51 research outputs found
Multiband Description of Optical Conductivity in Ferropnictide Superconductors
We study optical properties of the multiband superconductors with an s± order parameter symmetry. By comparing results of our theory with experimental data on optical conductivity for Ba0.68K0.32Fe2As2 single crystals, we show that satisfactory description of the novel superconductors can be obtained only considering a strong electron-boson coupling. We reexamine the effect of disorder and demonstrate that multiband superconductors are more robust with respect to it than naively expected by simple analogy with paramagnetic impurities in single-band superconductors. Moreover, disorder may give rise to new effects, in particular to a phase transition s±→s++. We discuss how the systematic study of disorder impact on the density of states and the optical conductivity may provide information on the underlying order parameter structur
Insight into nucleon structure from generalized parton distributions
The lowest three moments of generalized parton distributions are calculated
in full QCD and provide new insight into the behavior of nucleon
electromagnetic form factors, the origin of the nucleon spin, and the
transverse structure of the nucleon.Comment: 3 pages, Lattice2003(Theoretical developments
Moments of Nucleon Generalized Parton Distributions in Lattice QCD
Calculation of moments of generalized parton distributions in lattice QCD
requires more powerful techniques than those previously used to calculate
moments of structure functions. Hence, we present a novel approach that
exploits the full information content from a given lattice configuration by
measuring an overdetermined set of lattice observables to provide maximal
statistical constraints on the generalized form factors at a given virtuality,
t. In an exploratory investigation using unquenched QCD configurations at
intermediate sea quark masses, we demonstrate that our new technique is
superior to conventional methods and leads to reliable numerical signals for
the n=2 flavor singlet generalized form factors up to 3 GeV^2. The contribution
from connected diagrams in the flavor singlet sector to the total quark angular
momentum is measured to an accuracy of the order of one percent.Comment: 16 pages, 8 figures, LaTeX, minor elaboration of formalism and
singular value decomposition for non-specialists and addition of several
reference
Moments of nucleon spin-dependent generalized parton distributions
We present a lattice measurement of the first two moments of the
spin-dependent GPD H-tilde(x,xi,t). From these we obtain the axial coupling
constant and the second moment of the spin-dependent forward parton
distribution. The measurements are done in full QCD using Wilson fermions. In
addition, we also present results from a first exploratory study of full QCD
using Asqtad sea and domain-wall valence fermions.Comment: Lattice2003(Theory), 3 pages, 3 figures, to appear in the Proceedings
of Lattice 200
Hadronic physics with domain-wall valence and improved staggered sea quarks
With the advent of chiral fermion formulations, the simulation of light
valence quarks has finally become realistic for numerical simulations of
lattice QCD. The simulation of light dynamical quarks, however, remains one of
the major challenges and is still an obstacle to realistic simulations. We
attempt to meet this challenge using a hybrid combination of Asqtad sea quarks
and domain-wall valence quarks. Initial results for the proton form factor and
the nucleon axial coupling are presented.Comment: Two Talks presented at Lattice2004(spectrum), LaTex, 6 pages, 6 eps
figure
Comparative study of loop contributions in AdS and dS
The generic feature of non-conformal fields in Poincare patch of de Sitter
space is the presence of large IR loop corrections even for massive fields.
Moreover, in global de Sitter there are loop IR divergences for the massive
fields. Naive analytic continuation from de Sitter to Anti-de-Sitter might lead
one to conclude that something similar should happen in the latter space as
well. However, we show that there are no large IR effects in the one-loop
two-point functions in the Poincare patch of Anti-de-Sitter space even for the
zero mass minimally coupled scalar fields. As well there are neither large IR
effects nor IR divergences in global Anti-de-Sitter space even for the zero
mass.Comment: 12 pages. Minor changes, misprints are correcte
Leptonic - and -decays: mass effects, polarization effects and radiative corrections
We calculate the radiative corrections to the unpolarized and the four
polarized spectrum and rate functions in the leptonic decay of a polarized into a polarized electron. The new feature of our calculation is that we
keep the mass of the final state electron finite which is mandatory if one
wants to investigate the threshold region of the decay. Analytical results are
given for the energy spectrum and the polar angle distribution of the final
state electron whose longitudinal and transverse polarization is calculated. We
also provide analytical results on the integrated spectrum functions. We
analyze the limit of our general results and investigate the
quality of the approximation. In the case we
discuss in some detail the role of the anomalous helicity flip
contribution of the final electron which survives the limit. The
results presented in this 0203048 also apply to the leptonic decays of
polarized -leptons for which we provide numerical results.Comment: 39 pages, 11 postscript figures added. Updated version. Four
references added. A few text improvements. Final version to appear in
Phys.Rev.
Multiband model for penetration depth in MgB2
The results of first principles calculations of the electronic structure and
the electron-phonon interaction in MgB2 are used to study theoretically the
temperature dependence and anisotropy of the magnetic field penetration depth.
The effects of impurity scattering are essential for a proper description of
the experimental results. We compare our results with experimental data and we
argue that the two-band model describes the data rather well.Comment: submitted to Phys. Rev.
Search for heavy neutrinos mixing with tau neutrinos
We report on a search for heavy neutrinos (\nus) produced in the decay
D_s\to \tau \nus at the SPS proton target followed by the decay \nudecay in
the NOMAD detector. Both decays are expected to occur if \nus is a component
of .\
From the analysis of the data collected during the 1996-1998 runs with
protons on target, a single candidate event consistent with
background expectations was found. This allows to derive an upper limit on the
mixing strength between the heavy neutrino and the tau neutrino in the \nus
mass range from 10 to 190 . Windows between the SN1987a and Big Bang
Nucleosynthesis lower limits and our result are still open for future
experimental searches. The results obtained are used to constrain an
interpretation of the time anomaly observed in the KARMEN1 detector.\Comment: 20 pages, 7 figures, a few comments adde
Nonequilibrium Evolution of Correlation Functions: A Canonical Approach
We study nonequilibrium evolution in a self-interacting quantum field theory
invariant under space translation only by using a canonical approach based on
the recently developed Liouville-von Neumann formalism. The method is first
used to obtain the correlation functions both in and beyond the Hartree
approximation, for the quantum mechanical analog of the model. The
technique involves representing the Hamiltonian in a Fock basis of annihilation
and creation operators. By separating it into a solvable Gaussian part
involving quadratic terms and a perturbation of quartic terms, it is possible
to find the improved vacuum state to any desired order. The correlation
functions for the field theory are then investigated in the Hartree
approximation and those beyond the Hartree approximation are obtained by
finding the improved vacuum state corrected up to . These
correlation functions take into account next-to-leading and
next-to-next-to-leading order effects in the coupling constant. We also use the
Heisenberg formalism to obtain the time evolution equations for the equal-time,
connected correlation functions beyond the leading order. These equations are
derived by including the connected 4-point functions in the hierarchy. The
resulting coupled set of equations form a part of infinite hierarchy of coupled
equations relating the various connected n-point functions. The connection with
other approaches based on the path integral formalism is established and the
physical implications of the set of equations are discussed with particular
emphasis on thermalization.Comment: Revtex, 32 pages; substantial new material dealing with
non-equilibrium evolution beyond Hartree approx. based on the LvN formalism,
has been adde
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