5,248 research outputs found
Consequences of Approximate Symmetry of the Neutrino Mass Matrix
Assuming that the neutrino mass matrix is dominated by a term with the
permutation symmetry it is possible to explain
neutrino data only if the masses are quasi-degenerate. A sub-dominant term with
an approximate symmetry leads to an approximate tri-bimaximal form.
Experimental consequences are discussed.Comment: 7 pages, 2 figures, 1 table, RevTe
Neutrino masses and mixings in a Minimal S_3-invariant Extension of the Standard Model
The mass matrices of the charged leptons and neutrinos, that had been derived
in the framework of a Minimal S_3-invariant Extension of the Standard Model,
are here reparametrized in terms of their eigenvalues. The neutrino mixing
matrix, V_PMNS, is then computed and exact, explicit analytical expressions for
the neutrino mixing angles as functions of the masses of the neutrinos and
charged leptons are obtained. The reactor, theta_13, and the atmosferic,
theta_23, mixing angles are found to be functions only of the masses of the
charged leptons. The numerical values of theta_13{th} and theta_23{th} computed
from our theoretical expressions are found to be in excellent agreement with
the latest experimental determinations. The solar mixing angle, theta_12{th},
is found to be a function of both, the charged lepton and neutrino masses, as
well as of a Majorana phase phi_nu. A comparison of our theoretical expression
for the solar angle theta_12{th} with the latest experimental value
theta_12{exp} ~ 34 deg allowed us to fix the scale and origin of the neutrino
mass spectrum and obtain the mass values |m_nu1|=0.0507 eV, |m_nu2|=0.0499 eV
and |m_nu3|=0.0193 eV, in very good agreement with the observations of neutrino
oscillations, the bounds extracted from neutrinoless double beta decay and the
precision cosmological measurements of the CMB.Comment: To appear in the Proceedings of the XXIX Symposium on Nuclear
Physics, Cocoyoc, Mex., January 2006. Some typographical errors on formulae
correcte
Ultracold Bose gases in time-dependent 1D superlattices: response and quasimomentum structure
The response of ultracold atomic Bose gases in time-dependent optical
lattices is discussed based on direct simulations of the time-evolution of the
many-body state in the framework of the Bose-Hubbard model. We focus on
small-amplitude modulations of the lattice potential as implemented in several
recent experiment and study different observables in the region of the first
resonance in the Mott-insulator phase. In addition to the energy transfer we
investigate the quasimomentum structure of the system which is accessible via
the matter-wave interference pattern after a prompt release. We identify
characteristic correlations between the excitation frequency and the
quasimomentum distribution and study their structure in the presence of a
superlattice potential.Comment: 4 pages, 4 figure
Machine Learning Energies of 2 M Elpasolite (ABCD) Crystals
Elpasolite is the predominant quaternary crystal structure (AlNaKF
prototype) reported in the Inorganic Crystal Structure Database. We have
developed a machine learning model to calculate density functional theory
quality formation energies of all 2 M pristine ABCD elpasolite
crystals which can be made up from main-group elements (up to bismuth). Our
model's accuracy can be improved systematically, reaching 0.1 eV/atom for a
training set consisting of 10 k crystals. Important bonding trends are
revealed, fluoride is best suited to fit the coordination of the D site which
lowers the formation energy whereas the opposite is found for carbon. The
bonding contribution of elements A and B is very small on average. Low
formation energies result from A and B being late elements from group (II), C
being a late (I) element, and D being fluoride. Out of 2 M crystals, 90 unique
structures are predicted to be on the convex hull---among which NFAlCa,
with peculiar stoichiometry and a negative atomic oxidation state for Al
Crystal Structure Representations for Machine Learning Models of Formation Energies
We introduce and evaluate a set of feature vector representations of crystal
structures for machine learning (ML) models of formation energies of solids. ML
models of atomization energies of organic molecules have been successful using
a Coulomb matrix representation of the molecule. We consider three ways to
generalize such representations to periodic systems: (i) a matrix where each
element is related to the Ewald sum of the electrostatic interaction between
two different atoms in the unit cell repeated over the lattice; (ii) an
extended Coulomb-like matrix that takes into account a number of neighboring
unit cells; and (iii) an Ansatz that mimics the periodicity and the basic
features of the elements in the Ewald sum matrix by using a sine function of
the crystal coordinates of the atoms. The representations are compared for a
Laplacian kernel with Manhattan norm, trained to reproduce formation energies
using a data set of 3938 crystal structures obtained from the Materials
Project. For training sets consisting of 3000 crystals, the generalization
error in predicting formation energies of new structures corresponds to (i)
0.49, (ii) 0.64, and (iii) 0.37 eV/atom for the respective representations
U.S-RUSSIA RELATIONS: CHALLENGES AND IMPLICATIONS FOR GLOBAL SECURITY
The paper examines the cyclical nature of U.S-Russia relations in the 21st century; with special regards to the administration of U.S President Barrack Obama and Russia’s President Vladimir Putin. Specifically, the study examines the challenges and implications of confrontation between the two states. It takes the annexation of Crimea as the origin of headlong disagreement in recent time although the work acknowledges that Russia’s annexation of Crimea does not constitute the genesis of disagreement in post-cold war relations. In a qualitative manner, the study examines the historical antecedents of U.S- Russia relations and adopts the neo-realist tenets as theoretical guide for approximating the complex realities of U.S Russia relations. Structured interview with expert in the field of international relations particularly those specializing in the study of U.S-Russian relations, alongside documents from official websites of states were engaged. The paper discovers that of all the bonds of bilateral relations, it is the bond of U.S-Russia relations that directly have implications on global security especially because of the threat of nuclear misconception and war. Consequently, the study recommends that, U.S. and Russia must recognise the lasting quality of international politics and create a basis of understanding as such by acknowledging the legitimate interests and concerns of the other
Economic Development and the Utility of Local Content Legislation in the Oil and Gas Industry: Conflicts and Effects of Nigeria’s Local Content Act in the Context of International Investment Law
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