6,753 research outputs found
Credit crunches and credit allocation in a model of entrepreneurship
We study the effects of credit shocks in a model with heterogeneous entrepreneurs, financing constraints, and a realistic firm size distribution. As entrepreneurial firms can grow only slowly and rely heavily on retained earnings to expand the size of their business in this set-up, we show that, by reducing entrepreneurial firm size and earnings, negative shocks have a very persistent effect on real activity. In determining the speed of recovery from an adverse economic shock, the most important factor is the extent to which the shock erodes entrepreneurial wealth
Bequests and heterogeneity in retirement wealth
Households hold vastly heterogeneous amounts of wealth when they reach retirement, and differences in lifetime earnings explain only part of this variation. This paper studies the role of intergenerational transmission of ability, voluntary bequest motives, and the recipiency of accidental and intended bequests (both in terms of timing and size) in generating wealth dispersion at retirement, in the context of a rich quantitative model. Modeling voluntary bequests, and realistically calibrating them, not only generates more wealth dispersion at retirement and reduces the correlation between retirement wealth and lifetime income, but also generates a skewed bequest distribution that is close to the one in the observed data
Leptogenesis, Z' bosons, and the reheating temperature of the Universe
We study the impact for leptogenesis of new U(1) gauge bosons coupled to the
heavy Majorana neutrinos. They can significantly enhance the efficiency of
thermal scenarios in the weak washout regime as long as the Z' masses are not
much larger than the reheating temperature (), with the
highest efficiencies obtained for Z' bosons considerably heavier than the heavy
neutrinos (). We show how the allowed region of the parameter
space is modified in the presence of a Z' and we also obtain the minimum
reheating temperature that is required for these models to be successful.Comment: 14 pages, 6 figures; One figure added, discussion on the reheating
temperature extende
Credit crunches and credit allocation in a model of entrepreneurship
We study the effects of credit shocks in a model with heterogeneous entrepreneurs, financing constraints, and a realistic firm-size distribution. As entrepreneurial firms can grow only slowly and rely heavily on retained earnings to expand the size of their business, we show that, by reducing entrepreneurial firm size and earnings, negative shocks have a very persistent effect on real activity. In determining the speed of recovery from an adverse economic shock, the most important factor is the extent to which the shock erodes entrepreneurial wealth
Unifying Class-Based Representation Formalisms
The notion of class is ubiquitous in computer science and is central in many
formalisms for the representation of structured knowledge used both in
knowledge representation and in databases. In this paper we study the basic
issues underlying such representation formalisms and single out both their
common characteristics and their distinguishing features. Such investigation
leads us to propose a unifying framework in which we are able to capture the
fundamental aspects of several representation languages used in different
contexts. The proposed formalism is expressed in the style of description
logics, which have been introduced in knowledge representation as a means to
provide a semantically well-founded basis for the structural aspects of
knowledge representation systems. The description logic considered in this
paper is a subset of first order logic with nice computational characteristics.
It is quite expressive and features a novel combination of constructs that has
not been studied before. The distinguishing constructs are number restrictions,
which generalize existence and functional dependencies, inverse roles, which
allow one to refer to the inverse of a relationship, and possibly cyclic
assertions, which are necessary for capturing real world domains. We are able
to show that it is precisely such combination of constructs that makes our
logic powerful enough to model the essential set of features for defining class
structures that are common to frame systems, object-oriented database
languages, and semantic data models. As a consequence of the established
correspondences, several significant extensions of each of the above formalisms
become available. The high expressiveness of the logic we propose and the need
for capturing the reasoning in different contexts forces us to distinguish
between unrestricted and finite model reasoning. A notable feature of our
proposal is that reasoning in both cases is decidable. We argue that, by virtue
of the high expressive power and of the associated reasoning capabilities on
both unrestricted and finite models, our logic provides a common core for
class-based representation formalisms
Identifying Unconventional E Models at Colliders
Recently it was shown that, in the framework of superstring inspired \E
models, the presence of generation dependent discrete symmetries allows us to
construct a phenomenologically viable class of models in which the three
generations of fermions do not have the same embedding within the fundamental
{\bf 27} dimensional representation of E. In this scenario, these different
embeddings of the conventional fermions imply that the left-handed charged
leptons and the right-handed -type quarks are coupled in a non--universal
way to the new neutral gauge bosons present in these models. It
was also shown that a unique signature for this scenario, would be a deviation
from unity for the ratio of cross sections for the production of two different
lepton species in annihilation. However, several different scenarios
are possible, depending on the particular assignment chosen for ,
and and for the right-handed -type quarks, as well as on the type
of boson. Such scenarios can not be disentangled from one another by
means of cross section measurements alone. In this paper we examine the
possibility of identifying the pattern of embeddings through measurements of
polarized and unpolarized asymmetries for fermion pair-production at the 500
GeV Next Linear Collider (NLC). We show that it will be possible to
identify the different patterns of unconventional assignments for the
left-handed leptons and for the quark, for masses as large as
TeV.Comment: Plain Tex, 15 pages, + 9 figure available upon request
([email protected] or [email protected]), UM-TH 93--1
Baryonic masses based on the NJL model
We employ the Nambu Jona--Lasinio model to determine the vacuum pressure on
the quarks in a baryon and hence their density inside. Then we estimate the
baryonic masses by implementing the local density approximation for the mean
field quark energies obtained in a uniform and isotropic system. We obtain a
fair agreement with the experimental masses.Comment: 17 pages, 3 figures. to be published on EPJ
A Quantitative Analysis of Charmonium Suppression in Nuclear Collisions
Data from J/psi and psi' production in p-A collisions are used to determine
the cross section for absorption of pre-resonance charmonium in nuclear matter.
The J/psi suppression in O-Cu, O-U and S-U collisions is fully reproduced by
the corresponding nuclear absorption, while Pb-Pb collisions show an additional
suppression increasing with centrality. We study the onset of this change in
terms of hadronic comover interactions and conclude that so far no conventional
hadronic description can consistently account for all data. Deconfinement,
starting at a critical point determined by central S-U collisions, is in accord
with the observed suppression pattern.Comment: 37 pages, 12 figures, uses epsfig style, LaTe
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