189 research outputs found
Current constraints on the EFT for the \Lambda N --> N N transition
The relation between the low energy constants appearing in the effective
field theory description of the \Lambda N --> N N transition potential and the
parameters of the one-meson-exchange model previously developed are obtained.
We extract the relative importance of the different exchange mechanisms
included in the meson picture by means of a comparison to the corresponding
operational structures appearing in the effective approach. The ability of this
procedure to obtain the weak baryon-baryon-meson couplings for a possible
scalar exchange is also discussed.Comment: 10 pages, 6 figure
Functional approach to the non-mesonic decay of Lambda-hypernuclei
We present an evaluation of the non-mesonic decay widths for
Lambda-hypernuclei (Lambda N --> NN, Lambda NN --> NNN) within the framework of
the polarization propagator method. The full Lambda self-energy is evaluated
microscopically in nuclear matter by using the functional approach, which
supplies a theoretically well grounded approximation scheme for the
classification of the relevant diagrams, according to the prescriptions of the
bosonic loop expansion. We employ average Fermi momenta, suitably adapted to
different mass number regions (medium-light, medium and heavy hypernuclei).
Moreover, we study the dependence of the decay rates on the NN and Lambda-N
short range correlations. With a proper choice of the parameters which control
these correlations in the new approximation scheme, it is possible to reproduce
the experimental decay widths for A > 10 hypernuclei.Comment: 25 pages, 8 figure
Pi-K Scattering in Full QCD with Domain-Wall Valence Quarks
We calculate the pi+ K+ scattering length in fully-dynamical lattice QCD with
domain-wall valence quarks on MILC lattices with rooted staggered sea-quarks at
a lattice spacing of b=0.125 fm, lattice spatial size of L =2.5 fm and at pion
masses of m_pi=290, 350, 490 and 600 MeV. The lattice data, analyzed at
next-to-leading order in chiral perturbation theory, allows an extraction of
the full pi K scattering amplitude at threshold. Extrapolating to the physical
point gives m_pi a_3/2 = -0.0574 (+- 0.0016)(+0.0024 -0.0058) and m_pi a_1/2 =
0.1725 (+- 0.0017)(+0.0023 -0.0156) for the I=3/2 and I=1/2 scattering lengths,
respectively, where the first error is statistical and the second error is an
estimate of the systematic due to truncation of the chiral expansion.Comment: 14 pages, 9 figure
Unitary Limit of Two-Nucleon Interactions in Strong Magnetic Fields
Two-nucleon systems are shown to exhibit large scattering lengths in strong
magnetic fields at unphysical quark masses, and the trends toward the physical
values indicate that such features may exist in nature. Lattice QCD
calculations of the energies of one and two nucleons systems are performed at
pion masses of and 806 MeV in uniform, time-independent
magnetic fields of strength {\bf B}| \sim 10^{19}10^{20}$ Gauss to determine
the response of these hadronic systems to large magnetic fields. Fields of this
strength may exist inside magnetars and in peripheral relativistic heavy ion
collisions, and the unitary behavior at large scattering lengths may have
important consequences for these systems.Comment: Accepted journal versio
The Magnetic Structure of Light Nuclei from Lattice QCD
Lattice QCD with background magnetic fields is used to calculate the magnetic
moments and magnetic polarizabilities of the nucleons and of light nuclei with
, along with the cross-section for the transition , at the flavor SU(3)-symmetric point where the pion mass is MeV. These magnetic properties are extracted from nucleon and nuclear
energies in six uniform magnetic fields of varying strengths. The magnetic
moments are presented in a recent Letter. For the charged states, the
extraction of the polarizability requires careful treatment of Landau levels,
which enter non-trivially in the method that is employed. The nucleon
polarizabilities are found to be of similar magnitude to their physical values,
with fm and
fm, exhibiting a
significant isovector component. The dineutron is bound at these heavy quark
masses and its magnetic polarizability, fm differs significantly from twice that of the neutron. A
linear combination of deuteron scalar and tensor polarizabilities is determined
by the energies of the deuteron states, and is found to be
fm. The magnetic
polarizabilities of the three-nucleon and four-nucleon systems are found to be
positive and similar in size to those of the proton, fm, fm, fm. Mixing between the
deuteron state and the spin-singlet state induced by the background
magnetic field is used to extract the short-distance two-nucleon counterterm,
, of the pionless effective theory for systems (equivalent to
the meson-exchange current contribution in nuclear potential models), that
dictates the cross-section for the process near threshold.
Combined with previous determinations of NN scattering parameters, this enables
an ab initio determination of the threshold cross-section at these unphysical
masses.Comment: 49 pages, 24 figure
Ab initio calculation of the radiative capture process
Lattice QCD calculations of two-nucleon systems are used to isolate the
short-distance two-body electromagnetic contributions to the radiative capture
process , and the photo-disintegration processes
. In nuclear potential models, such contributions are
described by phenomenological meson-exchange currents, while in the present
work, they are determined directly from the quark and gluon interactions of
QCD. Calculations of neutron-proton energy levels in multiple background
magnetic fields are performed at two values of the quark masses, corresponding
to pion masses of and 806 MeV, and are combined with pionless
nuclear effective field theory to determine these low-energy inelastic
processes. Extrapolating to the physical pion mass, a cross section of
is obtained at an incident neutron speed of $v=2,200\
m/s\sigma^{expt}(np \to d\gamma)
= 334.2(0.5)\ mb$
Precise Determination of the I=2 pipi Scattering Length from Mixed-Action Lattice QCD
The I=2 pipi scattering length is calculated in fully-dynamical lattice QCD
with domain-wall valence quarks on the asqtad-improved coarse MILC
configurations (with fourth-rooted staggered sea quarks) at four light-quark
masses. Two- and three-flavor mixed-action chiral perturbation theory at
next-to-leading order is used to perform the chiral and continuum
extrapolations. At the physical charged pion mass, we find m_pi a_pipi(I=2) =
-0.04330 +- 0.00042, where the error bar combines the statistical and
systematic uncertainties in quadrature.Comment: 20 pages, 7 figure
Aplicación de redes neuronales artificiales a la previsión de series temporales no estacionarias o no invertibles
En los últimos tiempos se ha comprobado un aumento del interés en la aplicación de las Redes Neuronales Artificiales a la previsión de series temporales, intentando explotar las indudables ventajas de estas herramientas. En este artÃculo se calculan previsiones de series no estacionarias o no invertibles, que presentan dificultades cuando se intentan pronosticar utilizando la metodologÃa ARIMA de Box-Jenkins. Las ventajas de la aplicación de redes neuronales se aprecian con más claridad, cuando se trata de pronosticar sistemas multivariantes no estacionarios
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