192 research outputs found
A large- PNJL model with explicit Z symmetry
A PNJL model is built, in which the Polyakov-loop potential is explicitly
Z-symmetric in order to mimic a Yang-Mills theory with gauge group
SU(). The physically expected large- and large- behaviours of the
thermodynamic observables computed from the Polyakov-loop potential are used to
constrain its free parameters. The effective potential is eventually
U(1)-symmetric when is infinite. Light quark flavours are added by using
a Nambu-Jona-Lasinio (NJL) model coupled to the Polyakov loop (the PNJL model),
and the different phases of the resulting PNJL model are discussed in 't
Hooft's large- limit. Three phases are found, in agreement with previous
large- studies. When the temperature is larger than some deconfinement
temperature , the system is in a deconfined, chirally symmetric, phase for
any quark chemical potential . When however, the system is in a
confined phase in which chiral symmetry is either broken or not. The critical
line , signalling the restoration of chiral symmetry, has the same
qualitative features than what can be obtained within a standard PNJL
model.Comment: To appear in Phys Rev
Gluons in glueballs: Spin or helicity?
In the last decade, lattice QCD has been able to compute the low-lying
glueball spectrum with accuracy. Like other effective approaches of QCD,
potential models still have difficulties to cope with gluonic hadrons. Assuming
that glueballs are bound states of valence gluons with zero current mass, it is
readily understood that the use of a potential model, intrinsically non
covariant, could be problematic in this case. The main challenge for this kind
of model is actually to find a way to introduce properly the more relevant
degree of freedom of the gluon: spin or helicity. In this work, we use the
so-called helicity formalism of Jacob and Wick to describe two-gluon glueballs.
We show in particular that this helicity formalism exactly reproduces the
numbers which are observed in lattice QCD when the constituent gluons
have a helicity-1, without introducing extra states as it is the case in most
of the potential models. These extra states appear when gluons are seen as
spin-1 particles. Using a simple spinless Salpeter model with Cornell potential
within the helicity formalism, we obtain a glueball mass spectrum which is in
good agreement with lattice QCD predictions for helicity-1 gluons provided
instanton-induced interactions are taken into account.Comment: 2 figures, section III.B corrected but conclusions unchange
Glueballs and statistical mechanics of the gluon plasma
We study a pure gluon plasma in the context of quasiparticle models, where
the plasma is considered as an ideal gas of massive bosons. In order to
reproduce SU(3) gauge field lattice data within such a framework, we review
briefly the necessity to use a temperature-dependent gluon mass which accounts
for color interactions between the gluons near and agrees with
perturbative QCD at large temperatures. Consequently, we discuss the
thermodynamics of systems with temperature-dependent Hamiltonians and clarify
the situation about the possible solutions proposed in the literature to treat
consistently those systems. We then focus our attention to two possible
formulations which are thermodynamically consistent and we extract the gluon
mass from the equation of state obtained in SU(3) lattice QCD. We find that the
thermal gluon mass is similar in both statistical formalisms. Finally, an
interpretation of the gluon plasma as an ideal gas made of glueballs and gluons
is also presented. The glueball mass is consistently computed within a
relativistic formalism using a potential obtained from lattice QCD. We find
that the gluon plasma might be a glueball-rich medium for and suggest that glueballs could be detected in future
experiments dedicated to the quark-gluon plasma.Comment: minor corrections. Accepted for publication in PR
Short-range potentials from QCD at order
We systematically compute the effective short-range potentials arising from
second order QCD-diagrams related to bound states of quarks, antiquarks, and
gluons. Our formalism relies on the assumption that the exchanged gluons are
massless, while the constituent gluons as well as the lightest quarks acquire a
nonvanishing constituent mass because of confinement. The potentials we obtain
include the first relativistic corrections, thus spin-spin terms, spin-orbit
terms, etc. Such effective potentials are expected to be relevant for the
building of accurate potential models describing usual hadrons as well as
exotic ones like glueballs and hybrids. In particular, we compute
for the first time an effective quark-gluon potential, and show the existence
of a quadrupolar interaction term in this case. We also discuss the influence
of a possible nonzero mass for the exchanged gluons.Comment: 33 pages, 4 tables and 12 figures ; typos correcte
What makes SMEs more likely to collaborate? Analysing the role of regional policy
The last twenty years have witnessed the diffusion of regional innovation policies supporting networks of innovators. The underlying aim of these policies is to encourage firms, particularly SMEs, to undertake collaborations with organisations possessing complementary knowledge. Focusing on a set of SMEs that have participated, over time, in several innovation networks funded by the same regional government, the paper investigates how their relationships have evolved with respect to the following aspects: (i) reiteration of pre-existing relationships as opposed to experimentation of new relationships; (ii) collaboration with organisations possessing complementary rather than similar knowledge and competencies; (iii) creation of local relationships rather than experimentation of extra-local collaborations; (iv) reliance upon intermediaries to connect with other organisations. Our findings reveal that the involvement in these policy-supported networks changed the firms’ relational patterns, leading them to collaborate with a wider variety of agents than those with whom they were linked before the policies. Sectoral heterogeneity had a negative effect on the probability to collaborate, while co-localisation increased the likelihood to collaborate. Mutual involvement with intermediaries also had a positive effect. However, in the case of firm-to-university relationships only specialized intermediaries were likely to perform a positive role and, therefore, encourage networking
Government policy failure in public support for research and development
peer-reviewedPromoting Research and Development (R&D) and innovative activity is a key element of the EU Lisbon Agenda and is seen as playing a central part in stimulating economic development. In this paper we argue that, even allowing for benevolent policy-makers, informational asymmetries can lead to a misallocation of public support for R&D, hence government policy failure, with the potential to exacerbate preexisting market failures. Initially, we explore alternative allocation mechanisms for public support, which can help to minimize the scale of these government policy failures. Of these mechanisms (grants, tax credits, or allocation rules based on past performance), our results suggest that none is universally most efficient. Rather, the effectiveness of each allocation rule depends on the severity of financial constraints and on the level of innovative capabilities of the firms themselves.ACCEPTEDpeer-reviewe
Coordinated optimization of visual cortical maps (II) Numerical studies
It is an attractive hypothesis that the spatial structure of visual cortical
architecture can be explained by the coordinated optimization of multiple
visual cortical maps representing orientation preference (OP), ocular dominance
(OD), spatial frequency, or direction preference. In part (I) of this study we
defined a class of analytically tractable coordinated optimization models and
solved representative examples in which a spatially complex organization of the
orientation preference map is induced by inter-map interactions. We found that
attractor solutions near symmetry breaking threshold predict a highly ordered
map layout and require a substantial OD bias for OP pinwheel stabilization.
Here we examine in numerical simulations whether such models exhibit
biologically more realistic spatially irregular solutions at a finite distance
from threshold and when transients towards attractor states are considered. We
also examine whether model behavior qualitatively changes when the spatial
periodicities of the two maps are detuned and when considering more than 2
feature dimensions. Our numerical results support the view that neither minimal
energy states nor intermediate transient states of our coordinated optimization
models successfully explain the spatially irregular architecture of the visual
cortex. We discuss several alternative scenarios and additional factors that
may improve the agreement between model solutions and biological observations.Comment: 55 pages, 11 figures. arXiv admin note: substantial text overlap with
arXiv:1102.335
A reanalysis of Finite Temperature SU(N) Gauge Theory
We revise the , , lattice data on pure gauge theories at
finite temperature by means of a quasi-particle approach. In particular we
focus on the relation between the quasi-particle effective mass and the order
of the deconfinement transition, the scaling of the interaction measure with
, the role of gluon condensate, the screening mass.Comment: 7 pages, 7 figure
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