6,654 research outputs found
Evolution of structure of SiO2 nanoparticles upon cooling from the melt
Evolution of structure of spherical SiO2 nanoparticles upon cooling from the
melt has been investigated via molecular-dynamics (MD) simulations under
non-periodic boundary conditions (NPBC). We use the pair interatomic potentials
which have weak Coulomb interaction and Morse type short-range interaction. The
change in structure of SiO2 nanoparticles upon cooling process has been studied
through the partial radial distribution functions (PRDFs), coordination number
and bond-angle distributions at different temperatures. The core and surface
structures of nanoparticles have been studied in details. Our results show
significant temperature dependence of structure of nanoparticles. Moreover,
temperature dependence of concentration of structural defects in nanoparticles
upon cooling from the melt toward glassy state has been found and discussed.Comment: 12 pages, 6 figure
Charm Quark Mass from Inclusive Semileptonic B Decays
The MSbar charm quark mass is determined to be m_c(m_c)=1224+-17+-54 MeV from
a global fit to inclusive B meson decay data, where the first error is
experimental, and includes the uncertainty in alpha_s, and the second is an
estimate of theoretical uncertainties in the computation. We discuss the
implications of the pole mass renormalon in the determination of m_c.Comment: 7 pages, 2 tables; revtex4. References added, minor changes; version
to appear in PL
Phase Space Matching and Finite Lifetime Effects for Top-Pair Production Close to Threshold
The top-pair production cross section close to threshold in
collisions is strongly affected by the small lifetime of the top
quark. Since the cross section is defined through final states containing the
top decay products, a consistent definition of the cross section depends on
prescriptions how these final states are accounted for the cross section.
Experimentally, these prescriptions are implemented for example through cuts on
kinematic quantities such as the reconstructed top quark invariant masses. As
long as these cuts do not reject final states that can arise from the decay of
a top and an anti-top quark with a small off-shellness compatible with the
nonrelativistic power-counting, they can be implemented through imaginary phase
space matching conditions in NRQCD. The prescription-dependent cross section
can then be determined from the optical theorem using the forward
scattering amplitude. We compute the phase space matching conditions associated
to cuts on the top and anti-top invariant masses at next-to-next-to-leading
logarithmic (NNLL) order and partially at next-to-next-to-next-to-leading
logarithmic (NLL) order in the nonrelativistic expansion and, together
with finite lifetime and electroweak effects known from previous work, analyze
their numerical impact on the cross section. We show that the phase
space matching contributions are essential to make reliable NRQCD predictions,
particularly for energies below the peak region, where the cross section is
small. We find that irreducible background contributions associated to final
states that do not come from top decays are strongly suppressed and can be
neglected for the theoretical predictions.Comment: 62 pages, 21 figure
Electroweak Absorptive Parts in NRQCD Matching Conditions
Electroweak corrections associated with the instability of the top quark to
the next-to-next-to-leading logarithmic (NNLL) total top pair threshold cross
section in e+e- annihilation are determined. Our method is based on absorptive
parts in electroweak matching conditions of the NRQCD operators and the optical
theorem. The corrections lead to ultraviolet phase space divergences that have
to be renormalized and lead to NLL mixing effects. Numerically, the corrections
can amount to several percent and are comparable to the known NNLL QCD
corrections.Comment: 17 pages, revtex4, 4 postscript figures included; minor changes in
text and references, title modified in printed versio
Three-Loop Anomalous Dimension of the Heavy Quark Pair Production Current in Non-Relativistic QCD
The three-loop non-mixing contributions to the anomalous dimension of the
leading order quark pair production current in non-relativistic QCD are
computed. It is demonstrated that the renormalization procedure can only be
carried out consistently if the dynamics of both soft and the ultrasoft degrees
of freedom is present for all scales below the heavy quark mass, and if the
soft and ultrasoft renormalization scales are always correlated.Comment: 19 pages, revtex, 5 postscript figures include
An isogeometric analysis for elliptic homogenization problems
A novel and efficient approach which is based on the framework of
isogeometric analysis for elliptic homogenization problems is proposed. These
problems possess highly oscillating coefficients leading to extremely high
computational expenses while using traditional finite element methods. The
isogeometric analysis heterogeneous multiscale method (IGA-HMM) investigated in
this paper is regarded as an alternative approach to the standard Finite
Element Heterogeneous Multiscale Method (FE-HMM) which is currently an
effective framework to solve these problems. The method utilizes non-uniform
rational B-splines (NURBS) in both macro and micro levels instead of standard
Lagrange basis. Beside the ability to describe exactly the geometry, it
tremendously facilitates high-order macroscopic/microscopic discretizations
thanks to the flexibility of refinement and degree elevation with an arbitrary
continuity level provided by NURBS basis functions. A priori error estimates of
the discretization error coming from macro and micro meshes and optimal micro
refinement strategies for macro/micro NURBS basis functions of arbitrary orders
are derived. Numerical results show the excellent performance of the proposed
method
B decays in the upsilon expansion
Theoretical predictions for B decay rates are rewritten in terms of the
Upsilon(1S) meson mass instead of the b quark mass, using a modified
perturbation expansion. The theoretical consistency of this expansion is shown
both at low and high orders. Our method improves the behavior of the
perturbation series for semileptonic and nonleptonic inclusive decay modes, as
well as for exclusive decay form factors. The results are applied to the
determination of the semileptonic B branching ratio, charm counting, the ratio
of B -> X tau nu and B -> X e nu decay rates, and form factor ratios in B -> D*
e nu decay. We also comment on why it is not possible to separate perturbative
and nonperturbative effects in QCD.Comment: 21 page
First-Order Transition in XY Fully Frustrated Simple Cubic Lattice
We study the nature of the phase transition in the fully frustrated simple
cubic lattice with the XY spin model. This system is the Villain's model
generalized in three dimensions. The ground state is very particular with a
12-fold degeneracy. Previous studies have shown unusual critical properties.
With the powerful Wang-Landau flat-histogram Monte Carlo method, we carry out
in this work intensive simulations with very large lattice sizes. We show that
the phase transition is clearly of first order, putting an end to the
uncertainty which has lasted for more than twenty years
Phase Transition in Heisenberg Fully Frustrated Simple Cubic Lattice
The phase transition in frustrated spin systems is a fascinated subject in
statistical physics. We show the result obtained by the Wang-Landau flat
histogram Monte Carlo simulation on the phase transition in the fully
frustrated simple cubic lattice with the Heisenberg spin model. The degeneracy
of the ground state of this system is infinite with two continuous parameters.
We find a clear first-order transition in contradiction with previous studies
which have shown a second-order transition with unusual critical properties.
The robustness of our calculations allows us to conclude this issue putting an
end to the 20-year long uncertainty.Comment: submitted for publicatio
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