QCD in neutron stars and strange stars

This paper provides an overview of the possible role of Quantum Chromo Dynamics (QDC) for neutron stars and strange stars. The fundamental degrees of freedom of QCD are quarks, which may exist as unconfined (color superconducting) particles in the cores of neutron stars. There is also the theoretical possibility that a significantly large number of up, down, and strange quarks may settle down in a new state of matter known as strange quark matter, which, by hypothesis, could be more stable than atomic nuclei. In the latter case new classes of self-bound, color superconducting objects, ranging from strange quark nuggets to strange quark stars, should exist. The properties of such objects will be reviewed along with the possible existence of deconfined quarks in neutron stars. Implications for observational astrophysics are pointed out.Comment: 7 pages, 2 figures; presented at the T(R)OPICAL QCD Workshop, September 26 - October 2, 2010, Cairns, Australi

Sovereign debt (re)structuring-Where do we stand?

Argentina’s debt default in 2001 moved the international community to launch initiatives to develop procedures for the orderly restructuring of unsustainable sovereign debt. These initiatives come against the backdrop of the increase in public debt in emerging market economies over the last decade, of large-scale fi nancial crises linked to the level or structure of sovereign debt and, a recent development, of several cases of sovereign default on international bonds. These developments pose two challenges. Firstly, the complexity of sovereign debt dynamics makes it necessary to strengthen international institutions’ assessment capacity. The International Monetary Fund (IMF) has thus launched initiatives to improve the assessment of debt sustainability and balance sheet weaknesses. Secondly, the developments have prompted the re-examination of IMF facilities for managing debt crises. The absence of a clear framework in this area could create a moral hazard risk, as the international community could be tempted to avoid necessary restructuring by granting substantial fi nancial assistance. In practice, defining such a framework is complex, given that reforms in the area of restructuring influence the way in which debts are structured. In fact, the features of sovereign debt aim to reconcile two requirements: first, ensuring that the sovereign debtor honours the terms of the debt contract if it has the means to do so (ex ante efficiency); second, making sure that the cost of default is not excessive when the sovereign debtor is effectively unable to pay back (ex post effi ciency). The international community currently favours a market-based approach. Its first pillar is a result of the development and spread of collective action clauses (CAC), following the Quarles Report by the Group of 10. Incorporated into debt contracts, CAC aim to reduce the problems arising in inter-creditor co-ordination, by defi ning, in advance, the decision rules applicable in the event of re-negotiation. The “Principles for Stable Capital Flows and Fair Debt Restructuring in Emerging Markets”, whose defi nition was fostered notably by the Governor of the Banque de France and the Group of 20, form the second pillar. They supply guidelines for the parties involved to steer the exchange of information and facilitate co-ordination between debtors and creditors. The market-based approach is not exclusive of a formal regime, e.g. the IMF’s Sovereign Debt Restructuring Mechanism (SDRM), where such a regime is feasible. In the market-based approach, the IMF acts as an expert via the supply of information and analyses, and also as a “monitor” via the negotiation and monitoring of programmes. The provision of loans, which is in principle limited, aims mainly to facilitate renegotiation by giving credibility to the sovereign’s policy. By contrast, a more formal regime would suppose limiting IMF involvement, so as to guarantee the independence of the mechanism. Whatever the case, the efficiency of the overhauled restructuring framework, which is built on contractual provisions and general principles rather than on a formal regime, will depend on the Fund’s capacity to fully perform its three functions.

DCP-collected absolute target reflectance signatures assist accurate interpretation of ERTS-1 imagery

Data collection platforms (DCP's) are being used at a Black Hills, South Dakota, test site (MMC 226A) to record radiometric measurements needed to determine solar and atmospheric parameters that affect ERTS-1 multispectral scanner radiance measurements. A total of 72 channels of analog data transmitted from an unattended ground truth site via three DCP's at least six times a day. The system has operated with only minor problems since September, sending forth daily measurements of biophysical responses and atmospheric conditions. Comparisons of scene radiance data calculated from ERTS images with that measured on the ground show the image-measured values to be 35 percent higher for the green channel and 20 percent higher for the red channel for the same scene targets. Radiance values for channels 6 and 7 are nearly the same from the ground data and from the imagery

Directed-Loop Quantum Monte Carlo Method for Retarded Interactions

The directed-loop quantum Monte Carlo method is generalized to the case of retarded interactions. Using the path integral, fermion-boson or spin-boson models are mapped to actions with retarded interactions by analytically integrating out the bosons. This yields an exact algorithm that combines the highly-efficient loop updates available in the stochastic series expansion representation with the advantages of avoiding a direct sampling of the bosons. The application to electron-phonon models reveals that the method overcomes the previously detrimental issues of long autocorrelation times and exponentially decreasing acceptance rates. For example, the resulting dramatic speedup allows us to investigate the Peierls quantum phase transition on chains of up to $1282$ sites.Comment: 5 pages, 3 figures, supplemental material; final versio

Phonon spectral function of the one-dimensional Holstein-Hubbard model

We use the continuous-time interaction expansion (CT-INT) quantum Monte Carlo method to calculate the phonon spectral function of the one-dimensional Holstein-Hubbard model at half-filling. Our results are consistent with a soft-mode Peierls transition in the adiabatic regime, and the existence of a central peak related to long-range order in the Peierls phase. We explain a previously observed feature at small momenta in terms of a hybridization of charge and phonon excitations. Tuning the system from a Peierls to a metallic phase with a nonzero Hubbard interaction suppresses the central peak, but a significant renormalization of the phonon dispersion remains. In contrast, the dispersion is only weakly modified in the Mott phase. We discuss finite-size effects, the relation to the dynamic charge structure factor, as well as additional sum rules and their implications. Finally, we reveal the existence of a discrete symmetry in a continuum field theory of the Holstein model, which is spontaneously broken in the Peierls phase.Comment: 10 pages, 4 figure