9 research outputs found
All-Multiplicity One-Loop Corrections to MHV Amplitudes in QCD
We compute the complete one-loop corrections to the simplest class of QCD
gluon amplitudes, those with two color-adjacent opposite-helicity external
particles. We present results for an arbitrary number of external legs. The
computation uses the recently-developed on-shell recursion relations for the
rational parts, along with older unitarity-based results for the cut-containing
terms.Comment: 5 pages; corrected typos, minor text addition; final Phys Rev versio
Twistor-Inspired Construction of Electroweak Vector Boson Currents
We present an extension of the twistor-motivated MHV vertices and
accompanying rules presented by Cachazo, Svrvcek and Witten to the construction
of vector-boson currents coupling to an arbitrary source. In particular, we
give rules for constructing off-shell vector-boson currents with one fermion
pair and n gluons of arbitrary helicity. These currents may be employed
directly in the computation of electroweak amplitudes. The rules yield
expressions in agreement with previously-obtained results for Z,W,\gamma^* -->
qbar q + n gluons (analytically up to n=3, beyond via the Berends--Giele
recursion relations). We also confirm that the contribution to a seven-point
amplitude containing the non-abelian triple vector-boson coupling obtained
using the next-to-MHV currents matches the previous result in the literature.Comment: 22 pages, 4 figures, v2 minor corrections and added commentary on
multiple vector boson
Bootstrapping One-Loop QCD Amplitudes with General Helicities
The recently developed on-shell bootstrap for computing one-loop amplitudes
in non-supersymmetric theories such as QCD combines the unitarity method with
loop-level on-shell recursion. For generic helicity configurations, the
recursion relations may involve undetermined contributions from non-standard
complex singularities or from large values of the shift parameter. Here we
develop a strategy for sidestepping difficulties through use of pairs of
recursion relations. To illustrate the strategy, we present sets of recursion
relations needed for obtaining n-gluon amplitudes in QCD. We give a recursive
solution for the one-loop n-gluon QCD amplitudes with three or four
color-adjacent gluons of negative helicity and the remaining ones of positive
helicity. We provide an explicit analytic formula for the QCD amplitude
A_{6;1}(1^-,2^-,3^-,4^+,5^+,6^+), as well as numerical results for
A_{7;1}(1^-,2^-,3^-,4^+,5^+,6^+,7^+), A_{8;1}(1^-,2^-,3^-,4^+,5^+,6^+,7^+,8^+),
and A_{8;1}(1^-,2^-,3^-,4^-,5^+,6^+,7^+,8^+). We expect the on-shell bootstrap
approach to have widespread applications to phenomenological studies at
colliders.Comment: 77 pages, 17 figures; v2, corrected minor typos in text and small
equation
On-Shell Unitarity Bootstrap for QCD Amplitudes
We describe the recently developed on-shell bootstrap for computing one-loop
amplitudes in non-supersymmetric theories such as QCD. The method combines the
unitarity method with loop-level on-shell recursion. The unitarity method is
used to compute cut-containing parts of amplitudes, and on-shell recursion is
used for the remaining rational terms.Comment: 10 pages, 9 figures, write-up of talks given by Z. Bern and D. A.
Kosower at Loops & Legs 2006, Eisenach, Germany; v2: added referenc
Infrared-Finite Amplitudes for Massless Gauge Theories
We present a method to construct infrared-finite amplitudes for gauge
theories with massless fermions. Rather than computing -matrix elements
between usual states of the Fock space we construct order-by-order in
perturbation theory dressed states that incorporate all long-range
interactions. The -matrix elements between these states are shown to be free
from soft and collinear singularities. As an explicit example we consider the
process jets at next-to-leading order in the strong coupling. We
verify by explicit calculation that the amplitudes are infrared finite and
recover the well-known result for the total cross section hadrons.Comment: 46 page
Complete Genome Sequence of Serotype III Streptococcus agalactiae Sequence Type 17 Strain 874391.
Here we report the complete genome sequence of Streptococcus agalactiae strain 874391. This serotype III isolate is a member of the hypervirulent sequence type 17 (ST-17) lineage that causes a disproportionate number of cases of invasive disease in humans and mammals. A brief historical context of the strain is discussed
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Anesthetic action on the transmission delay between cortex and thalamus explains the beta-buzz observed under propofol anesthesia
In recent years, more and more surgeries under general anesthesia have been performed with the assistance of electroencephalogram (EEG) monitors. An increase in anesthetic concentration leads to characteristic changes in the power spectra of the EEG. Although tracking the anesthetic-induced changes in EEG rhythms can be employed to estimate the depth of anesthesia, their precise underlying mechanisms are still unknown. A prominent feature in the EEG of some patients is the emergence of a strong power peak in the β–frequency band, which moves to the α–frequency band while increasing the anesthetic concentration. This feature is called the beta-buzz. In the present study, we use a thalamo-cortical neural population feedback model to reproduce observed characteristic features in frontal EEG power obtained experimentally during propofol general anesthesia, such as this beta-buzz. First, we find that the spectral power peak in the α– and δ–frequency ranges depend on the decay rate constant of excitatory and inhibitory synapses, but the anesthetic action on synapses does not explain the beta-buzz. Moreover, considering the action of propofol on the transmission delay between cortex and thalamus, the model reveals that the beta-buzz may result from a prolongation of the transmission delay by increasing propofol concentration. A corresponding relationship between transmission delay and anesthetic blood concentration is derived. Finally, an analytical stability study demonstrates that increasing propofol concentration moves the systems resting state towards its stability threshold