2,508 research outputs found
A geometric approach to time evolution operators of Lie quantum systems
Lie systems in Quantum Mechanics are studied from a geometric point of view.
In particular, we develop methods to obtain time evolution operators of
time-dependent Schrodinger equations of Lie type and we show how these methods
explain certain ad hoc methods used in previous papers in order to obtain exact
solutions. Finally, several instances of time-dependent quadratic Hamiltonian
are solved.Comment: Accepted for publication in the International Journal of Theoretical
Physic
Evidence for SU(3) symmetry breaking from hyperon production
We examine the SU(3) symmetry breaking in hyperon semileptonic decays (HSD)
by considering two typical sets of quark contributions to the spin content of
the octet baryons: Set-1 with SU(3) flavor symmetry and Set-2 with SU(3) flavor
symmetry breaking in HSD. The quark distributions of the octet baryons are
calculated with a successful statistical model. Using an approximate relation
between the quark fragmentation functions and the quark distributions, we
predict polarizations of the octet baryons produced in annihilation
and semi-inclusive deeply lepton-nucleon scattering in order to reveal the
SU(3) symmetry breaking effect on the spin structure of the octet baryons. We
find that the SU(3) symmetry breaking significantly affects the hyperon
polarization. The available experimental data on the polarization
seem to favor the theoretical predictions with SU(3) symmetry breaking. We
conclude that there is a possibility to get a collateral evidence for SU(3)
symmetry breaking from hyperon production. The theoretical errors for our
predictions are discussed.Comment: 3 tables, 14 figure
Effective Two Higgs Doublets in Nonminimal Supersymmetric Models
The Higgs sectors of supersymmetric extensions of the Standard Model have two
doublets in the minimal version (MSSM), and two doublets plus a singlet in two
others: with (UMSSM) and without (NMSSM) an extra U(1)'. A very concise
comparison of these three models is possible if we assume that the singlet has
a somewhat larger breaking scale compared to the electroweak scale. In that
case, the UMSSM and the NMSSM become effectively two-Higgs-doublet models
(THDM), like the MSSM. As expected, the mass of the lightest CP-even neutral
Higgs boson has an upper bound in each case. We find that in the NMSSM, this
bound exceeds not very much that of the MSSM, unless tan(beta) is near one.
However, the upper bound in the UMSSM may be substantially enhanced.Comment: 8 pages, 1 table, 3 figure
Electroweak Corrections to the Charged Higgs Boson Decay into Chargino and Neutralino
The electroweak corrections to the partial widths of the decays including one-loop
diagrams of the third generation quarks and squarks, are investigated within
the Supersymmetric Standard Model. The relative corrections can reach the
values about 10%, therefore they should be taken into account for the precise
experimental measurement at future colliders.Comment: 21 pages, 6 eps figures, 1 Latex fil
Constraint methods for determining pathways and free energy of activated processes
Activated processes from chemical reactions up to conformational transitions
of large biomolecules are hampered by barriers which are overcome only by the
input of some free energy of activation. Hence, the characteristic and
rate-determining barrier regions are not sufficiently sampled by usual
simulation techniques. Constraints on a reaction coordinate r have turned out
to be a suitable means to explore difficult pathways without changing potential
function, energy or temperature. For a dense sequence of values of r, the
corresponding sequence of simulations provides a pathway for the process. As
only one coordinate among thousands is fixed during each simulation, the
pathway essentially reflects the system's internal dynamics. From mean forces
the free energy profile can be calculated to obtain reaction rates and insight
in the reaction mechanism. In the last decade, theoretical tools and computing
capacity have been developed to a degree where simulations give impressive
qualitative insight in the processes at quantitative agreement with
experiments. Here, we give an introduction to reaction pathways and
coordinates, and develop the theory of free energy as the potential of mean
force. We clarify the connection between mean force and constraint force which
is the central quantity evaluated, and discuss the mass metric tensor
correction. Well-behaved coordinates without tensor correction are considered.
We discuss the theoretical background and practical implementation on the
example of the reaction coordinate of targeted molecular dynamics simulation.
Finally, we compare applications of constraint methods and other techniques
developed for the same purpose, and discuss the limits of the approach
Regio- and stereoselective synthesis of acetallic tetrahydropyrans as building blocks for natural products preparation, via a tandem [4+3]-cycloaddition/ozonolysis Process
A highly versatile synthetic pathway is presented for the preparation of polyfunctionalized acetallic tetrahydropyrans from conveniently substituted 1-methoxy-8-oxabicyclo[3.2.1]- oct-6-en-3-one derivatives, as intermediates in the total synthesis of natural and unnatural products with structural, functional and/or biological importance. This synthetic methodology involves two key steps: a [4 + 3] cycloaddition reaction between an oxyallyl cation and 2-methoxyfuran as a diene, followed by oxidative and/or reductive ozonolysis of the cycloheptenone subunit. This sequence renders polyfunctionalized 2-methoxytetrahydropyranic products capable of being easily opened under acidic conditions. The key steps, cycloaddition and subsequent ozonolysis were both fully studied under different reaction conditions and using several substrates in order to optimize yields and stereoselectivities and to study the scope of the methodology. It is noteworthy that both reactions proceed with high diastereoselectivity and, in the case of the oxidative ozonolysis, outstanding regioselectivity as well. A chemical library of 14 polyfunctionalized tetrahydrofurans, having five or seven stereocenters, has been prepared using the detailed approach
TeV-scale seesaw from a multi-Higgs model
We suggest new simple model of generating tiny neutrino masses through a
TeV-scale seesaw mechanism without requiring tiny Yukawa couplings. This model
is a simple extension of the standard model by introducing extra one Higgs
singlet, and one Higgs doublet with a tiny vacuum expectation value.
Experimental constraints, electroweak precision data and no large flavor
changing neutral currents, are satisfied since the extra doublet only has a
Yukawa interaction with lepton doublets and right-handed neutrinos, and their
masses are heavy of order a TeV-scale. Since active light neutrinos are
Majorana particles, this model predicts a neutrinoless double beta decay.Comment: 21 pages, 8 figure
Single gluino production in the R-parity lepton number violating MSSM at the LHC
We examine the -violating signal of single gluino production
associated with a charged lepton or neutrino at the large hadron collider
(LHC), in the model of R-parity relaxed supersymmetric model. If the parameters
in the supersymmetric interactions are not too small, and the
mass of gluino is considered in the range from several GeV (as the Lightest
Supersymmetric Particle) to 800 GeV, the cross section of the single gluino
production via Drell-Yan processes can be in the order of
femto barn, and that via gluon fusion in the order of femto
barn. If the gluino decay can be well detected in the CERN LHC, this process
provides a prospective way to probe supersymmetry and violation.Comment: LaTex, 22 pages, 5 EPS file
Isospin breaking in the vector current of the nucleon
Extraction of the nucleon's strange form factors from experimental data
requires a quantitative understanding of the unavoidable contamination from
isospin violation. A number of authors have addressed this issue during the
past decade, and their work is reviewed here. The predictions from early models
are largely consistent with recent results that rely as much as possible on
input from QCD symmetries and related experimental data. The resulting bounds
on isospin violation are sufficiently precise to be of value to on-going
experimental and theoretical studies of the nucleon's strange form factors.Comment: 5 pages, 3 figures. Presented at the International Workshop "From
Parity Violation to Hadronic Structure and more...", Milos, Greece, 16-20 May
2006. Version 2 is only to update Refs. [21] and [25
Relativistic Random-Phase Approximation with density-dependent meson-nucleon couplings
The matrix equations of the relativistic random-phase approximation (RRPA)
are derived for an effective Lagrangian characterized by density-dependent
meson-nucleon vertex functions. The explicit density dependence of the
meson-nucleon couplings introduces rearrangement terms in the residual two-body
interaction, that are essential for a quantitative description of excited
states. Illustrative calculations of the isoscalar monopole, isovector dipole
and isoscalar quadrupole response of Pb, are performed in the fully
self-consistent RRPA framework, based on effective interactions with a
phenomenological density dependence adjusted to nuclear matter and ground-state
properties of spherical nuclei. The comparison of the RRPA results on multipole
giant resonances with experimental data constrains the parameters that
characterize the isoscalar and isovector channel of the density-dependent
effective interactions.Comment: RevTeX, 8 eps figures, submitted to Phys. Rev.
- …