7,284 research outputs found
Nucleon and gamma N -> Delta lattice form factors in a constituent quark model
A covariant quark model, based both on the spectator formalism and on vector
meson dominance, and previously calibrated by the physical data, is here
extended to the unphysical region of the lattice data by means of one single
extra adjustable parameter - the constituent quark mass in the chiral limit. We
calculated the Nucleon (N) and the gamma N -> Delta form factors in the
universe of values for that parameter described by quenched lattice QCD. A
qualitative description of the Nucleon and gamma N -> Delta form factors
lattice data is achieved for light pions.Comment: To appear in J.Phys.
The Non-Mesonic Weak Decay of Double-Lambda Hypernuclei: A Microscopic Approach
The non--mesonic weak decay of double-- hypernuclei is studied
within a microscopic diagrammatic approach. Besides the nucleon--induced
mechanism, , widely studied in single-- hypernuclei,
additional hyperon--induced mechanisms, ,
and , are
accessible in double-- hypernuclei and are investigated here. As in
previous works on single-- hypernuclei, we adopt a nuclear matter
formalism extended to finite nuclei via the local density approximation and a
one--meson exchange weak transition potential (including the ground state
pseudoscalar and vector octets mesons) supplemented by correlated and
uncorrelated two--pion--exchange contributions. The weak decay rates are
evaluated for hypernuclei in the region of the experimentally accessible light
hypernuclei Be and B. Our
predictions are compared with a few previous evaluations. The rate for the
decay is dominated by --, -- and
--exchange and turns out to be about 2.5\% of the free decay
rate, , while the total rate for the and decays, dominated by
--exchange, amounts to about 0.25\% of . The
experimental measurement of these decays would be essential for the beginning
of a systematic study of the non--mesonic decay of strangeness
hypernuclei. This field of research could also shed light on the possible
existence and nature of the --dibaryon.Comment: 17 pages, 2 figure
Cross-Correlation in the Auditory Coincidence Detectors of Owls
Interaural time difference (ITD) plays a central role in many auditory functions, most importantly in sound localization. The classic model for how ITD is computed was put forth by Jeffress (1948). One of the predictions of the Jeffress model is that the neurons that compute ITD should behave as cross-correlators. Whereas cross-correlation-like properties of the ITD-computing neurons have been reported, attempts to show that the shape of the ITD response function is determined by the spectral tuning of the neuron, a core prediction of cross-correlation, have been unsuccessful. Using reverse correlation analysis, we demonstrate in the barn owl that the relationship between the spectral tuning and the ITD response of the ITD-computing neurons is that predicted by cross-correlation. Moreover, we show that a model of coincidence detector responses derived from responses to binaurally uncorrelated noise is consistent with binaural interaction based on cross-correlation. These results are thus consistent with one of the key tenets of the Jeffress model. Our work sets forth both the methodology to answer whether cross-correlation describes coincidence detector responses and a demonstration that in the barn owl, the result is that expected by theory
Spin and angular momentum in the nucleon
Using the covariant spectator theory (CST), we present the results of a
valence quark-diquark model calculation of the nucleon structure function f(x)
measured in unpolarized deep inelastic scattering (DIS), and the structure
functions g1(x) and g2(x) measured in DIS using polarized beams and targets.
Parameters of the wave functions are adjusted to fit all the data. The fit
fixes both the shape of the wave functions and the relative strength of each
component. Two solutions are found that fit f(x) and g1(x), but only one of
these gives a good description of g2(x). This fit requires the nucleon CST wave
functions contain a large D-wave component (about 35%) and a small P-wave
component (about 0.6%). The significance of these results is discussed.Comment: 27 pages; 13 figure
Outer crust of a cold non-accreting magnetar
The outer crust structure and composition of a cold, non-accreting magnetar
is studied. We model the outer crust to be made of fully equilibrated matter
where ionized nuclei form a Coulomb crystal embedded in an electron gas. The
main effects of the strong magnetic field are those of quantizing the electron
motion in Landau levels and of modifying the nuclear single particle levels
producing, on average, an increased binding of nucleons in nuclei present in
the Coulomb lattice. The effect of an homogeneous and constant magnetic field
on nuclear masses has been predicted by using a covariant density functional,
in which induced currents and axial deformation due to the presence of a
magnetic field that breaks time-reversal symmetry have been included
self-consistently in the nucleon and meson equations of motion. Although not
yet observed, for G both effects contribute to produce
different compositions and to enlarge the range of pressures typically present
in common neutron stars. Specifically, in such a regime, the magnetic field
effects on nuclei favor the appearance of heavier nuclei at low pressures. As
increases, such heavier nuclei are also preferred up to larger pressures.
In the most extreme case, the whole outer crust is almost made of
Zr.Comment: Published versio
Covariant nucleon wave function with S, D, and P-state components
Expressions for the nucleon wave functions in the covariant spectator theory
(CST) are derived. The nucleon is described as a system with a off-mass-shell
constituent quark, free to interact with an external probe, and two spectator
constituent quarks on their mass shell. Integrating over the internal momentum
of the on-mass-shell quark pair allows us to derive an effective nucleon wave
function that can be written only in terms of the quark and diquark
(quark-pair) variables. The derived nucleon wave function includes
contributions from S, P and D-waves.Comment: 13 pages and 1 figur
Monetary Policy after the Great Moderation
The interferences among some financial, economic and monetary variables are checked as an indicator of economic performance in the long run and for the monetary policy applied between the Great Moderation (GM) of 1987-2001 and the Global Financial Crisis of 2007-2009. For achieving this target, some Granger causality tests are applied to GDP growth, credit growth, and lending interest of 36 countries of the EU and the OECD for the full sample of 1987-2012 and the sub-sample of 2002-2007. Results corroborate the interferences among these variables for the discretionary monetary policy applied immediately after the GM, within the "Ad Hoc Era" or "lax period", and independence when monetary policy was correctly applied and rules-based
How do income inequality and fiscal consolidation impact on banking crises? A post-Keynesian view
This is the first paper in estimating a population-averaged panel logit probability model to test the importance of the interaction between deficit in the public budgeting and income inequality on banking crises, for 36 developed countries from 1961-2011. New empirical evidence is shown on whether rising inequality is linked with financial crises, corroborating theoretical expectations of post-Keynesian authors. Policy measures are provided and tested empirically: whilst in general terms higher levels of income inequality could be associated with financial crises; countries with high levels could reduce the likelihood of a crisis better in a context of fiscal consolidation. One reason could be that governments could use this public surplus for reducing income inequality, which helps to reduce defaults and banking crises. These results could be useful for academics, and policy-makers
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