Instability of Non-Commutative SYM Theories at Finite Temperature

We extend our previous work on the quasi-particle excitations in N=4 non-commutative U(1) Yang-Mills theory at finite temperature. We show that above some critical temperature there is a tachyon in the spectrum of excitations. It is a collective transverse photon mode polarized in the non-commutative plane. Thus the theory seems to undergo a phase transition at high temperature. Furthermore we find that the group velocity of quasi-particles generically exceeds the speed of light at low momentum.Comment: 23 pages (harvmac b-mode), 5 figures, v2: discussion on U(N) theories and matter in fundamental representation added, misprints corrected, references added v3: reference adde

Short Note on the Unemployment Rate of the “French overseas regionsâ€

This article analyzes the hysteresis hypothesis in the unemployment rates of the four “French overseas regions†(Guadeloupe, Martinique, Guyana, Reunion) [FORs] over the period 1993-2008. We use standard univariate and panel unit root tests, among them Choi (2006) and Lopez (2009) that account for cross-sectional dependence and have improved performance when the number of countries and the time dimension of the data are limited. Our results cannot reject the null hypothesis of a unit root and so find evidence supporting hysteresis in the unemployment rates for the FORs.Hysteresis, Unemployment, Panel unit root test, cross-section dependence, convergence

Scalar Dark Matter Models with Significant Internal Bremsstrahlung

There has been interest recently on particle physics models that may give rise to sharp gamma ray spectral features from dark matter annihilation. Because dark matter is supposed to be electrically neutral, it is challenging to build weakly interacting massive particle models that may accommodate both a large cross section into gamma rays at, say, the Galactic center, and the right dark matter abundance. In this work, we consider the gamma ray signatures of a class of scalar dark matter models that interact with Standard Model dominantly through heavy vector-like fermions (the vector-like portal). We focus on a real scalar singlet S annihilating into lepton-antilepton pairs. Because this two-body final-state annihilation channel is d-wave suppressed in the chiral limit, we show that virtual internal bremsstrahlung emission of a gamma ray gives a large correction, both today and at the time of freeze-out. For the sake of comparison, we confront this scenario to the familiar case of a Majorana singlet annihilating into light lepton-antilepton pairs, and show that the virtual internal bremsstrahlung signal may be enhanced by a factor of (up to) two orders of magnitude. We discuss the scope and possible generalizations of the model.Comment: 25 pages, 10 figures, typos corrected, added references, matching version accepted by JCA

Kaléidoscope : l'instrumentalisation du problème de la drogue dans les Amériques

The purpose of this article is to shed new light on the issue of drug trafficking in the Americas. Specifically its objective is to demonstrate that the perception of drug trafficking as a national security threat is only one aspect of a much more complex socio-political phenomenon. First, this paper presents the multilateral mechanisms that serve as a frame work to facilitate regional cooperation on drug matters. It goes on to analyze the different approaches of regional drug trafficking. It concludes that a much more flexible approach is needed to gain an in-depth understanding of drug politics in the Americas

A Model to Describe Transport Properties in Bi2Sr2(CazPr1−z)Cu2O8+yBi_2Sr_2(Ca_zPr_{1-z})Cu_2O_{8+y}

A pseudo-spin model is proposed, as a means to describe some transport properties (resistivity and Hall mobility) in $Bi_2Sr_2(Ca_zPr_{1-z})Cu_2O_{8+y}$. Our model is based in a double-well potential where tunneling in a given site and interaction between different lattice sites are allowed only through the excited states. Doping of the pure system by the addition of $Pr$ increases the ratio between the activation energy and the tunneling constant. The model Hamiltonian displays some features which are present in the hydrogen-bonded ferroelectrics. Its dynamics is treated in the random phase approximation and the characteristic frequency (time) is used in a Drude formula in order to obtain some transport properties of the system, namely the electric resistivity and the Hall mobility. The quantities calculated in this work are compared with the experimental data of B. Beschoten, S. Sadewasser, G. G\"{u}ntherodt and C. Quitmann [Phys. Rev. Lett.77, 1837(1996)].Comment: 14 pages, 4 figure

Bidding strategies in reverse auctions for the automotive industry procurement

Thesis (M.Eng.)--Massachusetts Institute of Technology, Engineering Systems Division, 2000.Includes bibliographical references (leaves 48-51).The Information Era has brought along a myriad of revolutionary changes. The Internet in particular has given new forms to old paradigms. It is quite a challenge for companies to decide which of all these options best suit their business model and their strategies, hopefully one that also provides them with a competitive edge. One of these new alternatives is online auctions used as a procurement tool. And because of their novelty, it is still unclear whether they are a flawless method of decreasing costs. Although there are many issues concerning this topic, the one addressed here is how the bidders' behavior during the auction (the strategy used to place their bids) affects the outcome, for them as well as for the company setting up the event; and how managers on both sides can maximize the benefits. The basis for the study was a recent auction conducted by autoparts maker Visteon for the procurement of a plastic hose. On the supplier side, the results show that those companies that had a target price before entering the auction, would obtain the most benefits (less sacrifice in profits), since Visteon did not award the projects solely based on price. On the buyer side, having a rich mixture of bidders (diverse in size, location, quality concern for example) enables a more dynamic process (lower prices).by Michel Galland Lopez.M.Eng

A magnetospheric simulation at the space station

It is proposed that a strong magnet (terrella) be flown at or near the Space Station to create an artificial magnetosphere in a laboratory setting. The relative flow of the ionosphere past the terrella will constitute a plasma wind that will interact with the magnetic field of the terrella to produce a localized magnetosphere. This object could then be extensively studied using diagnostic probes attached to the Space Station, or with free flyers. The space and storage requirements would be minimal, since the experiment would be conducted outside the space station. The total equipment would consist of several terrella (with varying surface conductivities), approximately 3 small magnetometer/plasma diagnostic packages, and several gas canisters for upstream seeding. Power requirements would be approximately 60 watts. Several track mounted tethers, each approximately or 200 m long in length, with track parallel to the orbital motion and 100 m long, are also needed. Astronaut time needed would be minimal in the tethered configuration (approximately 4 man hours/week). A free flying configuration, while not needing the tether track, would require much more human interaction

Signatures from Scalar Dark Matter with a Vector-like Quark Mediator

We present a comprehensive study of a model where the dark matter is composed of a singlet real scalar that couples to the Standard Model predominantly via a Yukawa interaction with a light quark and a colored vector-like fermion. A distinctive feature of this scenario is that thermal freeze-out in the early universe may be driven by annihilation both into gluon pairs at one-loop ($gg$) and by virtual internal Bremsstrahlung of a gluon ($q \bar{q} g$). Such a dark matter candidate may also be tested through direct and indirect detection and at the LHC; viable candidates have either a mass nearly degenerate with that of the fermionic mediator or a mass above about 2 TeV.Comment: 30 pages, 10 figures. Matches published versio