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.

Universal Evolution of CKM Matrix Elements

We derive the two-loop evolution equations for the Cabibbo-Kobayashi-Maskawa matrix. We show that to leading order in the mass and CKM hierarchies the scaling of the mixings $|V_{ub}|^2$, $|V_{cb}|^2$, $|V_{td}|^2$, $|V_{ts}|^2$ and of the rephase-invariant CP-violating parameter $J$ is universal to all orders in perturbation theory. In leading order the other CKM elements do not scale. Imposing the constraint $\lambda _b=\lambda _{\tau}$ at the GUT scale determines the CKM scaling factor to be $\simeq 0.58$ in the MSSM.Comment: 17 pages + 2 figures not included (available upon request), revised version fixes discrepancy between S and S^{1/2}, no other changes, MAD/PH/72

Chiral Hierarchies, Compositeness and the Renormalization Group

A wide class of models involve the fine--tuning of significant hierarchies between a strong--coupling ``compositeness'' scale, and a low energy dynamical symmetry breaking scale. We examine the issue of whether such hierarchies are generally endangered by Coleman--Weinberg instabilities. A careful study using perturbative two--loop renormalization group methods finds that consistent large hierarchies are not generally disallowed.Comment: 22 pp + 5 figs (uuencoded and submitted separately), SSCL-Preprint-490; FERMI-PUB-93/035-

Chandra Observations of Gas Stripping in the Elliptical Galaxy NGC 4552 in the Virgo Cluster

We use a 54.4 ks Chandra observation to study ram-pressure stripping in NGC4552 (M89), an elliptical galaxy in the Virgo Cluster. Chandra images in the 0.5-2 keV band show a sharp leading edge in the surface brightness 3.1 kpc north of the galaxy center, a cool (kT =0.51^{+0.09}_{-0.06} keV) tail with mean density n_e ~5.4 +/- 1.7 x 10^{-3} cm^{-3} extending ~10 kpc to the south of the galaxy, and two 3-4 kpc horns of emission extending southward away from the leading edge. These are all features characteristic of supersonic ram-pressure stripping of galaxy gas, due to NGC4552's motion through the surrounding Virgo ICM. Fitting the surface brightness profile and spectra across the leading edge, we find the galaxy gas inside the edge is cooler (kT = 0.43^{+0.03}_{-0.02} keV) and denser (n_e ~ 0.010 cm^{-3}) than the surrounding Virgo ICM (kT = 2.2^{+0.7}_{-0.4} keV and n_e = 3.0 +/- 0.3 x 10^{-4} cm^{-3}). The resulting pressure ratio between the free-streaming ICM and cluster gas at the stagnation point is ~7.6^{+3.4}_{-2.0} for galaxy gas metallicities of 0.5^{+0.5}_{-0.3} Zsolar, which suggests that NGC4552 is moving supersonically through the cluster with a velocity v ~ 1680^{+390}_{-220} km/s (Mach 2.2^{+0.5}_{-0.3}) at an angle xi ~ 35 +/- 7 degrees towards us with respect to the plane of the sky.Comment: 31 pages, 12 figures, ApJ, in press; paper split into 2 parts, Paper I(sec 1-3) here, added figs and discussion to conform to published version; Paper II (sec. 4) in astro-ph/060440

Testable Flipped SU(5) x U(1)_X Models

The little hierarchy between the GUT scale and the string scale may give us some hints that can be tested at the LHC. To achieve string-scale gauge coupling unification, we introduce additional vector-like particles. We require that these vector-like particles be standard, form complete GUT multiplets, and have masses around the TeV scale or close to the string scale. Interestingly, only the flipped SU(5) x U(1)_X models can work elegantly. We consider all possible sets of vector-like particles with masses around the TeV scale. And we introduce vector-like particles with masses close to the string scale which can mimic the string-scale threshold corrections. We emphasize that all of these vector-like particles can be obtained in the interesting flipped SU(5) x U(1)_X string models from the four-dimensional free fermionic string construction. Assuming the low-energy supersymmetry, high-scale supersymmetry, and split supersymmetry, we show that the string-scale gauge coupling unification can indeed be achieved in the flipped SU(5) x U(1)_X models. These models can be tested at the LHC by observing simple sets of vector-like particles at the TeV scale. Moreover, we discuss a simple flipped SU(5) x U(1)_X model with string-scale gauge coupling unification and high-scale supersymmetry by introducing only one pair of the vector-like particles at the TeV scale, and we predict the corresponding Higgs boson masses. Also, we briefly comment on the string-scale gauge coupling unification in the model with low-energy supersymmetry by introducing only one pair of the vector-like particles at the intermediate scale. And we briefly comment on the mixings among the SM fermions and the corresponding extra vector-like particles.Comment: RevTex4, 25 pages, 4 figures, 4 tables, comments and references added, version to appear in NP

Hydrodynamical Simulations of the Lyman Alpha Forest: Model Comparisons

We investigate the properties of the Lyman alpha forest as predicted by numerical simulations for a range of currently viable cosmological models. This is done in order to understand the dependencies of the forest on cosmological parameters. Focusing on the redshift range from two to four, we show that: (1) most of the evolution in the distributions of optical depth, flux and column density can be understood by simple scaling relations, (2) the shape of optical depth distribution is a sensitive probe of the amplitude of density fluctuations on scales of a few hundred kpc, (3) the mean of the b distribution (a measure of the width of the absorption lines) is also very sensitive to fluctuations on these scales, and decreases as they increase. We perform a preliminary comparison to observations, where available. A number of other properties are also examined, including the evolution in the number of lines, the two-point flux distribution and the HeII opacity.Comment: 37 pages, 21 figures, submitted to Ap

Vacuum stability, neutrinos, and dark matter

Motivated by the discovery hint of the Standard Model (SM) Higgs mass around 125 GeV at the LHC, we study the vacuum stability and perturbativity bounds on Higgs scalar of the SM extensions including neutrinos and dark matter (DM). Guided by the SM gauge symmetry and the minimal changes in the SM Higgs potential we consider two extensions of neutrino sector (Type-I and Type-III seesaw mechanisms) and DM sector (a real scalar singlet (darkon) and minimal dark matter (MDM)) respectively. The darkon contributes positively to the $\beta$ function of the Higgs quartic coupling $\lambda$ and can stabilize the SM vacuum up to high scale. Similar to the top quark in the SM we find the cause of instability is sensitive to the size of new Yukawa couplings between heavy neutrinos and Higgs boson, namely, the scale of seesaw mechanism. MDM and Type-III seesaw fermion triplet, two nontrivial representations of $SU(2)_{L}$ group, will bring the additional positive contributions to the gauge coupling $g_{2}$ renormalization group (RG) evolution and would also help to stabilize the electroweak vacuum up to high scale.Comment: 18 pages, 15 figures; published versio

XMM-Newton Observation of an X-ray Trail Between the Spiral Galaxy NGC6872 and the Central Elliptical NGC6876 in the Pavo Group

We present XMM-Newton observations of a trail of enhanced X-rayemission extending along the 8'.7 X 4' region between the spiral NGC6872 and the dominant elliptical NGC6876 in the Pavo Group,the first known X-ray trail associated with a spiral galaxy in a poor galaxy group and, with projected length of 90 kpc, one of the longest X-ray trails observed in any system. The X-ray surface brightness in the trail region is roughly constant beyond ~20 kpc of NGC6876 in the direction of NGC6872. The trail is hotter (~ 1 keV) than the undisturbed Pavo IGM (~0.5 keV) and has low metal abundances (0.2 Zsolar). The 0.5-2 keV luminosity of the trail, measured using a 67 X 90 kpc rectangular region, is 6.6 X 10^{40} erg/s. We compare the properties of gas in the trail to the spectral properties of gas in the spiral NGC6872 and in the elliptical NGC6876 to constrain its origin. We suggest that the X-ray trail is either IGM gas gravitationally focused into a Bondi-Hoyle wake, a thermal mixture of ~64% Pavo IGM gas with ~36% galaxy gas that has been removed from the spiral NGC6872 by turbulent viscous stripping, or both, due to the spiral's supersonic motion at angle xi ~ 40 degrees with respect to the plane of the sky, past the Pavo group center (NGC6876) through the densest region of the Pavo IGM. Assuming xi = 40 degrees and a filling factor eta in a cylindrical volume with radius 33 kpc and projected length 90 kpc, the mean electron density and total hot gas mass in the trail is 9.5 X 10^{-4}*eta^{-1/2} cm^{-3} and 1.1 X 10^{10}*eta^{1/2} Msolar, respectively.Comment: typos corrected in Eq. 7 & 8, figures and discussion unchanged, 39 pages, 11 postscript figures, submitted to Ap

Axion Models with High-Scale Supersymmetry Breaking

Inspired by the possibility of high-scale supersymmetry breaking in the string landscape where the cosmological constant problem and the gauge hierarchy problem can be solved while the strong CP problem is still a challenge for naturalness, we propose a supersymmetric KSVZ axion model with an approximate universal intermediate-scale ($\sim 10^{11}$ GeV) supersymmetry breaking. To protect the global Peccei--Quinn (PQ) symmetry against quantum gravitational violation, we consider the gauged discrete $Z_N$ PQ symmetry. In our model the axion can be a cold dark matter candidate, and the intermediate supersymmetry breaking scale is directly related to the PQ symmetry breaking scale. Gauge coupling unification can be achieved at about $2.7\times 10^{16}$ GeV. The Higgs mass range is 130 GeV to 160 GeV. We briefly discuss other axion models with high-scale supersymmetry breaking where the stabilization of the axion solution is similar.Comment: RevTex4, 24 pages, 3 figures, typos corrected, references adde