Three-Point Functions at Finite Temperature

We study 3-point functions at finite temperature in the closed time path formalism. We give a general decomposition of the eight component tensor in terms of seven vertex functions. We derive a spectral representation for these seven functions in terms of two independent real spectral functions. We derive relationships between the seven functions and obtain a representation of the vertex tensor that greatly simplifies calculations in real time.Comment: 21 pages LaTeX; one ps-figure; Revised version, contains more references and discussio

On the order of the operators in the Douglas-Rachford algorithm

The Douglas-Rachford algorithm is a popular method for finding zeros of sums of monotone operators. By its definition, the Douglas-Rachford operator is not symmetric with respect to the order of the two operators. In this paper we provide a systematic study of the two possible Douglas-Rachford operators. We show that the reflectors of the underlying operators act as bijections between the fixed points sets of the two Douglas-Rachford operators. Some elegant formulae arise under additional assumptions. Various examples illustrate our results.Comment: 10 page

Leadership Structure and Corporate Governance in Switzerland

The question of whether the CEO should also serve as chairman of the board is one of the most hotly debated issues in the recent corporate governance discussion. While agencytheoretic arguments advocate a separation of decision and control functions, the empirical evidence focusing on U.S. companies is not conclusive. In this context evidence from a country with a different practice of CEO succession may provide important new insights with respect to the question of whether one leadership structure should generally be preferred to the other one. This article fills this gap by investigating the valuation effects of leadership structure in Switzerland where – in contrast to the U.S. – a separation of the CEO and chairman functions is common. Consistent with the majority of prior research focusing on the U.S., the authors found no evidence of a systematic and significant difference in valuation between firms with combined and firms with separated functions. They also investigated whether leadership structure is related to firm-level corporate governance characteristics and found a similar curvilinear relationship between leadership structure and managerial shareholdings as is observed between firm value and managerial shareholdings. An implication is that possible agency costs associated with a combined function are mitigated by a higher incentive alignment of the CEO/chairman through an adequate level of managerial shareholdings. Over the last few years corporate governance became an important investment criterion, which is for example reflected in the emergence of various corporate governance ratings. The authors of this article additionally investigated whether firm value is significantly related to firm level corporate governance as measured by a broad survey-based index for a representative sample of Swiss firms. They documented a positive and significant relationship between the corporate governance index and firm valuation. This finding is robust to controlling for a series of additional governance mechanisms related to ownership structure, board characteristics, and leverage as well as a potential endogeneity of these mechanisms.Leadership structure; Firm valuation; Corporate governance; Managerial shareholdings

Effects of jet quenching on the hydrodynamical evolution of quark-gluon plasma

We study the effects of jet quenching on the hydrodynamical evolution of the quark-gluon plasma (QGP) fluid created in a heavy-ion collision. In jet quenching, a hard QCD parton, before fragmenting into a jet of hadrons, deposits a fraction of its energy in the medium, leading to suppressed production of high-pT hadrons. Assuming that the deposited energy quickly thermalizes, we simulate the subsequent hydrodynamic evolution of the QGP fluid. For partons moving at supersonic speed, v_p > c_s, and sufficiently large energy loss, a shock wave forms leading to conical flow [1]. The PHENIX Collaboration recently suggested that observed structures in the azimuthal angle distribution [2] might be caused by conical flow. We show here that conical flow produces different angular structures than predicted in [1] and that, for phenomenologically acceptable values of parton energy loss, conical flow effects are too weak to explain the structures seen by PHENIX [2].Comment: 4 pages, 3 figures. Last figure changed, now showing angular distribution of pions instead of photons. Added comments on "lost jets" and pT-dependence of angular correlation

Jet trails and Mach cones: The interaction of microquasars with the ISM

A sub-set of microquasars exhibit high peculiar velocity with respect to the local standard of rest due to the kicks they receive when being born in supernovae. The interaction between the radio plasma released by microquasar jets from such high-velocity binaries with the ISM must lead to the production of trails and bow shocks similar to what is observed in narrow-angle tailed radio galaxies and pulsar wind nebulae. We present a set of numerical simulations of this interaction that illuminate the long term dynamical evolution and the observational properties of these microquasar bow shock nebulae and trails. We find that this interaction always produces a structure that consists of a bow shock, a trailing neck, and an expanding bubble. Using our simulations to model emission, we predict that the shock surrounding the bubble and the neck should be visible in H{\alpha} emission, the interior of the bubble should be visible in synchrotron radio emission, and only the bow shock is likely to be detectable in X-ray emission. We construct an analytic model for the evolution of the neck and bubble shape and compare this model with observations of X-ray binary SAX J1712.6-3739.Comment: 33 pages, 13 figures, 1 table; Accepted to Ap