14,225 research outputs found
Impersonating the Standard Model Higgs Boson: Alignment without Decoupling
In models with an extended Higgs sector there exists an alignment limit, in
which the lightest CP-even Higgs boson mimics the Standard Model Higgs. The
alignment limit is commonly associated with the decoupling limit, where all
non-standard scalars are significantly heavier than the boson. However,
alignment can occur irrespective of the mass scale of the rest of the Higgs
sector. In this work we discuss the general conditions that lead to "alignment
without decoupling", therefore allowing for the existence of additional
non-standard Higgs bosons at the weak scale. The values of for
which this happens are derived in terms of the effective Higgs quartic
couplings in general two-Higgs-doublet models as well as in supersymmetric
theories, including the MSSM and the NMSSM. Moreover, we study the information
encoded in the variations of the SM Higgs-fermion couplings to explore regions
in the parameter space.Comment: 42 Pages, 8 figure
The Influence of Metallicity on Star Formation in Protogalaxies
In cold dark matter cosmological models, the first stars to form are believed
to do so within small protogalaxies. We wish to understand how the evolution of
these early protogalaxies changes once the gas forming them has been enriched
with small quantities of heavy elements, which are produced and dispersed into
the intergalactic medium by the first supernovae. Our initial conditions
represent protogalaxies forming within a fossil H II region, a previously
ionized region that has not yet had time to cool and recombine. We study the
influence of low levels of metal enrichment on the cooling and collapse of
ionized gas in small protogalactic halos using three-dimensional, smoothed
particle hydrodynamics (SPH) simulations that incorporate the effects of the
appropriate chemical and thermal processes. Our previous simulations
demonstrated that for metallicities Z < 0.001 Z_sun, metal line cooling alters
the density and temperature evolution of the gas by less than 1% compared to
the metal-free case at densities below 1 cm-3) and temperatures above 2000 K.
Here, we present the results of high-resolution simulations using particle
splitting to improve resolution in regions of interest. These simulations allow
us to address the question of whether there is a critical metallicity above
which fine structure cooling from metals allows efficient fragmentation to
occur, producing an initial mass function (IMF) resembling the local Salpeter
IMF, rather than only high-mass stars.Comment: 3 pages, 2 figures, First Stars III conference proceeding
Solenoidal versus compressive turbulence forcing
We analyze the statistics and star formation rate obtained in high-resolution
numerical experiments of forced supersonic turbulence, and compare with
observations. We concentrate on a systematic comparison of solenoidal
(divergence-free) and compressive (curl-free) forcing, which are two limiting
cases of turbulence driving. Our results show that for the same RMS Mach
number, compressive forcing produces a three times larger standard deviation of
the density probability distribution. When self-gravity is included in the
models, the star formation rate is more than one order of magnitude higher for
compressive forcing than for solenoidal forcing.Comment: 1 page, to appear in the proceedings of the IAU General Assembly
Joint Discussion 14 "FIR2009: The ISM of Galaxies in the Far-Infrared and
Sub-Millimetre", ed. M. Cunningha
Vacuum Stability and Higgs Diphoton Decays in the MSSM
Current Higgs data at the Large Hadron Collider is compatible with a SM
signal at the 2 level, but the central value of the signal strength in
the diphoton channel is enhanced with respect to the SM expectation. If the
enhancement resides in the diphoton partial decay width, the data could be
accommodated in the Minimally Supersymmetric Standard Model (MSSM) with highly
mixed light staus. We revisit the issue of vacuum instability induced by large
mixing in the stau sector, including effects of a radiatively-corrected tau
Yukawa coupling. Further, we emphasize the importance of taking into account
the dependence in the stability bound. While the metastability of
the Universe constrains the possible enhancement in the Higgs to diphoton decay
width in the light stau scenario, an increase of the order of 50% can be
achieved in the region of large . Larger enhancements may be
obtained, but would require values of associated with
non-perturbative values of the tau Yukawa coupling at scales below the GUT
scale, thereby implying the presence of new physics beyond the MSSM.Comment: 25 pages, 7 figure
The speed of gravity in general relativity
The question is discussed of what is the speed of gravity (at the fundamental
non-perturbative level). The question is important, if nowhere else, in
discussing the problem of information "lost" in black holes. It turns out that
the duly defined "gravitational signal" generally may be causal, superluminal
and "semi-superluminal". In the class of globally hyperbolic spacetimes the two
last varieties coincide. And if some (often imposed, but not always satisfied)
conditions hold, the signals may be \emph{only} causal. In this sense the speed
of gravity does not exceed the speed of light.Comment: typos corrected, et
Nearby Clumpy, Gas Rich, Star Forming Galaxies: Local Analogs of High Redshift Clumpy Galaxies
Luminous compact blue galaxies (LCBGs) have enhanced star formation rates and
compact morphologies. We combine Sloan Digital Sky Survey data with HI data of
29 LCBGs at redshift z~0 to understand their nature. We find that local LCBGs
have high atomic gas fractions (~50%) and star formation rates per stellar mass
consistent with some high redshift star forming galaxies. Many local LCBGs also
have clumpy morphologies, with clumps distributed across their disks. Although
rare, these galaxies appear to be similar to the clumpy star forming galaxies
commonly observed at z~1-3. Local LCBGs separate into three groups: 1.
Interacting galaxies (~20%); 2. Clumpy spirals (~40%); 3. Non-clumpy,
non-spirals with regular shapes and smaller effective radii and stellar masses
(~40%). It seems that the method of building up a high gas fraction, which then
triggers star formation, is not the same for all local LCBGs. This may lead to
a dichotomy in galaxy characteristics. We consider possible gas delivery
scenarios and suggest that clumpy spirals, preferentially located in clusters
and with companions, are smoothly accreting gas from tidally disrupted
companions and/or intracluster gas enriched by stripped satellites. Conversely,
as non-clumpy galaxies are preferentially located in the field and tend to be
isolated, we suggest clumpy, cold streams, which destroy galaxy disks and
prevent clump formation, as a likely gas delivery mechanism for these systems.
Other possibilities include smooth cold streams, a series of minor mergers, or
major interactions.Comment: 22 pages, 5 figure
Infrared Emission from the Radio Supernebula in NGC 5253: A Proto-Globular Cluster?
Hidden from optical view in the starburst region of the dwarf galaxy NGC 5253
lies an intense radio source with an unusual spectrum which could be
interpreted variously as nebular gas ionized by a young stellar cluster or
nonthermal emission from a radio supernova or an AGN. We have obtained 11.7 and
18.7 micron images of this region at the Keck Telescope and find that it is an
extremely strong mid-infrared emitter. The infrared to radio flux ratio rules
out a supernova and is consistent with an HII region excited by a dense cluster
of young stars. This "super nebula" provides at least 15% of the total
bolometric luminosity of the galaxy. Its excitation requires 10^5-10^6 stars,
giving it the total mass and size (1-2 pc diameter) of a globular cluster.
However, its high obscuration, small size, and high gas density all argue that
it is very young, no more than a few hundred thousand years old. This may be
the youngest globular cluster yet observed.Comment: 6 pages, 2 color figures, Submitted to the ApJL, Revised 4/6/01 based
on referee's comment
Speed Limits in General Relativity
Some standard results on the initial value problem of general relativity in
matter are reviewed. These results are applied first to show that in a well
defined sense, finite perturbations in the gravitational field travel no faster
than light, and second to show that it is impossible to construct a warp drive
as considered by Alcubierre (1994) in the absence of exotic matter.Comment: 7 pages; AMS-LaTeX; accepted for publication by Classical and Quantum
Gravit
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