1,035 research outputs found
Two- and three-point functions in two-dimensional Landau-gauge Yang-Mills theory: Continuum results
We investigate the Dyson-Schwinger equations for the gluon and ghost
propagators and the ghost-gluon vertex of Landau-gauge gluodynamics in two
dimensions. While this simplifies some aspects of the calculations as compared
to three and four dimensions, new complications arise due to a mixing of
different momentum regimes. As a result, the solutions for the propagators are
more sensitive to changes in the three-point functions and the ansaetze used
for them at the leading order in a vertex a expansion. Here, we therefore go
beyond this common truncation by including the ghost-gluon vertex
self-consistently for the first time, while using a model for the three-gluon
vertex which reproduces the known infrared asymptotics and the zeros at
intermediate momenta as observed on the lattice. A separate computation of the
three-gluon vertex from the results is used to confirm the stability of this
behavior a posteriori. We also present further arguments for the absence of the
decoupling solution in two dimensions. Finally, we show how in general the
infrared exponent kappa of the scaling solutions in two, three and four
dimensions can be changed by allowing an angle dependence and thus an essential
singularity of the ghost-gluon vertex in the infrared.Comment: 24 pages; added references, improved choices of parameters for vertex
models; identical to version published in JHE
Gene expression drives the evolution of dominance.
Dominance is a fundamental concept in molecular genetics and has implications for understanding patterns of genetic variation, evolution, and complex traits. However, despite its importance, the degree of dominance in natural populations is poorly quantified. Here, we leverage multiple mating systems in natural populations of Arabidopsis to co-estimate the distribution of fitness effects and dominance coefficients of new amino acid changing mutations. We find that more deleterious mutations are more likely to be recessive than less deleterious mutations. Further, this pattern holds across gene categories, but varies with the connectivity and expression patterns of genes. Our work argues that dominance arises as a consequence of the functional importance of genes and their optimal expression levels
An A4 flavor model for quarks and leptons in warped geometry
We propose a spontaneous A4 flavor symmetry breaking scheme implemented in a
warped extra dimensional setup to explain the observed pattern of quark and
lepton masses and mixings. The main advantages of this choice are the
explanation of fermion mass hierarchies by wave function overlaps, the
emergence of tribimaximal neutrino mixing and zero quark mixing at the leading
order and the absence of tree-level gauge mediated flavor violations. Quark
mixing is induced by the presence of bulk flavons, which allow for cross-brane
interactions and a cross-talk between the quark and neutrino sectors, realizing
the spontaneous symmetry breaking pattern A4 --> nothing first proposed in
[X.G.\,He, Y.Y.\,Keum, R.R.\,Volkas, JHEP{0604}, 039 (2006)]. We show that the
observed quark mixing pattern can be explained in a rather economical way,
including the CP violating phase, with leading order cross-interactions, while
the observed difference between the smallest CKM entries V_{ub} and V_{td} must
arise from higher order corrections. We briefly discuss bounds on the
Kaluza-Klein scale implied by flavor changing neutral current processes in our
model and show that the residual little CP problem is milder than in flavor
anarchic models.Comment: 34 pages, 2 figures; version published in JHE
Many-body Landau-Zener dynamics in coupled 1D Bose liquids
The Landau-Zener model of a quantum mechanical two-level system driven with a
linearly time dependent detuning has served over decades as a textbook paradigm
of quantum dynamics. In their seminal work [L. D. Landau, Physik. Z. Sowjet. 2,
46 (1932); C. Zener, Proc. Royal Soc. London 137, 696 (1932)], Landau and Zener
derived a non-perturbative prediction for the transition probability between
two states, which often serves as a reference point for the analysis of more
complex systems. A particularly intriguing question is whether that framework
can be extended to describe many-body quantum dynamics. Here we report an
experimental and theoretical study of a system of ultracold atoms, offering a
direct many-body generalization of the Landau-Zener problem. In a system of
pairwise tunnel-coupled 1D Bose liquids we show how tuning the correlations of
the 1D gases, the tunnel coupling between the tubes and the inter-tube
interactions strongly modify the original Landau-Zener picture. The results are
explained using a mean-field description of the inter-tube condensate
wave-function, coupled to the low-energy phonons of the 1D Bose liquid.Comment: 13 pages, 10 figures
Linear approaches to intramolecular Förster Resonance Energy Transfer probe measurements for quantitative modeling
Numerous unimolecular, genetically-encoded Forster Resonance Energy Transfer (FRET) probes for monitoring biochemical activities in live cells have been developed over the past decade. As these probes allow for collection of high frequency, spatially resolved data on signaling events in live cells and tissues, they are an attractive technology for obtaining data to develop quantitative, mathematical models of spatiotemporal signaling dynamics. However, to be useful for such purposes the observed FRET from such probes should be related to a biological quantity of interest through a defined mathematical relationship, which is straightforward when this relationship is linear, and can be difficult otherwise. First, we show that only in rare circumstances is the observed FRET linearly proportional to a biochemical activity. Therefore in most cases FRET measurements should only be compared either to explicitly modeled probes or to concentrations of products of the biochemical activity, but not to activities themselves. Importantly, we find that FRET measured by standard intensity-based, ratiometric methods is inherently non-linear with respect to the fraction of probes undergoing FRET. Alternatively, we find that quantifying FRET either via (1) fluorescence lifetime imaging (FLIM) or (2) ratiometric methods where the donor emission intensity is divided by the directly-excited acceptor emission intensity (denoted R<sub>alt</sub>) is linear with respect to the fraction of probes undergoing FRET. This linearity property allows one to calculate the fraction of active probes based on the FRET measurement. Thus, our results suggest that either FLIM or ratiometric methods based on R<sub>alt</sub> are the preferred techniques for obtaining quantitative data from FRET probe experiments for mathematical modeling purpose
Flavourful Production at Hadron Colliders
We ask what new states may lie at or below the TeV scale, with sizable
flavour-dependent couplings to light quarks, putting them within reach of
hadron colliders via resonant production, or in association with Standard Model
states. In particular, we focus on the compatibility of such states with
stringent flavour-changing neutral current and electric-dipole moment
constraints. We argue that the broadest and most theoretically plausible
flavour structure of the new couplings is that they are hierarchical, as are
Standard Model Yukawa couplings, although the hierarchical pattern may well be
different. We point out that, without the need for any more elaborate or
restrictive structure, new scalars with "diquark" couplings to standard quarks
are particularly immune to existing constraints, and that such scalars may
arise within a variety of theoretical paradigms. In particular, there can be
substantial couplings to a pair of light quarks or to one light and one heavy
quark. For example, the latter possibility may provide a flavour-safe
interpretation of the asymmetry in top quark production observed at the
Tevatron. We thereby motivate searches for diquark scalars at the Tevatron and
LHC, and argue that their discovery represents one of our best chances for new
insight into the Flavour Puzzle of the Standard Model.Comment: 18 pp., 8 figures, references adde
Flavour in supersymmetry: horizontal symmetries or wave function renormalisation
We compare theoretical and experimental predictions of two main classes of
models addressing fermion mass hierarchies and flavour changing neutral
currents (FCNC) effects in supersymmetry: Froggatt-Nielsen (FN) U(1) gauged
flavour models and Nelson-Strassler/extra dimensional models with hierarchical
wave functions for the families. We show that whereas the two lead to identical
predictions in the fermion mass matrices, the second class generates a stronger
suppression of FCNC effects. We prove that, whereas at first sight the FN setup
is more constrained due to anomaly cancelation conditions, imposing unification
of gauge couplings in the second setup generates conditions which precisely
match the mixed anomaly constraints in the FN setup. Finally, we provide an
economical extra dimensional realisation of the hierarchical wave functions
scenario in which the leptonic FCNC can be efficiently suppressed due to the
strong coupling (CFT) origin of the electron mass.Comment: 23 page
Formation of Complex and Unstable Chromosomal Translocations in Yeast
Genome instability, associated with chromosome breakage syndromes and most human
cancers, is still poorly understood. In the yeast Saccharomyces
cerevisiae, numerous genes with roles in the preservation of genome
integrity have been identified. DNA-damage-checkpoint-deficient yeast cells that
lack Sgs1, a RecQ-like DNA helicase related to the human
Bloom's-syndrome-associated helicase BLM, show an increased rate of
genome instability, and we have previously shown that they accumulate recurring
chromosomal translocations between three similar genes, CAN1,
LYP1 and ALP1. Here, the chromosomal
location, copy number and sequence similarity of the translocation targets
ALP1 and LYP1 were altered to gain insight
into the formation of complex translocations. Among 844 clones with chromosomal
rearrangements, 93 with various types of simple and complex translocations
involving CAN1, LYP1 and ALP1
were identified. Breakpoint sequencing and mapping showed that the formation of
complex translocation types is strictly dependent on the location of the
initiating DNA break and revealed that complex translocations arise via a
combination of interchromosomal translocation and template-switching, as well as
from unstable dicentric intermediates. Template-switching occurred between
sequences on the same chromosome, but was inhibited if the genes were
transferred to different chromosomes. Unstable dicentric translocations
continuously gave rise to clones with multiple translocations in various
combinations, reminiscent of intratumor heterogeneity in human cancers. Base
substitutions and evidence of DNA slippage near rearrangement breakpoints
revealed that translocation formation can be accompanied by point mutations, and
their presence in different translocation types within the same clone provides
evidence that some of the different translocation types are derived from each
other rather than being formed de novo. These findings provide
insight into eukaryotic genome instability, especially the formation of
translocations and the sources of intraclonal heterogeneity, both of which are
often associated with human cancers
The diversification of Heliconius butterflies: what have we learned in 150 years?
Research into Heliconius butterflies has made a significant contribution to evolutionary biology. Here, we review our understanding of the diversification of these butterflies, covering recent advances and a vast foundation of earlier work. Whereas no single group of organisms can be sufficient for understanding life's diversity, after years of intensive study, research into Heliconius has addressed a wide variety of evolutionary questions. We first discuss evidence for widespread gene flow between Heliconius species and what this reveals about the nature of species. We then address the evolution and diversity of warning patterns, both as the target of selection and with respect to their underlying genetic basis. The identification of major genes involved in mimetic shifts, and homology at these loci between distantly related taxa, has revealed a surprising predictability in the genetic basis of evolution. In the final sections, we consider the evolution of warning patterns, and Heliconius diversity more generally, within a broader context of ecological and sexual selection. We consider how different traits and modes of selection can interact and influence the evolution of reproductive isolation.RMM is funded by a Junior Research Fellowship at King’s College, Cambridge. KMK is supported by the Balfour Studentship, University of Cambridge, SHMa by a Research Fellowship at St John's College, Cambridge, and SHMo by a Research Fellowship from the Royal Commission for the Exhibition of 1851. Our work on Heliconius has been additionally supported by the Agence Nationale de la Recherche (France), the Biology and Biotechnology Research Council (UK), the British Ecological Society, the European Research Council, the Natural Environment Research Council (UK), and the Smithsonian Tropical Research Institute.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1111/jeb.1267
FCNC Effects in a Minimal Theory of Fermion Masses
As a minimal theory of fermion masses we extend the SM by heavy vectorlike
fermions, with flavor-anarchical Yukawa couplings, that mix with chiral
fermions such that small SM Yukawa couplings arise from small mixing angles.
This model can be regarded as an effective description of the fermionic sector
of a large class of existing flavor models and thus might serve as a useful
reference frame for a further understanding of flavor hierarchies in the SM.
Already such a minimal framework gives rise to FCNC effects through exchange of
massive SM bosons whose couplings to the light fermions get modified by the
mixing. We derive general formulae for these corrections and discuss the bounds
on the heavy fermion masses. Particularly stringent bounds, in a few TeV range,
come from the corrections to the Z couplings.Comment: 19 pages, 1 figur
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