1,095 research outputs found
Progress towards a lattice determination of (moments of) nucleon structure functions
Using unimproved and non-perturbatively O(a) improved Wilson fermions,
results are given for the three lowest moments of unpolarised nucleon structure
functions. Renormalisation, chiral extrapolation and the continuum limit of the
matrix elements are briefly discussed. The simulations are performed for both
quenched and two flavours of unquenched fermions. No obvious sign of deviation
from linearity in the chiral extrapolations are found. (This is most clearly
seen in our quenched unimproved data, which extends to lighter quark mass.)
Possible quenching effects also seem to be small. The lowest moment thus
remains too large, so it seems to be necessary to reach smaller quark masses in
numerical simulations.Comment: 3 pages, Latex, 4 figures, Talk presented at
Lattice2001(matrixelement
Perturbative Renormalization of Improved Lattice Operators
We derive bases of improved operators for all bilinear quark currents up to
spin two (including the operators measuring the first moment of DIS Structure
Functions), and compute their one-loop renormalization constants for arbitrary
coefficients of the improvement terms. We have thus control over O(a)
corrections, and for a suitable choice of improvement coefficients we are only
left with errors of O(a^2).Comment: 4 pages, LaTeX + 1 eps file + epscrc2.sty (included). Talk given to
the Lattice 97 International Symposium, 22-26 July 1997, Edinburgh, UK. Minor
changes in notatio
Cerebellar Atrophy Related To Chronic Exposure To Toluene. Case Report.
A 31-year-old woman presented slowly progressing ataxia and neurasthenic symptoms after 14-year occupational exposure to low concentration toluene vapour. Examination disclosed only cerebellar signs. Cognitive functions were normal except moderate visuo-spatial and constructive deficit. CT imaging showed severe pancerebellar atrophy without pathological signs in other brain structures. Two years after she was removed from workplace, CT imaging and ataxia showed no worsening, while visuo-constructive function improved. The authors warn against possible neurotoxic risk associated with this kind of exposure.5290-
A combined experimental and computational study of the pressure dependence of the vibrational spectrum of solid picene C_22H_14
We present high-quality optical data and density functional perturbation
theory calculations for the vibrational spectrum of solid picene
(CH) under pressure up to 8 GPa. First-principles calculations
reproduce with a remarkable accuracy the pressure effects on both frequency and
intensities of the phonon peaks experimentally observed . Through a detailed
analysis of the phonon eigenvectors, We use the projection on molecular
eigenmodes to unambiguously fit the experimental spectra, resolving complicated
spectral structures, in a system with hundreds of phonon modes. With these
projections, we can also quantify the loss of molecular character under
pressure. Our results indicate that picene, despite a \sim 20 % compression of
the unit cell, remains substantially a molecular solid up to 8 GPa, with phonon
modes displaying a smooth and uniform hardening with pressure. The Grueneisen
parameter of the 1380 cm^{-1} a_1 Raman peak () is much lower
than the effective value () due to K doping. This is an
indication that the phonon softening in K doped samples is mainly due to charge
transfer and electron-phonon coupling.Comment: Replaced with final version (PRB
Higher-twist contributions to the Structure Functions coming from 4-fermion operators
We evaluate the contribution of a class of higher-twist operators to the
lowest moment of the Structure Functions, by computing appropriate matrix
elements of six four-fermion operators in the quenched approximation. Their
perturbative renormalization constants and mixing coefficients are calculated
in the 't Hooft-Veltman scheme of dimensional regularization, using codes
written in the algebraic manipulation computer language FORM.Comment: Talk presented at LATTICE99(matrix elements), Pisa (Italy), June 29 -
July 3; 3 pages; to be published in Nucl. Phys. B (Proc. Suppl.
Spectroscopy of HS: evidence of a new energy scale for superconductivity
The discovery of a superconducting phase in sulfur hydride under high
pressure with a critical temperature above 200 K has provided a new impetus to
the search for even higher . Theory predicted and experiment confirmed
that the phase involved is HS with Im-3m crystal structure. The observation
of a sharp drop in resistance to zero at , its downward shift with
magnetic field and a Meissner effect confirm superconductivity but the
mechanism involved remains to be determined. Here, we provide a first optical
spectroscopy study of this new superconductor. Experimental results for the
optical reflectivity of HS, under high pressure of 150 GPa, for several
temperatures and over the range 60 to 600 meV of photon energies, are compared
with theoretical calculations based on Eliashberg theory using DFT results for
the electron-phonon spectral density F(). Two significant
features stand out: some remarkably strong infrared active phonons at
160 meV and a band with a depressed reflectance in the superconducting state in
the region from 450 meV to 600 meV. In this energy range, as predicted by
theory, HS is found to become a better reflector with increasing
temperature. This temperature evolution is traced to superconductivity
originating from the electron-phonon interaction. The shape, magnitude, and
energy dependence of this band at 150 K agrees with our calculations. This
provides strong evidence of a conventional mechanism. However, the unusually
strong optical phonon suggests a contribution of electronic degrees of freedom.Comment: 10 pages, 8 figures. Main manuscript and supplementary informatio
Improvement for Quenched Wilson Fermions
We briefly describe some of our recent results for the mass spectrum and
matrix elements using improved fermions for quenched QCD. Where possible
a comparison is made between improved and Wilson fermions.Comment: 6 pages, Latex, 11 figures, epsf.sty and buckow1.sty needed
(buckow1.sty included). Talk presented at the 31st Ahrenshoop Symposium on
the Theory of Elementary Particles, September 1997, Buckow, German
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