4,072 research outputs found
Direct photon production from hadronic sources in high-energy heavy-ion collisions
The low direct photon production from a variety of the hadronic sources
is studied within the microscopic HSD transport approach for p+C, p+Pb and
Pb+Pb collisions at 160 A GeV. The direct photon emission from elementary
hadronic scatterings as well as meson-meson bremsstrahlung are incorporated.
The influence of in-medium effects such as a collisional broadening of the
vector-meson spectral functions on the photon emission rate is found to be
hardly observable in the final spectra which are dominated by bremsstrahlung
type processes. The uncertainties in the subtraction of the 'background' from
the photon decay of hadronic resonances inside the hot and dense fireball is
investigated, additionally. Our findings are relevant for the interpretation
and extraction of experimental data on direct photon production at low .Comment: 25 pages, 10 figures, to be published in Phys. Rev.
New type of stable particle like states in chiral magnets (Chiral bobbers)
We present a new type of a thermodynamically stable magnetic state at
interfaces and surfaces of chiral magnets. The state is a soliton solution of
micromagnetic equations localized in all three dimensions near a boundary and
contains a singularity, but nevertheless has a finite energy. Both features
combine to a quasi-particle state for which we expect unusual transport and
dynamical properties. It exhibits high thermal stability and thereby can be
considered as promising object for fundamental research and practical
applications in spintronic devices. We provide arguments that such a state can
be found in different B20-type alloys e.g. MnFeGe,
MnFeSi, FeCoSi.Comment: accepted in PR
Two-particle decays of B_c meson into charmonium states
The factorization of hard and soft contributions into the hadronic decays of
B_c meson at large recoils is explored in order to evaluate the decay rates
into the S, P and D-wave charmonia associated with rho and pi. The constraints
of approach applicability and uncertainties of numerical estimates are
discussed. The mode with the J/psi in the final state is evaluated taking into
account the cascade radiative electromagnetic decays of excited P-wave states,
that enlarges the branching ratio by 20-25%.Comment: 13 pages, LaTeX axodraw-style, 1 figure, 2 table
Physics of B_c mesons
In the framework of potential models for heavy quarkonium the mass spectrum
for the system () is considered. Spin-dependent splittings, taking
into account a change of a constant for effective coulomb interaction between
the quarks, and widths of radiative transitions between the () levels
are calculated. In the framework of QCD sum rules, masses of the lightest
vector and pseudoscalar states are estimated, scaling relation
for leptonic constants of heavy quarkonia is derived, and the leptonic constant
is evaluated. The decays are considered in the framework of
both the potential models and the QCD sum rules, where the significance of
Coulomb-like corrections is shown. The relations, following from the
approximate spin symmetry for the heavy quarks in the heavy quarkonium, are
analysed for the form factors of the semileptonic weak exclusive decays of
. The lifetime is evaluated with the account of the corrections to
the spectator mechanism of the decay, because of the quark binding into the
meson. The total and differential cross sections of the production in
different interactions are calculated. The analytic expressions for the
fragmentational production cross sections of are derived. The possibility
of the practical search in the current and future experiments at
electron-positron and hadron colliders is analysed.Comment: 81 page, latex, ihep.sty is required and attached in the end of the
file after \end{document}, figures are not availabl
Magnetic hopfions in solids
Hopfions are an intriguing class of string-like solitons, named according to
a classical topological concept classifying three-dimensional direction fields.
The search of hopfions in real physical systems is going on for nearly half a
century, starting with the seminal work of Faddeev. But so far realizations in
solids are missing. Here, we present a theory that identifies magnetic
materials featuring hopfions as stable states without the assistance of
confinement or external fields. Our results are based on an advanced
micromagnetic energy functional derived from a spin-lattice Hamiltonian.
Hopfions appear as emergent particles of the classical Heisenberg model.
Magnetic hopfions represent three-dimensional particle-like objects of
nanometre-size dimensions opening the gate to a new generation of spintronic
devices in the framework of a truly three-dimensional architecture. Our
approach goes beyond the conventional phenomenological models. We derive
material-realistic parameters that serve as concrete guidance in the search of
magnetic hopfions bridging computational physics with materials science
Forced Topological Nontrivial Field Configurations
The motion of a one-dimensional kink and its energy losses are considered as
a model of interaction of nontrivial topological field configurations with
external fields. The approach is based on the calculation of the zero modes
excitation probability in the external field. We study in the same way the
interaction of the t'Hooft-Polyakov monopole with weak external fields. The
basic idea is to treat the excitation of a monopole zero mode as the monopole
displacement. The excitation is found perturbatively. As an example we consider
the interaction of the t'Hooft-Polyakov monopole with an external uniform
magnetic field.Comment: 18 pages, 3 Postscript figures, RevTe
Kondo effect in a one-electron double quantum dot: Oscillations of the Kondo current in a weak magnetic field
We present transport measurements of the Kondo effect in a double quantum dot
charged with only one or two electrons, respectively. For the one electron case
we observe a surprising quasi-periodic oscillation of the Kondo conductance as
a function of a small perpendicular magnetic field |B| \lesssim 50mT. We
discuss possible explanations of this effect and interpret it by means of a
fine tuning of the energy mismatch of the single dot levels of the two quantum
dots. The observed degree of control implies important consequences for
applications in quantum information processing
Generation of Intrinsic Vibrational Gap Modes in Three-Dimensional Ionic Crystals
The existence of anharmonic localization of lattice vibrations in a perfect
3-D diatomic ionic crystal is established for the rigid-ion model by molecular
dynamics simulations. For a realistic set of NaI potential parameters, an
intrinsic localized gap mode vibrating in the [111] direction is observed for
fcc and zinc blende lattices. An axial elastic distortion is an integral
feature of this mode which forms more readily for the zinc blende than for the
fcc structure. Molecular dynamics simulations verify that in each structure
this localized mode may be stable for at least 200 cycles.Comment: 5 pages, 4 figures, RevTeX, using epsf.sty. To be published in Phys.
Rev. B. Also available at http://www.msc.cornell.edu/~kiselev
Mass spectra of doubly heavy Omega_QQ' baryons
We evaluate the masses of baryons composed of two heavy quarks and a strange
quark with account for spin-dependent splittings in the framework of potential
model with the KKO potential motivated by QCD with a three-loop beta-function
for the effective charge consistent with both the perturbative limit at short
distances and linear confinement term at long distances between the quarks. The
factorization of dynamics is supposed and explored in the nonrelativistic
Schroedinger equation for the motion in the system of two heavy quarks
constituting the doubly heavy diquark and the strange quark interaction with
the diquark. The limits of approach, its justification and uncertainties are
discussed. Excited quasistable states are classified by the quantum numbers of
heavy diquark composed by the heavy quarks of the same flavor.Comment: 14 pages, revtex4-file, 3 eps-figures, 5 tables, typos correcte
Algebraic properties of Gardner's deformations for integrable systems
An algebraic definition of Gardner's deformations for completely integrable
bi-Hamiltonian evolutionary systems is formulated. The proposed approach
extends the class of deformable equations and yields new integrable
evolutionary and hyperbolic Liouville-type systems. An exactly solvable
two-component extension of the Liouville equation is found.Comment: Proc. conf. "Nonlinear Physics: Theory and Experiment IV" (Gallipoli,
2006); Theor. Math. Phys. (2007) 151:3/152:1-2, 16p. (to appear
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