3,466 research outputs found
Unstable particle's wave-function renormalization prescription
We strictly define two set Wave-function Renormalization Constants (WRC)
under the LSZ reduction formula for unstable particles at the first time. Then
by introducing antiparticle's WRC and the CPT conservation law we obtain a new
wave-function renormalization condition which can be used to totally determine
the two set WRC. We calculate two physical processes to manifest the
consistence of the present wave-function renormalization prescription with the
gauge theory in standard model. We also prove that the conventional
wave-function renormalization prescription which discards the imaginary part of
unstable particle's WRC leads to physical amplitude gauge dependent.Comment: 10 pages, 3 figure
Constructing Gravity Amplitudes from Real Soft and Collinear Factorisation
Soft and collinear factorisations can be used to construct expressions for
amplitudes in theories of gravity. We generalise the "half-soft" functions used
previously to "soft-lifting" functions and use these to generate tree and
one-loop amplitudes. In particular we construct expressions for MHV tree
amplitudes and the rational terms in one-loop amplitudes in the specific
context of N=4 supergravity. To completely determine the rational terms
collinear factorisation must also be used. The rational terms for N=4 have a
remarkable diagrammatic interpretation as arising from algebraic link diagrams.Comment: 18 pages, axodraw, Proof of eq. 4.3 adde
b anti-b Higgs production at the LHC: Yukawa corrections and the leading Landau singularity
At tree-level Higgs production in association with a b-quark pair proceeds
through the small Yukawa bottom coupling in the Standard Model. Even in the
limit where this coupling vanishes, electroweak one-loop effects, through the
top-Higgs Yukawa coupling in particular, can still trigger this reaction. This
contribution is small for Higgs masses around 120GeV but it quickly picks up
for higher Higgs masses especially because the one-loop amplitude develops a
leading Landau singularity and new thresholds open up. These effects can be
viewed as the production of a pair of top quarks which rescatter to give rise
to Higgs production through WW fusion. We study the leading Landau singularity
in detail. Since this singularity is not integrable when the one-loop amplitude
is squared, we regulate the cross section by taking into account the width of
the internal top and W particles. This requires that we extend the usual box
one-loop function to the case of imaginary masses. We show how this can be
implemented analytically in our case. We study in some detail the cross section
at the LHC as a function of the Higgs mass and show how some distributions can
be drastically affected compared to the tree-level result.Comment: 48 pages, 20 figures. Phys.Rev.D accepted version. Conclusions
unchanged, minor changes and references adde
The power of monitoring stellar orbits
The center of the Milky Way hosts a massive black hole. The observational
evidence for its existence is overwhelming. The compact radio source Sgr A* has
been associated with a black hole since its discovery. In the last decade,
high-resolution, near-infrared measurements of individual stellar orbits in the
innermost region of the Galactic Center have shown that at the position of Sgr
A* a highly concentrated mass of 4 x 10^6 M_sun is located. Assuming that
general relativity is correct, the conclusion that Sgr A* is a massive black
hole is inevitable. Without doubt this is the most important application of
stellar orbits in the Galactic Center. Here, we discuss the possibilities going
beyond the mass measurement offered by monitoring these orbits. They are an
extremely useful tool for many scientific questions, such as a geometric
distance estimate to the Galactic Center or the puzzle, how these stars reached
their current orbits. Future improvements in the instrumentation will open up
the route to testing relativistic effects in the gravitational potential of the
black hole, allowing to take full advantage of this unique laboratory for
celestial mechanics.Comment: Proceedings of the Galactic Center Workshop 2009, Shangha
The Nambu-Jona-Lasinio model with staggered fermions
We investigate the neighbourhood of the chiral phase transition in a lattice
Nambu--Jona-Lasinio model, using both Monte Carlo methods and lattice
Schwinger-Dyson equations.Comment: Talks at LAT93, Dallas, U.S.A. Postscript file, 6 pages, figures
include
Tourism, jobs, capital accumulation and the economy: A dynamic analysis
This paper examines the effects of tourism on labor employment, capital accumulation and resident welfare for a small open economy with unemployment. A tourism boom improves the terms of trade, increases labor employment, but lowers capital accumulation. The reduction in the capital stock depends on the degree of factor intensity. When the traded sector is weakly capital intensive, the fall in capital would not be so severe and the expansion of tourism improves welfare. However, when the traded sector is strongly capital intensive, the fall in capital can be a dominant factor to lower welfare. This immiserizing result of tourism on resident welfare is confirmed by the German data.tourism ; employment ; capital accumulation ; welfare
An Inverse Compton Scattering Origin of X-ray Flares from Sgr A*
The X-ray and near-IR emission from Sgr A* is dominated by flaring, while a
quiescent component dominates the emission at radio and sub-mm wavelengths. The
spectral energy distribution of the quiescent emission from Sgr A* peaks at
sub-mm wavelengths and is modeled as synchrotron radiation from a thermal
population of electrons in the accretion flow, with electron temperatures
ranging up to \,MeV. Here we investigate the mechanism by which
X-ray flare emission is produced through the interaction of the quiescent and
flaring components of Sgr A*. The X-ray flare emission has been interpreted as
inverse Compton, self-synchrotron-Compton, or synchrotron emission. We present
results of simultaneous X-ray and near-IR observations and show evidence that
X-ray peak flare emission lags behind near-IR flare emission with a time delay
ranging from a few to tens of minutes. Our Inverse Compton scattering modeling
places constraints on the electron density and temperature distributions of the
accretion flow and on the locations where flares are produced. In the context
of this model, the strong X-ray counterparts to near-IR flares arising from the
inner disk should show no significant time delay, whereas near-IR flares in the
outer disk should show a broadened and delayed X-ray flare.Comment: 22 pages, 6 figures, 2 tables, AJ (in press
Maximally Supersymmetric Planar Yang-Mills Amplitudes at Five Loops
We present an ansatz for the planar five-loop four-point amplitude in
maximally supersymmetric Yang-Mills theory in terms of loop integrals. This
ansatz exploits the recently observed correspondence between integrals with
simple conformal properties and those found in the four-point amplitudes of the
theory through four loops. We explain how to identify all such integrals
systematically. We make use of generalized unitarity in both four and D
dimensions to determine the coefficients of each of these integrals in the
amplitude. Maximal cuts, in which we cut all propagators of a given integral,
are an especially effective means for determining these coefficients. The set
of integrals and coefficients determined here will be useful for computing the
five-loop cusp anomalous dimension of the theory which is of interest for
non-trivial checks of the AdS/CFT duality conjecture. It will also be useful
for checking a conjecture that the amplitudes have an iterative structure
allowing for their all-loop resummation, whose link to a recent string-side
computation by Alday and Maldacena opens a new venue for quantitative AdS/CFT
comparisons.Comment: 52 pages, 20 figures, revte
Quantum Electrodynamics at Large Distances II: Nature of the Dominant Singularities
Accurate calculations of macroscopic and mesoscopic properties in quantum
electrodynamics require careful treatment of infrared divergences: standard
treatments introduce spurious large-distances effects. A method for computing
these properties was developed in a companion paper. That method depends upon a
result obtained here about the nature of the singularities that produce the
dominant large-distance behaviour. If all particles in a quantum field theory
have non-zero mass then the Landau-Nakanishi diagrams give strong conditions on
the singularities of the scattering functions. These conditions are severely
weakened in quantum electrodynamics by effects of points where photon momenta
vanish. A new kind of Landau-Nakanishi diagram is developed here. It is geared
specifically to the pole-decomposition functions that dominate the macroscopic
behaviour in quantum electrodynamics, and leads to strong results for these
functions at points where photon momenta vanish.Comment: 40 pages, 11 encapsulated postscript figures, latexed,
math_macros.tex can be found on Archive. full postscript available from
http://theorl.lbl.gov/www/theorgroup/papers/35972.p
Exploiting Connections for Viral Replication.
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of the COVID-19 (coronavirus disease 2019) pandemic, is a positive strand RNA (+RNA) virus. Like other +RNA viruses, SARS-CoV-2 is dependent on host cell metabolic machinery to survive and replicate, remodeling cellular membranes to generate sites of viral replication. Viral RNA-containing double-membrane vesicles (DMVs) are a striking feature of +RNA viral replication and are abundant in SARS-CoV-2-infected cells. Their generation involves rewiring of host lipid metabolism, including lipid biosynthetic pathways. Viruses can also redirect lipids from host cell organelles; lipid exchange at membrane contact sites, where the membranes of adjacent organelles are in close apposition, has been implicated in the replication of several +RNA viruses. Here we review current understanding of DMV biogenesis. With a focus on the exploitation of contact site machinery by +RNA viruses to generate replication organelles, we discuss evidence that similar mechanisms support SARS-CoV-2 replication, protecting its RNA from the host cell immune response
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