1,888 research outputs found
Exact calculation of thermodynamical quantities of the integrable t-J model
The specific heat and the compressibility for the integrable t-J model are
calculated showing Luttinger liquid behavior for low temperatures. A
Trotter-Suzuki mapping and the quantum transfer matrix approach are utilized.
Using an algebraic Bethe ansatz this method permits the exact calculation of
the free energy and related quantities. A set of just two non-linear integral
equations determining these quantities is studied for various particle
densities and temperatures. The structure of the specific heat is discussed in
terms of the elementary charge as well as spin excitations.Comment: 4 pages, 5 Postscript figures, uses epsf.sty and revtex, tar'ed,
gzip'ed and uuencode
On the efficiency of stochastic volume sources for the determination of light meson masses
We investigate the efficiency of single timeslice stochastic sources for the
calculation of light meson masses on the lattice as one varies the quark mass.
Simulations are carried out with Nf = 2 flavours of non-perturbatively O(a)
improved Wilson fermions for pion masses in the range of 450 - 760 MeV. Results
for pseudoscalar and vector meson two-point correlation functions computed
using stochastic as well as point sources are presented and compared. At fixed
computational cost the stochastic approach reduces the variance considerably in
the pseudoscalar channel for all simulated quark masses. The vector channel is
more affected by the intrinsic stochastic noise. In order to obtain stable
estimates of the statistical errors and a more pronounced plateau for the
effective vector meson mass, a relatively large number of stochastic sources
must be used.Comment: 18 pages, 6 figure
Excited state TBA and functional relations in spinless Fermion model
The excited state thermodynamic Bethe ansatz (TBA) equations for the spinless
Fermion model are presented by the quantum transfer matrix (QTM) approach. We
introduce a more general family called T-functions and explore functional
relations among them (T-system) and their certain combinations (Y-system).
{}From their analytical property, we derive a closed set of non-linear integral
equations which characterize the correlation length of at
any finite temperatures. Solving these equations numerically, we explicitly
determine the correlation length, which coincides with earlier results with
high accuracy.Comment: 4 page
Global Wilson-Fisher fixed points
The Wilson-Fisher fixed point with universality in three dimensions is
studied using the renormalisation group. It is shown how a combination of
analytical and numerical techniques determine global fixed point solutions to
leading order in the derivative expansion for real or purely imaginary fields
with moderate numerical effort. Universal and non-universal quantitites such as
scaling exponents and mass ratios are computed, for all , together with
local fixed point coordinates, radii of convergence, and parameters which
control the asymptotic behaviour of the effective action. We also explain when
and why finite- results do not converge pointwise towards the exact
infinite- limit. In the regime of purely imaginary fields, a new link
between singularities of fixed point effective actions and singularities of
their counterparts by Polchinski are established. Implications for other
theories are indicated.Comment: 28 pages, 10 figures, v2: explanations and refs added, to appear
(NPB
Evidence that Polyunsaturated Aldehydes of Diatoms are Repellents for Pelagic Crustacean Grazers
Evidence is given that odour compounds of diatoms serve as potential repellents for crustacean grazers. Novel repellent-test and odour-test apparatus allowed the determination of repellent activity of diatom derived compounds, activated by freezing and thawing or mechanical disintegration, and pure compounds, respectively. Epilithic diatom biofilms when activated, produced odour compounds that were determined by GC-MS to be polyunsaturated aldehydes (PUA). 2(E),4(Z),7(Z)-Decatrienal and 2(E),4(Z)-octadienal were the major compounds, and 2(E),4(Z)-heptadienal was a minor compound. These PUA were each accompanied by small amounts of the E,E-isomers in positions 2 and 4. 2(E),4(E),7(Z)-Decatrienal was the most active repellent tested and exhibited a RC50 value (indicating the concentration of a compound necessary for a 50% reduction of swimming crustaceans in the assay vial) of 3.5μM in a defined water column. Quantitative analyses showed that upon activation diatom biofilms produced large amounts of eicosapentaenoic acid (EPA) of which only a minor part was degraded to PUA. The major part of EPA was retained in the cells whilst the major part of PUA was released into the surrounding water. The data are consistent with the hypothesis that diatoms damaged by grazers develop free EPA in the cells that is toxic to grazers, and release PUA into the water that serve as warning signals to grazers. Diatoms and other phytoplankton species, that have the capacity to form these compounds, might benefit from such a reaction because the producers live in colonies or assemblages and the death of one cell liberates a cloud of repellent compounds into the water which reduces the grazing pressure on the remaining cells. Such activated defence reactions may help explain food selection and avoidance in freshwater and marine ecosystem
Lattice calculations of the leading hadronic contribution to (g-2)_mu
We report on our ongoing project to calculate the leading hadronic
contribution to the anomalous magnetic moment of the muon a_mu^HLO using two
dynamical flavours of non-perturbatively O(a) improved Wilson fermions. In this
study, we changed the vacuum polarisation tensor to a combination of local and
point-split currents which significantly reduces the numerical effort.
Partially twisted boundary conditions allow us to improve the momentum
resolution of the vacuum polarisation tensor and therefore the determination of
the leading hadronic contribution to (g-2)_mu. We also extended the range of
ensembles to include a pion mass below 200 MeV which allows us to check the
non-trivial chiral behaviour of a_mu^HLO.Comment: 7 pages, 3 figures, 1 table, talk presented at the 30th International
Symposium on Lattice Field Theory (Lattice2012), Cairns, Australi
Hadronic form factors for rare semileptonic decays
We discuss first results for the computation of short distance contributions
to semileptonic form factors for the rare decays
and . Our simulations are based on RBC/UKQCD's
ensembles with domain wall light quarks and the Iwasaki gauge action.
For the valence -quark we chose the relativistic heavy quark action.Comment: 7 pages, 1 table, 3 figures, presented at the 33rd International
Symposium on Lattice Field Theory (Lattice2015), July 14-18, 2015, Kobe,
Japa
Generalization of form in visual pattern classification.
Human observers were trained to criterion in classifying compound Gabor signals with sym- metry relationships, and were then tested with each of 18 blob-only versions of the learning set. General- ization to dark-only and light-only blob versions of the learning signals, as well as to dark-and-light blob versions was found to be excellent, thus implying virtually perfect generalization of the ability to classify mirror-image signals. The hypothesis that the learning signals are internally represented in terms of a 'blob code' with explicit labelling of contrast polarities was tested by predicting observed generalization behaviour in terms of various types of signal representations (pixelwise, Laplacian pyramid, curvature pyramid, ON/OFF, local maxima of Laplacian and curvature operators) and a minimum-distance rule. Most representations could explain generalization for dark-only and light-only blob patterns but not for the high-thresholded versions thereof. This led to the proposal of a structure-oriented blob-code. Whether such a code could be used in conjunction with simple classifiers or should be transformed into a propo- sitional scheme of representation operated upon by a rule-based classification process remains an open question
Theoretical aspects of quantum electrodynamics in a finite volume with periodic boundary conditions
First-principles studies of strongly-interacting hadronic systems using
lattice quantum chromodynamics (QCD) have been complemented in recent years
with the inclusion of quantum electrodynamics (QED). The aim is to confront
experimental results with more precise theoretical determinations, e.g. for the
anomalous magnetic moment of the muon and the CP-violating parameters in the
decay of mesons. Quantifying the effects arising from enclosing QED in a finite
volume remains a primary target of investigations. To this end, finite-volume
corrections to hadron masses in the presence of QED have been carefully studied
in recent years. This paper extends such studies to the self-energy of moving
charged hadrons, both on and away from their mass shell. In particular, we
present analytical results for leading finite-volume corrections to the
self-energy of spin-0 and spin- particles in the presence of QED
on a periodic hypercubic lattice, once the spatial zero mode of the photon is
removed, a framework that is called . By altering
modes beyond the zero mode, an improvement scheme is introduced to eliminate
the leading finite-volume corrections to masses, with potential applications to
other hadronic quantities. Our analytical results are verified by a dedicated
numerical study of a lattice scalar field theory coupled to
. Further, this paper offers new perspectives on the
subtleties involved in applying low-energy effective field theories in the
presence of , a theory that is rendered non-local
with the exclusion of the spatial zero mode of the photon, clarifying recent
discussions on this matter.Comment: 57 pages, 10 figures, version accepted for publication in Phys. Rev.
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