16,129 research outputs found
Direct and indirect exciton mixing in a slightly asymmetric double quantum well
We studied, theoretically, the optical absorption spectra for a slightly
asymmetric double quantum well (DQW), in the presence of electric and magnetic
fields. Recent experimental results for a 10.18/3.82/9.61 nm GaAs Al(_{.33}
)Ga(_{.67})As DQW show clearly the different behavior in the luminescence peaks
for the indirect exciton (IX) and left direct exciton (DX) as a function of the
external electric field. We show that the presence of a peak near the (DX)
peak, attributed to an impurity bound left (DX) in the experimental results,
could be a consequence of the non-trivial mixing between excitonic states.Comment: 8 pages and 8 figure
Calibrating the Na\"ive Cornell Model with NRQCD
Along the years, the Cornell Model has been extraordinarily successful in
describing hadronic phenomenology, in particular in physical situations for
which an effective theory of the strong interactions such as NRQCD cannot be
applied. As a consequence of its achievements, a relevant question is whether
its model parameters can somehow be related to fundamental constants of QCD. We
shall give a first answer in this article by comparing the predictions of both
approaches. Building on results from a previous study on heavy meson
spectroscopy, we calibrate the Cornell model employing NRQCD predictions for
the lowest-lying bottomonium states up to NLO, in which the bottom mass is
varied within a wide range. We find that the Cornell model mass parameter can
be identified, within perturbative uncertainties, with the MSR mass at the
scale GeV. This identification holds for any value of or
the bottom mass, and for all perturbative orders investigated. Furthermore, we
show that: a) the "string tension" parameter is independent of the bottom mass,
and b) the Coulomb strength of the Cornell model can be related to the
QCD strong coupling constant at a characteristic non-relativistic
scale. We also show how to remove the renormalon of the static QCD
potential and sum-up large logs related to the renormalon subtraction by
switching to the low-scale, short-distance MSR mass, and using R-evolution. Our
R-improved expression for the static potential remains independent of the heavy
quark mass value and agrees with lattice QCD results for values of the radius
as large as fm, and with the Cornell model potential at long distances.
Finally we show that for moderate values of , the R-improved NRQCD and
Cornell static potentials are in head-on agreement.Comment: 22 pages, 13 figures, 3 table
Charmonium resonances in the 3.9 GeV/ energy region and the puzzle
An interesting controversy has emerged challenging the widely accepted nature
of the and the resonances, which had initially been
assigned to the and states,
respectively. To unveil their inner structure, the properties of the
and charmonium states in the energy
region of these resonances are analyzed in the framework of a constituent quark
model. Together with the bare states, threshold effects due to the
opening of nearby meson-meson channels are included in a coupled-channels
scheme calculation. We find that the structure of both states is dominantly
molecular with a probability of bare states lower than . Our
results favor the hypothesis that and resonances arise as
different decay mechanisms of the same state. Moreover we
found an explanation for the recently discovered MeV as a
state and rediscovery the lost as an
additional state in the family.Comment: 6 pages, 3 table
Molecular components in P-wave charmed-strange mesons
Results obtained by various experiments show that the
and mesons are very narrow states located below the and
thresholds, respectively. This is markedly in contrast with the
expectations of naive quark models and heavy quark symmetry. Motivated by a
recent lattice study which addresses the mass shifts of the ground
states with quantum numbers () and
() due to their coupling with -wave
thresholds, we perform a similar analysis within a
nonrelativistic constituent quark model in which quark-antiquark and
meson-meson degrees of freedom are incorporated. The quark model has been
applied to a wide range of hadronic observables and thus the model parameters
are completely constrained. The coupling between quark-antiquark and
meson-meson Fock components is done using a model in which its only
free parameter has been elucidated performing a global fit to the
decay widths of mesons that belong to different quark sectors, from light to
heavy. We observe that the coupling of the meson sector to
the threshold is the key feature to simultaneously lower the
masses of the corresponding and states
predicted by the naive quark model and describe the meson as the
state of the doublet predicted by heavy quark
symmetry, reproducing its strong decay properties. Our calculation allows to
introduce the coupling with the -wave channel and the
computation of the probabilities associated with the different Fock components
of the physical state.Comment: 11 pages, 3 figures, 7 table
Thermal entanglement witness for materials with variable local spin lengths
We show that the thermal entanglement in a spin system using only magnetic
susceptibility measurements is restricted to the insulator materials. We
develop a generalization of the thermal entanglement witness that allows us to
get information about the system entanglement with variable local spin lengths
that can be used experimentally in conductor or insulator materials. As an
application, we study thermal entanglement for the half-filled Hubbard model
for linear, square and cubic clusters. We note that it is the itinerancy of
electrons that favors the entanglement. Our results suggest a weak dependence
between entanglement and external spin freedom degrees.Comment: 4 pages, 3 figure
Molecular components in and mesons
Different experiments have confirmed that the and
mesons are very narrow states located, respectively, below the
and thresholds. This is markedly in contrast with the
expectations of naive quark models and heavy quark symmetry. We address the
mass shifts of the ground states with quantum numbers
() and () using a
nonrelativistic constituent quark model in which quark-antiquark and
meson-meson degrees of freedom are incorporated. The quark model has been
applied to a wide range of hadronic observables and thus the model parameters
are completely constrained. We observe that the coupling of the
meson sector to the threshold is a key feature in
lowering the masses of the corresponding and
states predicted by the naive quark model, but also in
describing the meson as the state of the
doublet predicted by heavy quark symmetry and thus
reproducing its strong decay properties. Two features of our formalism cannot
be address nowadays by other approaches: the coupling of the -wave
threshold in the channel and the
computation of the probabilities associated with different Fock components in
the physical state.Comment: Contribution to the proceedings of the 14th International Workshop on
Meson Production, Properties and Interaction (MESON2016). June 2-7, 2016.
Krakow, Polan
GOVERNMENT REVENUES AND EXPENDITURES IN GUINEA-BISSAU: CAUSALITY AND COINTEGRATION
The paper establishes empirically the temporal causality and long run relationship between government expenditures and government revenues for the case of Guinea-Bissau - a low income country under stress (LICUS) in Africa. A macroeconomic model is developed to lay out the hypothesis of a spend-tax behavior in the country¡¯s public finances management system. Empirical validation is carried out by means of a traditional Granger-causality test and the estimation of an error correction model between expenditures and revenues.Public Finances, Causality Tests, Cointegration Analysis
- …