Two-loop Renormalization Group Equations in the Standard Model

Two-loop renormalization group equations in the standard model are re-calculated. A new coefficient is found in the beta-function of the quartic coupling and a class of gauge invariants are found to be absent in the beta-functions of hadronic Yukawa couplings. The two-loop beta-function of the Higgs mass parameter is presented in complete form.Comment: 4 pages, RevTe

Two-loop Renormalization Group Equations in General Gauge Field Theories

The complete set of two-loop renormalization group equations in general gauge field theories is presented. This includes the \beta functions of parameters with and without a mass dimension

Finite-mass helium atoms. ii- the 23p state plus

Energy states of helium isoelectronic series investigated by 50-term variational wave function with two nonlinear parameter

Irregular sloshing cold fronts in the nearby merging groups NGC 7618 and UGC 12491: evidence for Kelvin-Helmholtz instabilities

We present results from two \sim30 ks Chandra observations of the hot atmospheres of the merging galaxy groups centered around NGC 7618 and UGC 12491. Our images show the presence of arc-like sloshing cold fronts wrapped around each group center and \sim100 kpc long spiral tails in both groups. Most interestingly, the cold fronts are highly distorted in both groups, exhibiting 'wings' along the fronts. These features resemble the structures predicted from non-viscous hydrodynamic simulations of gas sloshing, where Kelvin-Helmholtz instabilities (KHIs) distort the cold fronts. This is in contrast to the structure seen in many other sloshing and merger cold fronts, which are smooth and featureless at the current observational resolution. Both magnetic fields and viscosity have been invoked to explain the absence of KHIs in these smooth cold fronts, but the NGC 7618/UGC 12491 pair are two in a growing number of both sloshing and merger cold fronts that appear distorted. Magnetic fields and/or viscosity may be able to suppress the growth of KHIs at the cold fronts in some clusters and groups, but clearly not in all. We propose that the presence or absence of KHI-distortions in cold fronts can be used as a measure of the effective viscosity and/or magnetic field strengths in the ICM.Comment: ApJ, accepted. Uses emulateapj styl

Effects of a Soft X-ray Background on Structure Formation at High Redshift

We use three dimensional hydrodynamic simulations to investigate the effects of a soft X-ray background, that could have been produced by an early generation of mini-quasars, on the subsequent cooling and collapse of high redshift pregalactic clouds. The simulations use an Eulerian adaptive mesh refinement technique with initial conditions drawn from a flat Lambda-dominated cold dark matter model cosmology to follow the nonequilibrium chemistry of nine chemical species in the presence of both a soft ultraviolet Lyman-Werner H_2 photodissociating flux and a soft X-ray background extending to 7.2 keV. Although we vary the normalization of the X-ray background by two orders of magnitude, the positive feedback effect of the X-rays on cooling and collapse of the pregalactic cloud expected due to the increased electron fraction is quite mild, only weakly affecting the mass threshold for collapse and the fraction of gas within the cloud that is able to cool. Inside most of the cloud we find that H_2 is in photodissociation equilibrium with the soft UV flux. The net buildup of the electron density needed to enhance H_2 formation occurs too slowly compared to the H_2 photodissociation and dynamical timescales within the cloud to overcome the negative impact of the soft UV photodissociating flux on cloud collapse. However, we find that even in the most extreme cases the first objects to form do rely on molecular hydrogen as coolant and stress that our results do not justify the neglect of these objects in models of galaxy formation.Comment: 17 pages, 8 figures, accepted in MNRA

The Lyman Alpha Forest in Hierarchical Cosmologies

The comparison of quasar absorption spectra with numerically simulated spectra from hierarchical cosmological models of structure formation promises to be a valuable tool to discriminate among these models. We present simulation results for the column density, Doppler b parameter, and optical depth probability distributions for five popular cosmological models.Comment: 4 pages, 3 figures, uses aipproc.sty, to appear in the Proceedings of the 9th Annual October Astrophysics Conference in Maryland, "After the Dark Ages: When Galaxies Were Young (the Universe at 2<z<5)", ed. S. S. Holt and E. P. Smith, October 12-14, 199

Higgs Boson Bounds in Three and Four Generation Scenarios

In light of recent experimental results, we present updated bounds on the lightest Higgs boson mass in the Standard Model (SM) and in the Minimal Supersymmetric extension of the Standard Model (MSSM). The vacuum stability lower bound on the pure SM Higgs boson mass when the SM is taken to be valid up to the Planck scale lies above the MSSM lightest Higgs boson mass upper bound for a large amount of SUSY parameter space. If the lightest Higgs boson is detected with a mass M_{H} < 134 GeV (150 GeV) for a top quark mass M_{top} = 172 GeV (179 GeV), it may indicate the existence of a fourth generation of fermions. The region of inconsistency is removed and the MSSM is salvagable for such values of M_{H} if one postulates the existence of a fourth generation of leptons and quarks with isodoublet degenerate masses M_{L} and M_{Q} such that 60 GeV 170 GeV.Comment: 7 pages, 4 figures. To be published in Physical Review

Constraints on Mass Spectrum of Fourth Generation Fermions and Higgs Bosons

We reanalyze constraints on the mass spectrum of the chiral fourth generation fermions and the Higgs bosons for the standard model (SM4) and the two Higgs doublet model (THDM). We find that the Higgs mass in the SM4 should be larger than roughly the fourth generation up-type quark mass, while the light CP even Higgs mass in the THDM can be smaller. Various mass spectra of the fourth generation fermions and the Higgs bosons are allowed. The phenomenology of the fourth generation models is still rich.Comment: 15 pages, 16 figures; some points clarified, references added, to appear in Phys.Rev.