^2 > 0.6 (GeV/c)^2 is approximately 0.87, independent of Q^2, which is consistent with the predicted depletion by short-range correlations. The total 1p and 1s strength for E_m < 80 MeV approaches 100% of IPSM, consistent with a continuum contribution for 30 < E_m < 80 MeV of about 12% of IPSM. Similarly, a scale factor of 1.12 brings RDWIA calculations into good agreement with 12C(e,e'p) data for transparency. We also analyzed low Q^2 data from which a recent NDWIA analysis suggested that spectroscopic factors might depend strongly upon the resolution of the probe. We find that momentum distributions for their empirical Woods-Saxon wave functions fit to low Q^2 data for parallel kinematics are too narrow to reproduce data for quasiperpendicular kinematics, especially for larger Q^2, and are partly responsible for reducing fitted normalization factors.Comment: 19 pages, 14 figures, to be submitted to PR

    The Final Merger of Black-Hole Binaries

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    Recent breakthroughs in the field of numerical relativity have led to dramatic progress in understanding the predictions of General Relativity for the dynamical interactions of two black holes in the regime of very strong gravitational fields. Such black-hole binaries are important astrophysical systems and are a key target of current and developing gravitational-wave detectors. The waveform signature of strong gravitational radiation emitted as the black holes fall together and merge provides a clear observable record of the process. After decades of slow progress, these mergers and the gravitational-wave signals they generate can now be routinely calculated using the methods of numerical relativity. We review recent advances in understanding the predicted physics of events and the consequent radiation, and discuss some of the impacts this new knowledge is having in various areas of astrophysics.Comment: 57 pages; 9 figures. Updated references & fixed typos. Published version is at http://www.annualreviews.org/doi/abs/10.1146/annurev.nucl.010909.08324

    Black-hole binaries, gravitational waves, and numerical relativity

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    Understanding the predictions of general relativity for the dynamical interactions of two black holes has been a long-standing unsolved problem in theoretical physics. Black-hole mergers are monumental astrophysical events, releasing tremendous amounts of energy in the form of gravitational radiation, and are key sources for both ground- and space-based gravitational-wave detectors. The black-hole merger dynamics and the resulting gravitational waveforms can only be calculated through numerical simulations of Einstein's equations of general relativity. For many years, numerical relativists attempting to model these mergers encountered a host of problems, causing their codes to crash after just a fraction of a binary orbit could be simulated. Recently, however, a series of dramatic advances in numerical relativity has allowed stable, robust black-hole merger simulations. This remarkable progress in the rapidly maturing field of numerical relativity, and the new understanding of black-hole binary dynamics that is emerging is chronicled. Important applications of these fundamental physics results to astrophysics, to gravitational-wave astronomy, and in other areas are also discussed.Comment: 54 pages, 42 figures. Some typos corrected & references updated. Essentially final published versio

    Odbojna polarizacija u elektro-tvorbi mezona

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    We briefly review the objectives and techniques for several experiments upon recoil polarization in electroproduction of pseudoscalar mesons that will be performed in Hall A at Jefferson Laboratory.Daje se pregled ciljeva i metoda više mjerenja odbojne polarizacije u elektro-tvorbi pseudoskalarnih mezona koja će se izvoditi u hali A Jeffersonovog Laboratorija

    Homes Affordable for Good: Covenants and Ground Leases as Long-Term Resale-Restriction Devices

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    Covenants and ground leases have been, and continue to be, used to create shared spaces that are fundamentally, and often invidiously, exclusive. Famously made a dead letter in the case of Shelley v. Kraemer, covenants banning resale to nonwhite households put the force of law behind the segregated birth of America’s suburbs. Today, gated residential communities and shopping malls assure a degree of class exclusivity through covenants and commercial ground leases, respectively. These same legal mechanisms, however, are now deployed to assure long-term inclusion as well. Developers of affordable housing are creating homes that are not only beneficial to the original homeowners but also available for future generations of qualified home buyers. When selling the newly developed homes, they are having subsidized homeowners promise to pass the good deals on to future home buyers. These resale restrictions allow single-family homes to be sold, and later resold, to low and moderate-income households in neighborhoods that would otherwise be unavailable to them. Affordability protections of 15 years or less are relatively common and can be achieved through a number of legal arrangements. Common law and statutory hostility to long-term private arrangements that limit alienability, however, have made the search for perpetual affordability more challenging. Those seeking to sustain economic diversity in residential communities over multiple generations of homeowners have turned to covenants authorized by statute and ground leases as the vehicles by which these promises can be enforced. As stand-alone enforceable promises that run with land, covenants have become the primary vehicle for Inclusionary Zoning programs that seek to preserve the mixed-income nature of affected for-profit housing developments for the long haul. Community Land Trusts have generally preferred the ground lease, a standard device for shopping mall creation, to ensure that subsidized single-family homes developed by nonprofit housing organizations can remain affordable forever. As economic diversity in communities is given its proper place as a long-term goal for America’s metropolitan areas, 21st century real estate law will need to integrate both covenants and ground lease reversion interests as stable, effective means of enforcing affordability-preserving resale restrictions. In addition to arguing for the importance of both covenants and ground leases as affordability conservation mechanisms, this article will analyze and evaluate each device as to its effectiveness in achieving the development goal of creating and sustaining economically diverse communities of choice

    Article 27 and Mexican Land Reform: The Legacy of Zapata\u27s Dream

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    This student note takes an historical look at indigenous land tenure in Mexico and the role that limited alienability has played in sustaining indigenous agriculture from the time of the Aztecs up until the reforms enacted by the Salinas administration in Mexico in the early 1990\u27s. As the piece was in edits, the Zapatista rebellion broke out and the text was amended to note the role that land tenure played in the uprising

    Land Trusts That Conserve Communities

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    Much has been written about land trusts that conserve wilderness, agriculture or other environmentally beneficial uses that would be threatened by unfettered development. In the context of inner-cities, Community Land Trusts (CLTs) conserve neighborhoods. Like their environmental and agricultural counterparts, CLTs employ use restrictions to prioritize communally beneficial development. Conserving communities, however, requires other legal tools as well. CLTs create and sustain permanently affordable homes to break the market’s bias toward socioeconomic homogeneity. CLTs also make room, literally, for green space, sites of shared culture and other productive activities that the market tends to commercialize or marginalize. By sustaining a range of housing opportunities, CLTs decommodify community membership. By managing commons land with a light touch, they allow that diverse population to celebrate and deepen personal creativity even while promoting cohesion. Most importantly, as democratically controlled organizations, CLTs and their community partner organizations do not offer these primary goods as gifts but instead give community members only the opportunities to fight for them and continually discern good and better ways of retaining them. The process of sustaining community by owning land itself sustains community. If adjustment of alienability and commons management comprise the substance of community stewardship, then the development and the governance of the land trust itself is its transformative process. The substance and process of connecting community and land evoke an understanding of human flourishing that challenges conventional welfare economics approaches. This article argues that Community Land Trusts are better appreciated, evaluated and guided by neo-Aristotelian social philosophies that appreciate the importance of the community and land in the urban neighborhood context. As an advocate for policies focused on human capabilities, Amartya Sen returns market economics to its roots in a moral philosophy of the human good and demonstrates the need for holistic, broad-based development, albeit one that is thoroughly committed to personal freedom. Alasdair MacIntyre insists that popularly controlled, community institutions are needed to foster and sustain the networks of giving and receiving that will inculcate the “virtues of acknowledged dependence” essential to an authentic and productive politics. While Sen’s writings develop a broader information base for judging the gains of CLTs, MacIntyre’s work finds indispensable communal institutions like CLTs that preserve the gains of citizens continually contending with both the state and the market. Drawing upon the actual struggles and achievements of communities in Boston, Los Angeles and Syracuse, this article will show how land trusts conserve communities and the significance of long-term community control of neighborhood land resources for the stable growth of inner-city communities and the people who make them up. Part II will discuss how short-term investment thinking is harming inner-city neighborhoods and the measures three community land trusts have taken to conserve their communities. Part III of the article will examine the neo-Aristotelian thought of Amartya Sen and Alasdair MacIntyre as providing a rationale for community conservation institutions in a world divided between the market and the state. The article will conclude by showing how a theoretical awareness of the significance of local communities in human flourishing informs the precise corporate and property relationships inner-city neighborhoods should look to for creating and sustaining economically diverse communities of choice
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