Mixing of pseudoscalar mesons and isospin symmetry breaking

Mixing of the pseudoscalar mesons is discussed in the quark-flavor basis with the hypothesis that the basis decay constants follow the pattern of particle state mixing. The divergences of the axial vector currents which embody the axial vector anomaly, combined with this hypothesis provide a calculational scheme for the parameters describing the mixing of the pion, eta and eta' mesons. Phenomenological applications of this mixing scheme are presented with particular interest focussed on isospin symmetry breaking in QCD estimated as eta and eta' admixtures to the pion. In contrast to previous work a possible difference in the basis decay constants f_u and f_d is considered and consequences of this potentially large effect on the strength of isospin symmetry breaking is discussed.Comment: 10 pages, (using LATEX with w-ijmpa.sty), invited talk presented at MESON 2004, 8th Intern. Workshop on Meson Production, Properties and Interaction

Lessons from cosmic history: The case for a linear star formation -- H2 relation

Observations show that star formation in galaxies is closely correlated with the abundance of molecular hydrogen. Modeling this empirical relation from first principles proves challenging, however, and many questions regarding its properties remain open. For instance, the exact functional form of the relation is still debated and it is also unknown whether it applies at z>4, where CO observations are sparse. Here, we analyze how the shape of the star formation -- gas relation affects the cosmic star formation history and global galaxy properties using an analytic model that follows the average evolution of galaxies in dark matter halos across cosmic time. We show that a linear relation with an H2 depletion time of ~2.5 Gyr, as found in studies of nearby galaxies, results in good agreement with current observations of galaxies at both low and high redshift. These observations include the evolution of the cosmic star formation rate density, the z~4-9 UV luminosity function, the evolution of the mass -- metallicity relation, the relation between stellar and halo mass, and the gas-to-stellar mass ratios of galaxies. In contrast, the short depletion times that result from adopting a highly super-linear star formation -- gas relation lead to large star formation rates, substantial metal enrichment (~0.1 solar), and low gas-to-stellar mass ratios already at z~10, in disagreement with observations. These results can be understood in terms of an equilibrium picture of galaxy evolution in which gas inflows, outflows, and star formation drive the metallicities and gas fractions toward equilibrium values that are determined by the ratio of the accretion time to the gas depletion time. In this picture, the cosmic modulation of the accretion rate is the primary process that drives the evolution of stellar masses, gas masses, and metallicities of galaxies from high redshift until today.Comment: 22 pages, 13 figures, minor revision after referee repor

The Argo Simulation: I. Quenching of Massive Galaxies at High Redshift as a Result of Cosmological Starvation

Observations show a prevalence of high redshift galaxies with large stellar masses and predominantly passive stellar populations. A variety of processes have been suggested that could reduce the star formation in such galaxies to observed levels, including quasar mode feedback, virial shock heating, or galactic winds driven by stellar feedback. However, the main quenching mechanisms have yet to be identified. Here we study the origin of star formation quenching using Argo, a cosmological, hydrodynamical zoom-in simulation that follows the evolution of a massive galaxy at $z\geq{}2$. This simulation adopts the same sub-grid recipes of the Eris simulations, which have been shown to form realistic disk galaxies, and, in one version, adopts also a mass and spatial resolution identical to Eris. The resulting galaxy has properties consistent with those of observed, massive (M_* ~ 1e11 M_sun) galaxies at z~2 and with abundance matching predictions. Our models do not include AGN feedback indicating that supermassive black holes likely play a subordinate role in determining masses and sizes of massive galaxies at high z. The specific star formation rate (sSFR) of the simulated galaxy matches the observed M_* - sSFR relation at early times. This period of smooth stellar mass growth comes to a sudden halt at z=3.5 when the sSFR drops by almost an order of magnitude within a few hundred Myr. The suppression is initiated by a leveling off and a subsequent reduction of the cool gas accretion rate onto the galaxy, and not by feedback processes. This "cosmological starvation" occurs as the parent dark matter halo switches from a fast collapsing mode to a slow accretion mode. Additional mechanisms, such as perhaps radio mode feedback from an AGN, are needed to quench any residual star formation of the galaxy and to maintain a low sSFR until the present time.Comment: 20 pages, 12 figures, 2 tables, accepted for publication in MNRA

The Unemployment Effect of Exchange Rate Volatility in Industrial Countries

Using data on 17 industrial countries from 1982 to 2003 and controlling for a wide array of factors, this paper finds that higher exchange rate volatility increases the unemployment rate. The magnitude of the effect is small. The results are robust to variations in specification.exchange rate volatility; unemployment