155 research outputs found
Roper Excitation in Alpha-Proton Scattering
We study the Roper excitation in the reaction. We consider
all processes which may be relevant in the Roper excitation region, namely,
Roper excitation in the target, Roper excitation in the projectile, and double
excitation processes. The theoretical investigation shows that the
Roper excitation in the proton target mediated by an isoscalar exchange is the
dominant mechanism in the process. We determine an effective isoscalar
interaction by means of which the experimental cross section is well
reproduced. This should be useful to make predictions in related reactions and
is a first step to construct eventually a microscopic
transition potential, for which the present reaction does not offer enough
information.Comment: Latex 17 pages; figures available by request; Phys. Rev. C in prin
Neutrino masses and mixing parameters in a left-right model with mirror fermions
In this work we consider a left-right model containing mirror fermions with
gauge group SU(3). The model has several free parameters which here we have
calculated by using the recent values for the squared-neutrino mass
differences. Lower bound for the mirror vacuum expectation value helped us to
obtain crude estimations for some of these parameters. Also we estimate the
order of magnitude of the masses of the standard and mirror neutrinos.Comment: 13 pages, version submitted to European Physical Journal
Ricci flow, quantum mechanics and gravity
It has been argued that, underlying any given quantum-mechanical model, there
exists at least one deterministic system that reproduces, after
prequantisation, the given quantum dynamics. For a quantum mechanics with a
complex d-dimensional Hilbert space, the Lie group SU(d) represents classical
canonical transformations on the projective space CP^{d-1} of quantum states.
Let R stand for the Ricci flow of the manifold SU(d-1) down to one point, and
let P denote the projection from the Hopf bundle onto its base CP^{d-1}. Then
the underlying deterministic model we propose here is the Lie group SU(d),
acted on by the operation PR. Finally we comment on some possible consequences
that our model may have on a quantum theory of gravity.Comment: 8 page
Neutrino mixing and masses in a left-right model with mirror fermions
In the framework of a left-right model containing mirror fermions with gauge
group SU(3),
we estimate the neutrino masses, which are found to be consistent with their
experimental bounds and hierarchy. We evaluate the decay rates of the Lepton
Flavor Violation (LFV) processes , and . We obtain upper limits for the
flavor-changing branching ratios in agreement with their present experimental
bounds. We also estimate the decay rates of heavy Majorana neutrinos in the
channels , and , which are roughly equal for large values of the heavy
neutrino mass. Starting from the most general Majorana neutrino mass matrix,
the smallness of active neutrino masses turns out from the interplay of the
hierarchy of the involved scales and the double application of seesaw
mechanism. An appropriate parameterization on the structure of the neutrino
mass matrix imposing a symmetric mixing of electron neutrino with muon and tau
neutrinos leads to Tri-bimaximal mixing matrix for light neutrinos.Comment: Accepted by European Physical Journal
The Imaginary Part of Nucleon Self-energy in hot nuclear matter
A semiphenomenological approach to the nucleon self-energy in nuclear matter
at finite temperatures is followed. It combines elements of Thermo Field
Dynamics for the treatment of finite temperature with a model for the
self-energy, which evaluates the second order diagrams taking the needed
dynamics of the NN interaction from experiment. The approach proved to be
accurate at zero temperature to reproduce Im(Sigma) and other properties of
nucleons in matter. In the present case we apply it to determine Im(Sigma) at
finite temperatures. An effective NN cross section is deduced which can be
easily used in analyses of heavy ion reactions.Comment: 15 pages, 6 postscripts figures, to be published in Nucl. Phys.
Automation on the generation of genome scale metabolic models
Background: Nowadays, the reconstruction of genome scale metabolic models is
a non-automatized and interactive process based on decision taking. This
lengthy process usually requires a full year of one person's work in order to
satisfactory collect, analyze and validate the list of all metabolic reactions
present in a specific organism. In order to write this list, one manually has
to go through a huge amount of genomic, metabolomic and physiological
information. Currently, there is no optimal algorithm that allows one to
automatically go through all this information and generate the models taking
into account probabilistic criteria of unicity and completeness that a
biologist would consider. Results: This work presents the automation of a
methodology for the reconstruction of genome scale metabolic models for any
organism. The methodology that follows is the automatized version of the steps
implemented manually for the reconstruction of the genome scale metabolic model
of a photosynthetic organism, {\it Synechocystis sp. PCC6803}. The steps for
the reconstruction are implemented in a computational platform (COPABI) that
generates the models from the probabilistic algorithms that have been
developed. Conclusions: For validation of the developed algorithm robustness,
the metabolic models of several organisms generated by the platform have been
studied together with published models that have been manually curated. Network
properties of the models like connectivity and average shortest mean path of
the different models have been compared and analyzed.Comment: 24 pages, 2 figures, 2 table
A parallel algorithm for the partial single-input pole assignment problem
AbstractFor a linear control system, we introduce a parallel algorithm to assign a desired subset of eigenvalues to a single-input linear invariant dynamic system. We obtain a sequential algorithm as a particular case. The proposed algorithms are conceptually simple and are based on the computation of left eigenvectors of the state matrix. In addition, the parallel algorithm parallelizes easily as the numerical examples show
Isoscalar Roper Excitation in Reactions in the Region
Recent experiments at Saturne at showed that the
reaction on the proton shows two distinctive peaks, which were associated to
projectile excitation and Roper target excitation. A subsequent
theoretical analysis has shown that this picture is qualitatively correct but
there are important interference effects between the two mechanisms.
Futhermore, at this energy the ratio of strengths for the Roper and
peak is about . In the present paper we show that by going to the region the interference effects become negligible, the signal for the
Roper excitation is increased by more than an order of magnitude and the ratio
of cross sections at the peaks for Roper and excitation becomes of the
order of unity, thus making this range of energies ideal for studies of
isoscalar Roper excitation.Comment: Latex 10 pages; figures available by request; Phys. Lett. B submitte
Quantum Dynamics on the Worldvolume from Classical su(n) Cohomology
A key symmetry of classical p-branes is invariance under worldvolume diffeomorphisms. Under the assumption that the worldvolume, at fixed values of the time, is a compact, quantisable Kähler manifold, we prove that the Lie algebra of volume-preserving diffeomorphisms of the worldvolume can be approximated by su(n), for n → ∞. We also prove, under the same assumptions regarding the worldvolume at fixed time, that classical Nambu brackets on the worldvolume are quantised by the multibrackets corresponding to cocycles in the cohomology of the Lie algebra su(n)
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