Superluminal pions in a hadronic fluid

We study the propagation of pions at finite temperature and finite chemical potential in the framework of the linear sigma model with 2 quark flavors and $N_c$ colors. The velocity of massless pions in general differs from that of light. One-loop calculations show that in the chiral symmetry broken phase pions, under certain conditions, propagate faster than light.Comment: 8 pages, 3 figures included. Considerably revised, discussions expanded, one figure added, typos corrected, results unchanged. To be published in Phys. Rev.

General-Relativistic Thomas-Fermi model

A system of self-gravitating massive fermions is studied in the framework of the general-relativistic Thomas-Fermi model. We study the properties of the free energy functional and its relation to Einstein's field equations. A self-gravitating fermion gas we then describe by a set of Thomas-Fermi type self-consistency equations.Comment: 7 pages, LaTex, to appear in Gen. Rel. Gra

Is there a Supermassive Black Hole at the Center of the Milky Way?

This review outlines the observations that now provide an overwhelming scientific case that the center of our Milky Way Galaxy harbors a supermassive black hole. Observations at infrared wavelength trace stars that orbit about a common focal position and require a central mass (M) of 4 million solar masses within a radius of 100 Astronomical Units. Orbital speeds have been observed to exceed 5,000 km/s. At the focal position there is an extremely compact radio source (Sgr A*), whose apparent size is near the Schwarzschild radius (2GM/c^2). This radio source is motionless at the ~1 km/s level at the dynamical center of the Galaxy. The mass density required by these observations is now approaching the ultimate limit of a supermassive black hole within the last stable orbit for matter near the event horizon.Comment: Invited review submitted to International Journal of Modern Physics D; 23 pages; 10 figure

Ghost Condensate Busting

Applying the Thomas-Fermi approximation to renormalizable field theories, we construct ghost condensation models that are free of the instabilities associated with violations of the null-energy condition.Comment: 9 pages, minor corrections, a reference added, the discussion on consistency of the Thomas-Fermi approximation expanded, to appear in JCA

Generalized Chaplygin Gas Model: Dark Energy - Dark Matter Unification and CMBR Constraints

The generalized Chaplygin gas (GCG) model allows for an unified description of the recent accelerated expansion of the Universe and the evolution of energy density perturbations. This dark energy - dark matter unification is achieved through an exotic background fluid whose equation of state is given by $p = - A/\rho^{\alpha}$, where $A$ is a positive constant and $0 < \alpha \le 1$. Stringent constraints on the model parameters can be obtained from recent WMAP and BOOMERanG bounds on the locations of the first few peaks and troughs of the Cosmic Microwave Background Radiation (CMBR) power spectrum as well as SNe Ia data.Comment: 9 pages, 2 figures; essay selected for an honorable mention by the Gravity Research Foundation, 200

Chiral-symmetry restoration in the linear sigma model at nonzero temperature and baryon density

We study the chiral phase transition in the linear sigma model with 2 quark flavors and $N_c$ colors. One-loop calculations predict a first-order phase transition at both $\mu=0$ and $\mu\neq 0$. We also discuss the phase diagram and make a comparison with a thermal parametrization of existing heavy-ion experimental data.Comment: 12 pages, 6 ps-figures, LaTe

Quenched QCD at finite density

Simulations of quenched $QCD$ at relatively small but {\it nonzero} chemical potential $\mu$ on $32 \times 16^3$ lattices indicate that the nucleon screening mass decreases linearly as $\mu$ increases predicting a critical chemical potential of one third the nucleon mass, $m_N/3$, by extrapolation. The meson spectrum does not change as $\mu$ increases over the same range, from zero to $m_\pi/2$. Past studies of quenched lattice QCD have suggested that there is phase transition at $\mu = m_\pi/2$. We provide alternative explanations for these results, and find a number of technical reasons why standard lattice simulation techniques suffer from greatly enhanced fluctuations and finite size effects for $\mu$ ranging from $m_\pi/2$ to $m_N/3$. We find evidence for such problems in our simulations, and suggest that they can be surmounted by improved measurement techniques.Comment: 23 pages, Revte

Comment on "Quantum Scattering of Heavy Particles from a 10 K Cu(111) Surface"

In the original paper Althoff et al. (see ibid., vol.79, p.4429 (1997)) reported a study of scattering of thermal Ne, Ar, and Kr atoms from a Cu(111) surface in which they assessed the corresponding Debye-Waller factor (DWF) as a function of the particle mass m in a wide range of substrate temperature T. The experiments were interpreted by the semiclassical DWF theory in which the projectile moves on the classical recoilless trajectory and the surface vibrations are quantized. Siber and Gumhalter claim that the experiments described by Althoff et al. were carried out in the quantum scattering regime in which the semiclassical scalings of Althoff et al. do not hold and the semiclassical DWE significantly deviates from the exact quantum one both in the low and high T limits. Hence, it is claimed, the quantum scattering data of Althoff et al. cannot be reliably interpreted by the semiclassical theory.Comment: 1 page (2 figures) - comment in Phys. Rev. Let

Transition from a quark-gluon plasma in the presence of a sharp front

The effect of a sharp front separating the quark-gluon plasma phase from the hadronic phase is investigated. Energy-momentum conservation and baryon number conservation constrain the possible temperature jump across the front. If one assumes that the temperature in the hadronic phase is $T\simeq$ 200 MeV , as has been suggested by numerous results from relativistic ion collisions, one can determine the corresponding temperature in the quark phase with the help of continuity equations across the front. The calculations reveal that the quark phase must be in a strongly supercooled state. The stability of this solution with respect to minor modifications is investigated. In particular the effect of an admixture of hadronic matter in the quark phase (e.g. in the form of bubbles) is considered in detail. In the absence of admixture the transition proceeds via a detonation transition and is accompanied by a substantial super-cooling of the quark-gluon plasma phase. The detonation is accompanied by less supercooling if a small fraction of bubbles is allowed. By increasing the fraction of bubbles the supercooling becomes weaker and eventually the transition proceeds via a smoother deflagration wave.Comment: 10 pages, manuscript in TeX, 9 figures available as Postscript files, CERN-TH 6923/9

Effect of Helicobacter Pylori Eradication on Extent of Duodenal Gastric Metaplasia and Grade of Gastritis

The extent of the regression of duodenal gastric metaplasia (DGM) after the eradication of Helicobacter pylori infection is controversial. Therefore, we decided to assess the degree of DGM before, sex weeks and one year after H. pylori eradication. 105 consecutive Helicobacter pylori positive patients with endoscopically proven duodenal ulcer, with DGM and Helicobacter pylori infection were recruited for this study. The diagnosis of Helicobacter pylori infection was based on CLO-test and histology, and DGM was assessed on four bulb biopsies taken before, sex weeks and one year after Helicobacter pylori eradication. Histological assessment of Helicobacter pylori associated gastritis was performed according to the Sydney classification. Follow up study on 98 patients before, six weeks and one year after the eradication of Helicobacter pylori showed that the mean extent of DGM did not change significantly after eradication and did not differ when compared with 14 patients with persisting infection. Our results show that the inflammatory process related to Helicobacter pylori does not play the main role in the development of DGM