5,663 research outputs found
Linear Time-Dependent Invariants for Scalar Fields and Noether's Theorem
The infinite number of time-dependent linear in field and conjugated momenta
invariants is derived for the scalar field using the Noether's theorem
procedure.Comment: LaTeX, 6 pages, preprint of Instituto de Ciencias Nucleares, UNAM,
Departamento de F\ii sica and Matem\'aticas Aplicadas, M\'exico D. F. ,
M\'exico No. 04 (1994
Squeezing in a 2-D generalized oscillator
A two-dimensional generalized oscillator with time-dependent parameters is
considered to study the two-mode squeezing phenomena. Specific choices of the
parameters are used to determine the dispersion matrix and analytic
expressions, in terms of standard hermite polynomials, of the wavefunctions and
photon distributions. (to be publish in the Third Workshop on Squeezed States
and Uncertainty Relations, Baltimore, USA, (August 1993))Comment: LaTeX, 5 pages, preprint of Instituto de Ciencias Nucleares, UNAM
Departamento de F\ii sica and Matem\'aticas Aplicadas, No. 02 (1994
Characterization of the known T type dwarfs towards the Sigma Orionis cluster
(Abridged) A total of three T type candidates (SOri70, SOri73, and
SOriJ0538-0213) lying in the line of sight towards Sigma Orionis were
characterized by means of near-infrared photometric, astrometric, and
spectroscopic studies. H-band methane images were collected for all three
sources and an additional sample of 15 field T type dwarfs using LIRIS/WHT.
J-band spectra of resolution of ~500 were obtained for SOriJ0538-0213 with
ISAAC/VLT, and JH spectra of resolution of ~50 acquired with WFC3/HST were
employed for the spectroscopic classification of SOri70 and 73. Proper motions
with a typical uncertainty of +/-3 mas/yr and a time interval of ~7-9 yr were
derived. Using the LIRIS observations of the field T dwarfs, we calibrated this
imager for T spectral typing via methane photometry. The three SOri objects
were spectroscopically classified as T4.5+/-0.5 (SOri73), T5+/-0.5
(SOriJ0538-0213), and T7 (SOri70). The similarity between the
observed JH spectra and the methane colors and the data of field ultra-cool
dwarfs of related classifications suggests that SOri70, 73, and
SOriJ053804.65-021352.5 do not deviate significantly in surface gravity in
relation to the field. Additionally, the detection of KI at ~1.25 microns in
SOriJ0538-0213 points to a high-gravity atmosphere. Only the K-band reddish
nature of SOri70 may be consistent with a low gravity atmosphere. The proper
motions of SOri70 and 73 are measurable and are larger than that of the cluster
by >3.5 sigma. The proper motion of SOriJ0538-0213 is consistent with a null
displacement. These observations suggest that none of the three T dwarfs are
likely Sigma Orionis members, and that either planetary-mass objects with
masses below ~4 MJup may not exist free-floating in the cluster or they may lie
at fainter near-infrared magnitudes than those of the targets (this is H>20.6
mag), thus remaining unidentified to date.Comment: Accepted for publication in A&A (2014), corrected typo
Excavando huesos en los museos. El caso de la necrĆ³polis de "Los Millares"
X Congreso Nacional de PaleopatologĆa. Univesidad AutĆ³noma de Madrid, septiembre de 200
Pion re-scattering in pi0 production
This work discusses the approximations frequently used to calculate the
contribution to pion production in proton-proton scattering near threshold from
irreducible pion re-scattering. The reference result is obtained from the
quantum mechanical reduction of the corresponding Feynman diagram to
Time-Ordered Field Theory diagrams. We conclude that the Distorted Wave Born
Approximation is quite adequate at threshold energies and even above. We
discuss the choices for the energy of the exchanged pion, both for its
propagator and for the piN amplitude describing the re-scattering vertex. The
calculation employs a physical model for nucleons and pions -- pseudo-vector
coupling for the piNN vertex and realistic amplitudes for the piN re-scattering
and for the NN transitions in the initial and final states.Comment: 30 pages, 11 figures; To appear in Phys. Rev.
Strengthening regional supply chains for agricultural transformation in Colombia
Over the last 2 decades, Colombia has gained valuable experience in supporting agricultural supply chains. The government views this approach primarily as a means of achieving agricultural transformation, and to this end the Ministry of Agriculture and Rural Development (MADR, its Spanish acronym) has designed a set of innovative policies. Under these policies, regional committees plan and implement development initiatives in collaboration with the local public and private sectors. While achieving several notable successes, the committees have also come up against various challenges: boosting operational funds; developing stronger relationships with regional governments, private actors, and support systems; strengthening small-scale producer organizations; and, above all, creating a long-term regional vision for supply-chain development. Colombia must meet these challenges effectively if its public policies are to achieve the desired effect of enhancing agricultural productivity
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