1,234 research outputs found
Nilpotent invariants in N=4 SYM
It is shown that there are no nilpotent invariants in N=4 analytic superspace
for points. It is argued that there is (at least) one such invariant
for n=5 points which is not invariant under U(1)_Y. The consequences of these
results are that the n=2 and 3 point correlation functions of the N=4
gauge-invariant operators which correspond to KK multiplets in AdS supergravity
are given exactly by their tree level expressions, the 4 point correlation
functions of such operators are invariant under U(1)_Y and correlation
functions with points have non-trivial dependence on the Yang-Mills
coupling constant.Comment: 9 page
Simplifications of four-point functions in N=4 supersymmetric Yang-Mills theory at two loops
The superconformal Ward identities combined with N=2 harmonic analyticity are
used to evaluate two-loop four-point correlation functions of gauge-invariant
operators in D=4, N=4 supersymmetric Yang-Mills theory in terms of the
well-known one-loop box integral. The result is confirmed by a direct numerical
computation
Explicit construction of nilpotent covariants in N=4 SYM
Some aspects of correlation functions in N=4 SYM are discussed. Using N=4
harmonic superspace we study two and three-point correlation functions which
are of contact type and argue that these contact terms will not affect the
non-renormalisation theorem for such correlators at non-coincident points. We
then present a perturbative calculation of a five-point function at two loops
in N=2 harmonic superspace and verify that it reproduces the derivative of the
previously found four-point function with respect to the coupling. The
calculation of this four-point function via the five-point function turns out
to be significantly simpler than the original direct calculation. This
calculation also provides an explicit construction of an N=2 component of an
N=4 five-point nilpotent covariant that violates U(1)_Y symmetry.Comment: 20 pages, standard late
Fermi Liquid Properties of a Two Dimensional Electron System With the Fermi Level Near a van Hove Singularity
We use a diagrammatic approach to study low energy physics of a two
dimensional electron system where the Fermi level is near van-Hove singularies
in the energy spectrum. We find that in most regions of the
phase diagram the system behaves as a normal Fermi liquid rather than a
marginal Fermi liquid. Particularly, the imaginary part of the self energy is
much smaller than the excitation energy, which implies well defined
quasiparticle excitations, and single particle properties are only weakly
affected by the presence of the van-Hove singularities. The relevance to high
temperature superconductivity is also discussed.Comment: 10 pages, 4 postscript figure
Higher-Derivative Terms in N=2 Supersymmetric Effective Actions
We show how to systematically construct higher-derivative terms in effective
actions in harmonic superspace despite the infinite redundancy in their
description due to the infinite number of auxiliary fields. Making an
assumption about the absence of certain superspace Chern-Simons-like terms
involving vector multiplets, we write all 3- and 4-derivative terms on Higgs,
Coulomb, and mixed branches. Among these terms are several with only
holomorphic dependence on fields, and at least one satisfies a
non-renormalization theorem. These holomorphic terms include a novel
3-derivative term on mixed branches given as an integral over 3/4 of
superspace. As an illustration of our method, we search for Wess-Zumino terms
in the low energy effective action of N=2 supersymmetric QCD. We show that such
terms occur only on mixed branches. We also present an argument showing that
the combination of space-time locality with supersymmetry implies locality in
the anticommuting superspace coordinates of for unconstrained superfields.Comment: 30 pages. Added references and simplified final form of WZ ter
Four-point functions in N=2 superconformal field theories
Four-point correlation functions of hypermultiplet bilinear composites are
analysed in N=2 superconformal field theory using the superconformal Ward
identities and the analyticity properties of the composite operator
superfields. It is shown that the complete amplitude is determined by a single
arbitrary function of the two conformal cross-ratios of the space-time
variables.Comment: 36 pp LaTeX2e, uses amsfonts, amssymb. Some references adde
Vertical transport and electroluminescence in InAs/GaSb/InAs structures: GaSb thickness and hydrostatic pressure studies
We have measured the current-voltage (I-V) of type II InAs/GaSb/InAs double
heterojunctions (DHETs) with 'GaAs like' interface bonding and GaSb thickness
between 0-1200 \AA. A negative differential resistance (NDR) is observed for
all DHETs with GaSb thickness 60 \AA below which a dramatic change in the
shape of the I-V and a marked hysteresis is observed. The temperature
dependence of the I-V is found to be very strong below this critical GaSb
thickness. The I-V characteristics of selected DHETs are also presented under
hydrostatic pressures up to 11 kbar. Finally, a mid infra-red
electroluminescence is observed at 1 bar with a threshold at the NDR valley
bias. The band profile calculations presented in the analysis are markedly
different to those given in the literature, and arise due to the positive
charge that it is argued will build up in the GaSb layer under bias. We
conclude that the dominant conduction mechanism in DHETs is most likely to
arise out of an inelastic electron-heavy-hole interaction similar to that
observed in single heterojunctions (SHETs) with 'GaAs like' interface bonding,
and not out of resonant electron-light-hole tunnelling as proposed by Yu et al.
A Zener tunnelling mechanism is shown to contribute to the background current
beyond NDR.Comment: 8 pages 12 fig
Three-Neutrino Mixing after the First Results from K2K and KamLAND
We analyze the impact of the data on long baseline \nu_\mu disappearance from
the K2K experiment and reactor \bar\nu_e disappearance from the KamLAND
experiment on the determination of the leptonic three-generation mixing
parameters. Performing an up-to-date global analysis of solar, atmospheric,
reactor and long baseline neutrino data in the context of three-neutrino
oscillations, we determine the presently allowed ranges of masses and mixing
and we consistently derive the allowed magnitude of the elements of the
leptonic mixing matrix. We also quantify the maximum allowed contribution of
\Delta m^2_{21} oscillations to CP-odd and CP-even observables at future long
baseline experiments.Comment: Some typos correcte
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