182 research outputs found
A Calculation of the plane wave string Hamiltonian from N=4 super-Yang-Mills theory
Berenstein, Maldacena, and Nastase have proposed, as a limit of the strong
form of the AdS/CFT correspondence, that string theory in a particular plane
wave background is dual to a certain subset of operators in the N=4
super-Yang-Mills theory. Even though this is a priori a strong/weak coupling
duality, the matrix elements of the string theory Hamiltonian, when expressed
in gauge theory variables, are analytic in the 't Hooft coupling constant. This
allows one to conjecture that, like the masses of excited string states, these
can be recovered using perturbation theory in Yang-Mills theory.
In this paper we identify the difference between the generator of scale
transformations and a particular U(1) R-symmetry generator as the operator dual
to the string theory Hamiltonian for nonvanishing string coupling. We compute
its matrix elements and find that they agree with the string theory prediction
provided that the state-operator map is modified for nonvanishing string
coupling. We construct this map explicitly and calculate the anomalous
dimensions of the new operators. We identify the component arising from the
modification of the state-operator map with the contribution of the string
theory contact terms to the masses of string states.Comment: 38 pages, Latex; v2: Comparison with string theory changed in light
of corrections to string theory results in hep-th/0206073 v3; state-operator
map modified; Physical interpretation and conclusions unchange
Analytical and Numerical Simulation of Multipactor within a Helical Resonant Filter
Multipactor analysis of a helical resonant filter has been performed using CST Particle Studio and analytically using a 1-D particle tracking code, based on the Runge-Kutta-Nystrom method. A comparison of results is presented
Discrete Wavelet Transform Analysis of the Electroretinogram in Autism Spectrum Disorder and Attention Deficit Hyperactivity Disorder
Background: To evaluate the electroretinogram waveform in autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD) using a discrete wavelet transform (DWT) approach. Methods: A total of 55 ASD, 15 ADHD and 156 control individuals took part in this study. Full field light-adapted electroretinograms (ERGs) were recorded using a Troland protocol, accounting for pupil size, with five flash strengths ranging from -0.12 to 1.20 log photopic cd.s.m-2. A DWT analysis was performed using the Haar wavelet on the waveforms to examine the energy within the time windows of the a- and b-waves and the oscillatory potentials (OPs) which yielded six DWT coefficients related to these parameters. The central frequency bands were from 20-160 Hz relating to the a-wave, b-wave and OPs represented by the coefficients: a20, a40, b20, b40, op80, and op160, respectively. In addition, the b-wave amplitude and percentage energy contribution of the OPs (%OPs) in the total ERG broadband energy was evaluated. Results: There were significant group differences (p < 0.001) in the coefficients corresponding to energies in the b-wave (b20, b40) and OPs (op80 and op160) as well as the b-wave amplitude. Notable differences between the ADHD and control groups were found in the b20 and b40 coefficients. In contrast, the greatest differences between the ASD and control group were found in the op80 and op160 coefficients. The b-wave amplitude showed both ASD and ADHD significant group differences from the control participants, for flash strengths greater than 0.4 log photopic cd.s.m-2 (p < 0.001). Conclusion: This methodological approach may provide insights about neuronal activity in studies investigating group differences where retinal signaling may be altered through neurodevelopment or neurodegenerative conditions. However, further work will be required to determine if retinal signal analysis can offer a classification model for neurodevelopmental conditions in which there is a co-occurrence such as ASD and ADHD
The electroretinogram b-wave amplitude: a differential physiological measure for Attention Deficit Hyperactivity Disorder and Autism Spectrum Disorder
Background: Attention Deficit Hyperactivity Disorder (ADHD) is the most prevalent childhood neurodevelopmental disorder. It shares some genetic risk with Autism Spectrum Disorder (ASD), and the conditions often occur together. Both are potentially associated with abnormal glutamate and GABA neurotransmission, which can be modelled by measuring the synaptic activity in the retina with an electroretinogram (ERG). Reduction of retinal responses in ASD has been reported, but little is known about retinal activity in ADHD. In this study, we compared the light-adapted ERGs of individuals with ADHD, ASD and controls to investigate whether retinal responses differ between these neurodevelopmental conditions. / Methods: Full field light-adapted ERGs were recorded from 15 ADHD, 57 ASD (without ADHD) and 59 control participants, aged from 5.4 to 27.3 years old. A Troland protocol was used with a random series of nine flash strengths from â0.367 to 1.204 log photopic cd.s.mâ2. The time-to-peak and amplitude of the a- and b-waves and the parameters of the Photopic Negative Response (PhNR) were compared amongst the three groups of participants, using generalised estimating equations. / Results: Statistically significant elevations of the ERG b-wave amplitudes, PhNR responses and faster timings of the b-wave time-to-peak were found in those with ADHD compared with both the control and ASD groups. The greatest elevation in the b-wave amplitudes associated with ADHD were observed at 1.204 log phot cd.s.mâ2 flash strength (p <.0001), at which the b-wave amplitude in ASD was significantly lower than that in the controls. Using this measure, ADHD could be distinguished from ASD with an area under the curve of 0.88. / Conclusions: The ERG b-wave amplitude appears to be a distinctive differential feature for both ADHD and ASD, which produced a reversed pattern of b-wave responses. These findings imply imbalances between glutamate and GABA neurotransmission which primarily regulate the b-wave formation. Abnormalities in the b-wave amplitude could provisionally serve as a biomarker for both neurodevelopmental conditions
Redox-active ferrocene-modified Cowpea mosaic virus nanoparticles
A naturally occurring nanoparticle, the plant virus Cowpea mosaic virus, can be decorated with ferrocene derivatives, of various linker lengths with amine and carboxylategroups, on the external surface using a range of conjugation strategies. The multiple, organometallic, redox-active ferrocene moieties on the outer surface of the virus are electrochemically independent with reduction potentials that span a potential window of 0.16 V that are dependent on the site of modification and the nature of the ferrocene derivative. The number of ferrocenes coupled to each virus ranges from about 100 to 240 depending upon the conjugation site and the linker length and these redox active units can provide multielectron reservoirs
Towards a holographic dual of large-N_c QCD
We study N_f D6-brane probes in the supergravity background dual to N_c
D4-branes compactified on a circle with supersymmetry-breaking boundary
conditions. In the limit in which the resulting Kaluza--Klein modes decouple,
the gauge theory reduces to non-supersymmetric, four-dimensional QCD with N_c
colours and N_f << N_c flavours. As expected, this decoupling is not fully
realised within the supergravity/Born--Infeld approximation. For N_f = 1 and
massless quarks, m_q = 0, we exhibit spontaneous chiral symmetry breaking by a
quark condensate, \neq 0, and find the associated massless
`pion' in the spectrum. The latter becomes massive for m_q > 0, obeying the
Gell-Mann--Oakes--Renner relation: M_pi^2= - m_q / \f_pi^2. In
the case N_f > 1 we provide a holographic version of the Vafa--Witten theorem,
which states that the U(N_f) flavour symmetry cannot be spontaneously broken.
Further we find N_f^2 - 1 unexpectedly light pseudo-scalar mesons in the
spectrum. We argue that these are not (pseudo) Goldstone bosons and speculate
on the string mechanism responsible for their lightness. We then study the
theory at finite temperature and exhibit a phase transition associated with a
discontinuity in the chiral condensate. D6/anti-D6 pairs are also briefly
discussed.Comment: 43 pages, LaTeX; v3: Scalar vs. pseudo-scalar nature of mesons
clarified, references added. v4: Small change in Acknowledgment
A toy model for the AdS/CFT correspondence
We study the large N gauged quantum mechanics for a single Hermitian matrix
in the Harmonic oscillator potential well as a toy model for the AdS/CFT
correspondence. We argue that the dual geometry should be a string in two
dimensions with a curvature of stringy size. Even though the dual geometry is
not weakly curved, one can still gain knowledge of the system from a detailed
study of the open-closed string duality. We give a mapping between the basis of
states made of traces (closed strings) and the eigenvalues of the matrix
(D-brane picture) in terms of Schur polynomials. We connect this model with the
study of giant gravitons in AdS_5 x S^5. We show that the two giant gravitons
that expand along AdS_5 and S^5 can be interpreted in the matrix model as
taking an eigenvalue from the Fermi sea and exciting it very much, or as making
a hole in the Fermi sea respectively. This is similar to recent studies of the
c=1 string. This connection gives new insight on how to perform calculations
for giant gravitons.Comment: 19 pages JHEP, 4 figures. v2: comments added, typos fixed, additional
refs. v3: The paper has been largely revised, to make the relation as a limit
of N=4 SYM clear, also some proofs have been written in full rather than
sketched. This updated version reflects the changes that were made in the
published versio
Moduli Space Metric of N=2 Supersymmetric SU(N) Gauge Theory and the Enhancon
We compute the moduli space metric of SU(N) Yang-Mills theory with N=2
supersymmetry in the vicinity of the point where the classical moduli vanish.
This gauge theory may be realized as a set of N D7-branes wrapping a K3
surface, near the enhancon locus. The moduli space metric determines the
low-energy worldvolume dynamics of the D7 branes near this point, including
stringy corrections. Non-abelian gauge symmetry is not restored on the
worldvolume at the enhancon point, but rather the gauge group remains
U(1)^{N-1} and light electric and magnetically charged particles coexist. We
also study the moduli space metric for a single probe brane in the background
of N-1 branes near the enhancon point. We find quantum corrections to the
supergravity probe metric that are not suppressed at large separations, but are
down by 1/N factors, due to the response of the N-1 enhancon branes to the
probe. A singularity appears before the probe reaches the enhancon point where
a dyon becomes massless. We compute the masses of W-bosons and monopoles in a
large N limit near this critical point.Comment: 20 pages, 2 figure
Rolling Tachyon Boundary State, Conserved Charges and Two Dimensional String Theory
The boundary state associated with the rolling tachyon solution on an
unstable D-brane contains a part that decays exponentially in the asymptotic
past and the asymptotic future, but it also contains other parts which either
remain constant or grow exponentially in the past or future. We argue that the
time dependence of the latter parts is completely determined by the requirement
of BRST invariance of the boundary state, and hence they contain information
about certain conserved charges in the system. We also examine this in the
context of the unstable D0-brane in two dimensional string theory where these
conserved charges produce closed string background associated with the discrete
states, and show that these charges are in one to one correspondence with the
symmetry generators in the matrix model description of this theory.Comment: LaTeX file, 37 pages; v3: references added; v4: minor change
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