885 research outputs found
Constructing the Tree-Level Yang-Mills S-Matrix Using Complex Factorization
A remarkable connection between BCFW recursion relations and constraints on
the S-matrix was made by Benincasa and Cachazo in 0705.4305, who noted that
mutual consistency of different BCFW constructions of four-particle amplitudes
generates non-trivial (but familiar) constraints on three-particle coupling
constants --- these include gauge invariance, the equivalence principle, and
the lack of non-trivial couplings for spins >2. These constraints can also be
derived with weaker assumptions, by demanding the existence of four-point
amplitudes that factorize properly in all unitarity limits with complex
momenta. From this starting point, we show that the BCFW prescription can be
interpreted as an algorithm for fully constructing a tree-level S-matrix, and
that complex factorization of general BCFW amplitudes follows from the
factorization of four-particle amplitudes. The allowed set of BCFW deformations
is identified, formulated entirely as a statement on the three-particle sector,
and using only complex factorization as a guide. Consequently, our analysis
based on the physical consistency of the S-matrix is entirely independent of
field theory. We analyze the case of pure Yang-Mills, and outline a proof for
gravity. For Yang-Mills, we also show that the well-known scaling behavior of
BCFW-deformed amplitudes at large z is a simple consequence of factorization.
For gravity, factorization in certain channels requires asymptotic behavior
~1/z^2.Comment: 35 pages, 6 figure
Early assessment of vestibular function after unilateral cochlear implant surgery
Introduction : Cochlear implantation (CI) has been reported to negatively effect on the vestibular function. The study of the vestibular function has variably been conducted by different types of diagnostic tools. The combined use of modern, rapidly performable diagnostic tools could reveal useful for standardizing the evaluation protocol.
Methods: In a group of 28 subjects undergoing CI, the video Head Impulse Test (vHIT), the cervical Vestibular Evoked Myogenic Potentials (cVEMPS) and the short-form of Dizziness Handicap Inventory (DHI) questionnaire were investigated pre-operatively and post-operatively (implant on and off) in both the implanted and the contralateral, non-implanted ear. All surgeries were performed with a round window approach (RWA), except for three otosclerosis cases were the extended RWA (eRWA) was used.
Results: The vHIT of the lateral semicircular canal showed a pre-operative vestibular involvement in nearly 50% of the cases, whilst the three canals were contemporarily affected in only 14% of them. In all the hypo-functional subjects, cVEMPs were absent. A low VOR gain in all the investigated SSCC was found in 4 subjects (14%). In those subjects, (21.7%) in whom cVEMPs were pre-operatively present and normal in the operated side, absence of response was post-operatives recorded.
Discussion/Conclusion: The vestibular protocol applied for the study showed to be appropriate for distinguishing between the CI operated and the non-operated ear. In this regard, cVEMPs showed to be more sensitive than vHIT for revealing a vestibular sufferance after CI, although without statistical significance. Finally, the use of the RWA surgery was apparently not avoiding signs of vestibular impairment to occur
A Superbubble Feedback Model for Galaxy Simulations
We present a new stellar feedback model that reproduces superbubbles.
Superbubbles from clustered young stars evolve quite differently to individual
supernovae and are substantially more efficient at generating gas motions. The
essential new components of the model are thermal conduction, sub-grid
evaporation and a sub-grid multi-phase treatment for cases where the simulation
mass resolution is insufficient to model the early stages of the superbubble.
The multi-phase stage is short compared to superbubble lifetimes. Thermal
conduction physically regulates the hot gas mass without requiring a free
parameter. Accurately following the hot component naturally avoids overcooling.
Prior approaches tend to heat too much mass, leaving the hot ISM below K
and susceptible to rapid cooling unless ad-hoc fixes were used. The hot phase
also allows feedback energy to correctly accumulate from multiple, clustered
sources, including stellar winds and supernovae.
We employ high-resolution simulations of a single star cluster to show the
model is insensitive to numerical resolution, unresolved ISM structure and
suppression of conduction by magnetic fields. We also simulate a Milky Way
analog and a dwarf galaxy. Both galaxies show regulated star formation and
produce strong outflows.Comment: 13 pages, 13 figures; replaced with version accepted to MNRA
Delayed Effect of Active Pressure Treatment on Endolymphatic Hydrops
OBJECTIVE: To identify eventual correlations between the effect of low-pressure treatment and endolymphatic hydrops in Ménière patients.
MATERIAL AND METHODS: The study group consisted of subjects affected by definite Ménière disease (2015) and a severe degree of disability, who received a ventilation tube with or without a low-pressure treatment before undergoing a surgical procedure (vestibular neurectomy). After the placement of the ventilation tube, the subjects were either left alone with the tube or received 1 month of self-administered low-pressure therapy with a portable device. In all subjects, an electrocochleography (ECochG) was performed and specific questionnaires - Dizziness Handicap Inventory (DHI) and Functional Scale Level (FSL) - were completed before starting either arm of treatment, at the end of treatment, and then 3 and 6 months later.
RESULTS: All selected subjects presented with an ECochG pattern that was indicative of endolymphatic hydrops before starting either treatment. At the end of pressure treatment, 80% showed symptomatic improvement while maintaining the hydropic ECochG pattern. At the 3-month control stage, the hydropic pattern resulted normalized (<0.5) in all the improved subjects.
CONCLUSIONS: Although 1 month of low-pressure treatment provided a positive symptomatological outcome, normalization of the hydropic ECochG parameters occurred only at a later time. Therefore, it is possible to assume that endolymphatic hydrops could be concurrent with a non-symptomatic stage of Ménière disease, and that the anti-hydropic effect of the low-pressure treatment, if any, would present with a certain delay after its completion
Effects of sub-optimal temperatures on seed germination of three warm-season turfgrasses with perspectives of cultivation in transition zone
Warm-season turfgrass species prevail in tropical and subtropical areas, but can also be grown in the transition zone. In this case, cold tolerance is a key aspect for germination and successful turfgrass establishment. The germination response to sub-optimal temperatures was investigated for Cynodon dactylon (cvs Jackpot, La Paloma, Transcontinental, Yukon, Riviera), Buchloe dactyloides (cv SWI 2000) and Paspalum vaginatum (cv Pure Dynasty). Four temperature regimes were applied, i.e., 20/30 °C, 15/25 °C, 10/20 °C and 5/15 °C, with a 12:12 h (light:dark) photoperiod. Germination assays were performed twice, with six replicates (Petri dishes) per treatment in each experiment, fifty seeds per dish. The final germinated percentages at last inspection time (FGP) were obtained for each Petri dish and processed by using a generalized linear mixed model (binomial error and logit link). Germination curves were fitted to each Petri dish by using time-to-event methods and germination rates (GR) for the 10th, 20th and 30th percentiles were derived and used to fit a linear thermal-time model. For all cultivars, FGP decreased with decreasing mean daily temperatures. Base temperatures (Tb) ranged between 11.4 °C and 17.0 °C, while the thermal time to obtain 30% germination ranged from 51.3 °C day for SWI 2000 to 144.0 °C day for Pure Dynasty. The estimated parameters were used to predict germination time in the field, considering the observed soil temperatures in Legnaro. The estimated date for the beginning of germination in the field would range from early April for SWI 2000 and Transcontinental to mid-May for Riviera. These results might be used as a practical support for planning spring sowing, which is crucial for successful turfgrass establishment, especially without irrigation
Evolution of giant molecular clouds across cosmic time
Giant molecular clouds (GMCs) are well studied in the local Universe, however, exactly how their properties vary during galaxy evolution is poorly understood due to challenging resolution requirements, both observational and computational. We present the first time-dependent analysis of GMCs in a Milky Way-like galaxy and an Large Magellanic Cloud (LMC)-like dwarf galaxy of the FIRE-2 (Feedback In Realistic Environments) simulation suite, which have sufficient resolution to predict the bulk properties of GMCs in cosmological galaxy formation self-consistently. We show explicitly that the majority of star formation outside the galactic centre occurs within self-gravitating gas structures that have properties consistent with observed bound GMCs. We find that the typical cloud bulk properties such as mass and surface density do not vary more than a factor of 2 in any systematic way after the first Gyr of cosmic evolution within a given galaxy from its progenitor. While the median properties are constant, the tails of the distributions can briefly undergo drastic changes, which can produce very massive and dense self-gravitating gas clouds. Once the galaxy forms, we identify only two systematic trends in bulk properties over cosmic time: a steady increase in metallicity produced by previous stellar populations and a weak decrease in bulk cloud temperatures. With the exception of metallicity, we find no significant differences in cloud properties between the Milky Way-like and dwarf galaxies. These results have important implications for cosmological star and star cluster formation and put especially strong constraints on theories relating the stellar initial mass function to cloud properties
Master field treatment of metric perturbations sourced by the trailing string
We present decoupled, separable forms of the linearized Einstein equations
sourced by a string trailing behind an external quark moving through a thermal
state of N=4 super-Yang-Mills theory. We solve these equations in the
approximations of large and small wave-numbers.Comment: 40 pages, 3 figure
Amplitudes and Spinor-Helicity in Six Dimensions
The spinor-helicity formalism has become an invaluable tool for understanding
the S-matrix of massless particles in four dimensions. In this paper we
construct a spinor-helicity formalism in six dimensions, and apply it to derive
compact expressions for the three, four and five point tree amplitudes of
Yang-Mills theory. Using the KLT relations, it is a straightforward process to
obtain amplitudes in linearized gravity from these Yang-Mills amplitudes; we
demonstrate this by writing down the gravitational three and four point
amplitudes. Because there is no conserved helicity in six dimensions, these
amplitudes describe the scattering of all possible polarization states (as well
as Kaluza-Klein excitations) in four dimensions upon dimensional reduction. We
also briefly discuss a convenient formulation of the BCFW recursion relations
in higher dimensions.Comment: 26 pages, 2 figures. Minor improvements of the discussio
Tree amplitudes of noncommutative U(N) Yang-Mills Theory
Following the spirit of S-matrix program, we proposed a modified
Britto-Cachazo-Feng-Witten recursion relation for tree amplitudes of
noncommutative U(N) Yang-Mills theory. Starting from three-point amplitudes,
one can use this modified BCFW recursion relation to compute or analyze
color-ordered tree amplitudes without relying on any detail information of
noncommutative Yang-Mills theory. After clarifying the color structure of
noncommutative tree amplitudes, we wrote down the noncommutative analogies of
U(1)-decoupling, Kleiss-Kuijf and Bern-Carrasco-Johansson relations for
color-ordered tree amplitudes, and proved them using the modified BCFW
recursion relation.Comment: 24 pages, 3 figures. v2 References added. v3 some typos correcte
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