22,181 research outputs found
The Vacuum Structure of Light-Front -Theory
We discuss the vacuum structure of -theory in 1+1 dimensions
quantised on the light-front . To this end, one has to solve a
non-linear, operator-valued constraint equation. It expresses that mode of the
field operator having longitudinal light-front momentum equal to zero, as a
function of all the other modes in the theory. We analyse whether this zero
mode can lead to a non-vanishing vacuum expectation value of the field
and thus to spontaneous symmetry breaking. In perturbation theory, we get no
symmetry breaking. If we solve the constraint, however, non-perturbatively,
within a mean-field type Fock ansatz, the situation changes: while the vacuum
state itself remains trivial, we find a non-vanishing vacuum expectation value
above a critical coupling. Exactly the same result is obtained within a
light-front Tamm-Dancoff approximation, if the renormalisation is done in the
correct way.Comment: 28 pages LaTeX, 1 Postscript figur
Hadron-Hadron Interactions from Lattice QCD: isospin-1 scattering length
We present results for the interaction of two kaons at maximal isospin. The
calculation is based on flavour gauge configurations generated by
the European Twisted Mass Collaboration with pion masses ranging from about
to at three values of the lattice spacing. The
elastic scattering length is calculated at several values of the
bare strange and light quark masses. We find as
the result of a combined extrapolation to the continuum and to the physical
point, where the first error is statistical, and the three following are
systematical. This translates to .Comment: 28 pages, 18 tables, 14 figure
The solubility and oxidation state of nickel in silicate melt at low oxygen fugacities: Results using a mechanically assisted equilibration technique
The solubility of Ni in a silicate melt has been measured using a new, mechanically assisted equilibration technique over a wide range of controlled ƒO2 values. The melt composition corresponds to the 1 atm eutectic in the system CaAl2Si2O8-CaMgSi2O6 + 10 wt% CaO. The experiments were performed at 1300°C and over an ƒO2 range of 10−8.5 to 10−13.75, and over a temperature range of 1270 to 1390°C at a constant gas mixing ratio ( ). The experiment consists of a sample of melt contained within a crucible of Ni metal and held in a 1 atm gas mixing furnace. A Ni spindle is entered into the sample from above and continuously rotated at a constant angular velocity using a viscometer head. The stirring of the sample serves to accelerate the approach to equilibrium between the liquid sample and the metal crucible (and spindle). This arrangement allows relatively rapid equilibration of Ni content following changes to higher or lower ƒO2 values. Samples of the melt may be taken at any time for analysis and thus the equilibrium solubility of Ni in the silicate melt may be determined from unambiguous experimental reversals. The Ni contents of samples, analysed both by INAA and by ICP-AES, range from 25 to 5300 ppm.
The data presented in this paper indicate that the oxidation state of Ni in the investigated melt is Ni2+ over the entire range of Æ’O2 investigated. This conclusion contrasts with recent reports in the literature of an inflection in the Æ’O2 dependence of Ni solubility, which has been interpreted as solution of neutral Ni at low Æ’O2 (Morse et al., 1991; Colson, 1992; Ehlers et al., 1992). We also present data for the temperature dependence of Ni solubility in the investigated melt. The solubility decreases with increasing temperature at constant Æ’O2. The present results are in good agreement with the metal-loop-equilibration experiments reported by Holzheid et al. (1994)
Distribution of label spacings for genome mapping in nanochannels
In genome mapping experiments, long DNA molecules are stretched by confining
them to very narrow channels, so that the locations of sequence-specific
fluorescent labels along the channel axis provide large-scale genomic
information. It is difficult, however, to make the channels narrow enough so
that the DNA molecule is fully stretched. In practice its conformations may
form hairpins that change the spacings between internal segments of the DNA
molecule, and thus the label locations along the channel axis. Here we describe
a theory for the distribution of label spacings that explains the heavy tails
observed in distributions of label spacings in genome mapping experiments.Comment: 18 pages, 4 figures, 1 tabl
Hadron-Hadron Interactions from Lattice QCD: isospin-2 scattering length
We present results for the scattering length using
twisted mass lattice QCD for three values of the lattice spacing and a range of
pion mass values. Due to the use of Laplacian Heaviside smearing our
statistical errors are reduced compared to previous lattice studies. A detailed
investigation of systematic effects such as discretisation effects, volume
effects, and pollution of excited and thermal states is performed. After
extrapolation to the physical point using chiral perturbation theory at NLO we
obtain .Comment: Edited for typos, overhauled figures, more detailed comparison to
existing lattice result
Estimating entanglement measures in experiments
We present a method to estimate entanglement measures in experiments. We show
how a lower bound on a generic entanglement measure can be derived from the
measured expectation values of any finite collection of entanglement witnesses.
Hence witness measurements are given a quantitative meaning without the need of
further experimental data. We apply our results to a recent multi-photon
experiment [M. Bourennane et al., Phys. Rev. Lett. 92, 087902 (2004)], giving
bounds on the entanglement of formation and the geometric measure of
entanglement in this experiment.Comment: 4 pages, 1 figure, v2: final versio
Magnification relations for Kerr lensing and testing Cosmic Censorship
A Kerr black hole with mass parameter m and angular momentum parameter a
acting as a gravitational lens gives rise to two images in the weak field
limit. We study the corresponding magnification relations, namely the signed
and absolute magnification sums and the centroid up to post-Newtonian order. We
show that there are post-Newtonian corrections to the total absolute
magnification and centroid proportional to a/m, which is in contrast to the
spherically symmetric case where such corrections vanish. Hence we also propose
a new set of lensing observables for the two images involving these
corrections, which should allow measuring a/m with gravitational lensing. In
fact, the resolution capabilities needed to observe this for the Galactic black
hole should in principle be accessible to current and near-future
instrumentation. Since a/m >1 indicates a naked singularity, a most interesting
application would be a test of the Cosmic Censorship conjecture. The technique
used to derive the image properties is based on the degeneracy of the Kerr lens
and a suitably displaced Schwarzschild lens at post-Newtonian order. A simple
physical explanation for this degeneracy is also given.Comment: 13 pages, version 2: references added, minor changes. To appear in
Phys. Rev.
Isospin-0 s-wave scattering length from twisted mass lattice QCD
We present results for the isospin-0 s-wave scattering length
calculated with Osterwalder-Seiler valence quarks on Wilson twisted mass gauge
configurations. We use three ensembles with unitary (valence) pion
mass at its physical value (250MeV), at 240MeV (320MeV) and
at 330MeV (400MeV), respectively. By using the stochastic Laplacian
Heaviside quark smearing method, all quark propagation diagrams contributing to
the isospin-0 correlation function are computed with sufficient
precision. The chiral extrapolation is performed to obtain the scattering
length at the physical pion mass. Our result agrees reasonably well with various experimental measurements and
theoretical predictions. Since we only use one lattice spacing, certain
systematics uncertainties, especially those arising from unitary breaking, are
not controlled in our result.Comment: 21 pages, 5 figures, 6 table
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