168 research outputs found
Beta Functions of Orbifold Theories and the Hierarchy Problem
We examine a class of gauge theories obtained by projecting out certain
fields from an N=4 supersymmetric SU(N) gauge theory. These theories are
non-supersymmetric and in the large N limit are known to be conformal. Recently
it was proposed that the hierarchy problem could be solved by embedding the
standard model in a theory of this kind with finite N. In order to check this
claim one must find the conformal points of the theory. To do this we calculate
the one-loop beta functions for the Yukawa and quartic scalar couplings. We
find that with the beta functions set to zero the one-loop quadratic
divergences are not canceled at sub-leading order in N; thus the hierarchy
between the weak scale and the Planck scale is not stabilized unless N is of
the order 10^28 or larger. We also find that at sub-leading orders in N
renormalization induces new interactions, which were not present in the
original Lagrangian.Comment: 21 pages, LaTeX, 6 figures. Minor clarifications, references adde
Impact of massive neutrinos on the Higgs self-coupling and electroweak vacuum stability
The presence of right-handed neutrinos in the type I seesaw mechanism may
lead to significant corrections to the RG evolution of the Higgs self-coupling.
Compared to the Standard Model case, the Higgs mass window can become narrower,
and the cutoff scale become lower. Naively, these effects decrease with
decreasing right-handed neutrino mass. However, we point out that the unknown
Dirac Yukawa matrix may impact the vacuum stability constraints even in the low
scale seesaw case not far away from the electroweak scale, hence much below the
canonical seesaw scale of 10^15 GeV. This includes situations in which
production of right-handed neutrinos at colliders is possible. We illustrate
this within a particular parametrization of the Dirac Yukawas and with explicit
low scale seesaw models. We also note the effect of massive neutrinos on the
top quark Yukawa coupling, whose high energy value can be increased with
respect to the Standard Model case.Comment: 17 pages, 7 figures, minor revisions, version to appear in JHE
Collisional Dark Matter and the Origin of Massive Black Holes
If the cosmological dark matter is primarily in the form of an elementary
particle which has cross section and mass for self-interaction having a ratio
similar to that of ordinary nuclear matter, then seed black holes (formed in
stellar collapse) will grow in a Hubble time, due to accretion of the dark
matter, to a mass range 10^6 - 10^9 solar masses. Furthermore, the dependence
of the final black hole mass on the galaxy velocity dispersion will be
approximately as observed and the growth rate will show a time dependence
consistent with observations. Other astrophysical consequences of collisional
dark matter and tests of the idea are noted.Comment: 7 pages, no figures, LaTeX2e, Accepted for publication in Phys. Rev.
Lett. Changed conten
Limit Cycles in Four Dimensions
We present an example of a limit cycle, i.e., a recurrent flow-line of the
beta-function vector field, in a unitary four-dimensional gauge theory. We thus
prove that beta functions of four-dimensional gauge theories do not produce
gradient flows. The limit cycle is established in perturbation theory with a
three-loop calculation which we describe in detail.Comment: 12 pages, 1 figure. Significant revision of the interpretation of our
result. Improved description of three-loop calculatio
Feedback from the IR Background in the Early Universe
It is commonly believed that the earliest stages of star-formation in the
Universe were self-regulated by global radiation backgrounds - either by the
ultraviolet Lyman-Werner (LW) photons emitted by the first stars (directly
photodissociating H_2), or by the X-rays produced by accretion onto the black
hole (BH) remnants of these stars (heating the gas but catalyzing H_2
formation). Recent studies have suggested that a significant fraction of the
first stars may have had low masses (a few M_sun). Such stars do not leave BH
remnants and they have softer spectra, with copious infrared (IR) radiation at
photon energies around 1eV. Similar to LW and X-ray photons, these photons have
a mean-free path comparable to the Hubble distance, building up an early IR
background. Here we show that if soft-spectrum stars, with masses of a few
M_sun, contributed more than 1% of the UV background (or their mass fraction
exceeded 90%), then their IR radiation dominated radiative feedback in the
early Universe. The feedback is different from the UV feedback from high-mass
stars, and occurs through the photo-detachment of H^- ions, necessary for
efficient H_2 formation. Nevertheless, we find that the baryon fraction which
must be incorporated into low-mass stars in order to suppress H_2-cooling is
only a factor of few higher than for high-mass stars.Comment: Accepted for publication in MNRAS (Letters). 5 pages with 2 figure
Four Generations: SUSY and SUSY Breaking
We revisit four generations within the context of supersymmetry. We compute
the perturbativity limits for the fourth generation Yukawa couplings and show
that if the masses of the fourth generation lie within reasonable limits of
their present experimental lower bounds, it is possible to have perturbativity
only up to scales around 1000 TeV. Such low scales are ideally suited to
incorporate gauge mediated supersymmetry breaking, where the mediation scale
can be as low as 10-20 TeV. The minimal messenger model, however, is highly
constrained. While lack of electroweak symmetry breaking rules out a large part
of the parameter space, a small region exists, where the fourth generation stau
is tachyonic. General gauge mediation with its broader set of boundary
conditions is better suited to accommodate the fourth generation.Comment: 27 pages, 5 figure
High-Resolution Quantification of Focal Adhesion Spatiotemporal Dynamics in Living Cells
Focal adhesions (FAs) are macromolecular complexes that provide a linkage between the cell and its external environment. In a motile cell, focal adhesions change size and position to govern cell migration, through the dynamic processes of assembly and disassembly. To better understand the dynamic regulation of focal adhesions, we have developed an analysis system for the automated detection, tracking, and data extraction of these structures in living cells. This analysis system was used to quantify the dynamics of fluorescently tagged Paxillin and FAK in NIH 3T3 fibroblasts followed via Total Internal Reflection Fluorescence Microscopy (TIRF). High content time series included the size, shape, intensity, and position of every adhesion present in a living cell. These properties were followed over time, revealing adhesion lifetime and turnover rates, and segregation of properties into distinct zones. As a proof-of-concept, we show how a single point mutation in Paxillin at the Jun-kinase phosphorylation site Serine 178 changes FA size, distribution, and rate of assembly. This study provides a detailed, quantitative picture of FA spatiotemporal dynamics as well as a set of tools and methodologies for advancing our understanding of how focal adhesions are dynamically regulated in living cells. A full, open-source software implementation of this pipeline is provided at http://gomezlab.bme.unc.edu/tools
Observing the First Stars and Black Holes
The high sensitivity of JWST will open a new window on the end of the
cosmological dark ages. Small stellar clusters, with a stellar mass of several
10^6 M_sun, and low-mass black holes (BHs), with a mass of several 10^5 M_sun
should be directly detectable out to redshift z=10, and individual supernovae
(SNe) and gamma ray burst (GRB) afterglows are bright enough to be visible
beyond this redshift. Dense primordial gas, in the process of collapsing from
large scales to form protogalaxies, may also be possible to image through
diffuse recombination line emission, possibly even before stars or BHs are
formed. In this article, I discuss the key physical processes that are expected
to have determined the sizes of the first star-clusters and black holes, and
the prospect of studying these objects by direct detections with JWST and with
other instruments. The direct light emitted by the very first stellar clusters
and intermediate-mass black holes at z>10 will likely fall below JWST's
detection threshold. However, JWST could reveal a decline at the faint-end of
the high-redshift luminosity function, and thereby shed light on radiative and
other feedback effects that operate at these early epochs. JWST will also have
the sensitivity to detect individual SNe from beyond z=10. In a dedicated
survey lasting for several weeks, thousands of SNe could be detected at z>6,
with a redshift distribution extending to the formation of the very first stars
at z>15. Using these SNe as tracers may be the only method to map out the
earliest stages of the cosmic star-formation history. Finally, we point out
that studying the earliest objects at high redshift will also offer a new
window on the primordial power spectrum, on 100 times smaller scales than
probed by current large-scale structure data.Comment: Invited contribution to "Astrophysics in the Next Decade: JWST and
Concurrent Facilities", Astrophysics & Space Science Library, Eds. H.
Thronson, A. Tielens, M. Stiavelli, Springer: Dordrecht (2008
Photodissociation of H2 in Protogalaxies: Modeling Self-Shielding in 3D Simulations
The ability of primordial gas to cool in proto-galactic haloes exposed to
Lyman-Werner (LW) radiation is critically dependent on the self-shielding of
H_2. We perform radiative transfer calculations of LW line photons,
post-processing outputs from three-dimensional adaptive mesh refinement (AMR)
simulations of haloes with T_vir > 10^4 K at redshifts around z=10. We
calculate the optically thick photodissociation rate numerically, including the
effects of density, temperature, and velocity gradients in the gas, as well as
line overlap and shielding of H_2 by HI, over a large number of sight-lines. In
low-density regions (n<10^4 cm^-3) the dissociation rates exceed those obtained
using most previous approximations by more than an order of magnitude; the
correction is smaller at higher densities. We trace the origin of the
deviations primarily to inaccuracies of (i) the most common fitting formula
(Draine & Bertoldi 1996) for the suppression of the dissociation rate and (ii)
estimates for the effective shielding column density from local properties of
the gas. The combined effects of gas temperature and velocity gradients are
comparatively less important, typically altering the spherically averaged rate
only by a factor of less than two. We present a simple modification to the DB96
fitting formula for the optically thick rate which improves agreement with our
numerical results to within approx. 15 per cent, and can be adopted in future
simulations. We find that estimates for the effective shielding column can be
improved by using the local Sobolev length. Our correction to the H_2
self-shielding reduces the critical LW flux to suppress H_2-cooling in
T_vir>10^4 K haloes by an order of magnitude; this increases the number of such
haloes in which supermassive (approx. M=10^5 M_sun) black holes may have
formed.Comment: 17 pages, 11 figures. Submitted to MNRA
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