667 research outputs found
Condition for gapless color-antitriplet excitations in NJL models
We present an exact condition for the existence of gapless quasiparticle
excitations in NJL models of color superconducting quark matter with a
quark-quark interaction in the scalar color-antitriplet channel. The condition
can be represented by a rotated ellipse in the plane of mass and chemical
potential differences for the paired quark fields.Comment: Accepted for publication in PRC. 5 pages, 4 figures; Corrected typos
and added one more term to the series expansion in (19
Accessibility of color superconducting quark matter phases in heavy-ion collisions
We discuss a hybrid equation of state (EoS) that fulfills constraints for
mass-radius relationships and cooling of compact stars. The quark matter EoS is
obtained from a Polyakov-loop Nambu--Jona-Lasinio (PNJL) model with color
superconductivity, and the hadronic one from a relativistic mean-field (RMF)
model with density-dependent couplings (DD-RMF). For the construction of the
phase transition regions we employ here for simplicity a Maxwell construction.
We present the phase diagram for symmetric matter which exhibits two remarkable
features: (1) a "nose"-like structure of the hadronic-to-quark matter phase
border with an increase of the critical density at temperatures below T ~ 150
MeV and (2) a high critical temperature for the border of the two-flavor color
superconducting (2SC) phase, T_c > 160 MeV. We show the trajectories of
heavy-ion collisions in the plane of excitation energy vs. baryon density
calculated using the UrQMD code and conjecture that for incident energies of 4
... 8 A GeV as provided, e.g., by the Nuclotron-M at JINR Dubna or by lowest
energies at the future heavy-ion collision experiments CBM@FAIR and NICA@JINR,
the color superconducting quark matter phase becomes accessible.Comment: 5 pages, 1 figure, Poster presented at the XXVI. Max Born Symposium
"Three Days of Strong Interactions", Wroclaw (Poland), July 9-11, 200
Subsidies in Oligopoly Markets: A Welfare Comparison Between Symmetric and Asymmetric Costs
Oligopolistic industries generally produce less than is socially desirable. As a result, the use of production subsidies is often suggested as a means of raising production toward the efficient level in imperfectly competitive markets. In cases where firms are equally efficient in producing the good, the common prescription is a policy of uniform subsidization, as suggested by Besley (1989).(1) When firms in an industry differ in cost efficiency, however, uniform subsidization involves subsidizing inefficient firms in the same manner as efficient firms. Consequently, uniform policy may be undesirable from a social perspective, particularly when the price-cost margins of inefficient firms are small. In an oligopolistic industry comprising firms that differ in cost effectiveness, one might ideally like to subsidize only the most efficient firm(s) and perhaps tax or even exclude inefficient firms from the market. Yet, in many situations, treating rival firms in an industry differently is politically infeasible. It is therefore important to understand how the welfare implications of uniform policy in an asymmetric-cost industry diverge from the case of equal cost efficiency. Different welfare implications are likely to arise because changes in regulatory structure can affect market structure in an asymmetric-cost industry (see Dierickx, Matutes, and Neven 1988; Kimmel 1992). Several papers in the public finance literature have addressed the issue of tax incidence on the rivalry and profitability of firms in oligopoly markets. Katz and Rosen (1985) show, in a conjectural variations model with symmetric firms, that a uniform tax on production can lead to an outcome with larger after-tax profits for firms. This result is also supported by Dierickx, Matutes, and Neven (1988); Kimmel (1992); and Seade (1985) for the case when the cost efficiency of firms differs. However, these papers do not directly analyze the industry profit and social welfare effects of a change in the tax or subsidy program. This article identifies relevant implications for tax policy by comparing welfare changes in the asymmetric-cost case to a benchmark case of symmetric costs. It is shown that, relative to the symmetric-cost case, the welfare effect is smaller when demand is nonconvex in the asymmetric-cost case, while the opposite is true for the case of convex demand. The greater the cost asymmetry in the industry and the more collusive firm behavior, the greater is this difference in welfare impact
1-2-3-flavor color superconductivity in compact stars
We suggest a scenario where the three light quark flavors are sequentially
deconfined under increasing pressure in cold asymmetric nuclear matter, e.g.,
as in neutron stars. The basis for our analysis is a chiral quark matter model
of Nambu--Jona-Lasinio (NJL) type with diquark pairing in the spin-1 single
flavor (CSL) and spin-0 two/three flavor (2SC/CFL) channels, and a
Dirac-Brueckner Hartree-Fock (DBHF) approach in the nuclear matter sector. We
find that nucleon dissociation sets in at about the saturation density, n_0,
when the down-quark Fermi sea is populated (d-quark dripline) due to the flavor
asymmetry imposed by beta-equilibrium and charge neutrality. At about 3n_0
u-quarks appear forming a two-flavor color superconducting (2SC) phase, while
the s-quark Fermi sea is populated only at still higher baryon density. The
hybrid star sequence has a maximum mass of 2.1 M_sun. Two- and three-flavor
quark matter phases are found only in gravitationally unstable hybrid star
solutions.Comment: 4 pages, 2 figures, to appear in the proceedings of Quark Matter
2008: 20th International Conference on Ultra-Relativistic Nucleus Nucleus
Collisions (QM 2008), Jaipur, India, 4-10 Feb 200
Color superconducting quark matter in compact stars
Recent indications for high neutron star masses (M \sim 2 M_sun) and large
radii (R > 12 km) could rule out soft equations of state and have provoked a
debate whether the occurence of quark matter in compact stars can be excluded
as well. We show that modern quantum field theoretical approaches to quark
matter including color superconductivity and a vector meanfield allow a
microscopic description of hybrid stars which fulfill the new, strong
constraints. For these objects color superconductivity turns out to be an
essential ingredient for a successful description of the cooling phenomenology
in accordance with recently developed tests. We discuss the energy release in
the neutrino untrapping transition as a new aspect of the problem that hybrid
stars masquerade themselves as neutron stars. Quark matter searches in future
generations of low-temperature/high-density nucleus-nucleus collision
experiments such as low-energy RHIC and CBM @ FAIR might face the same problem
of an almost crossover behavior of the deconfinement transition. Therefore,
diagnostic tools shall be derived from effects of color superconductivity.Comment: 8 pages, 3 figures, To appear in the proceedings of EXOCT 2007:
International Symposium on Exotic States of Nuclear Matter, Catania, Italy,
11-15 Jun 200
Compact stars in the standard model - and beyond
In the context of the standard model of particle physics, there is a definite
upper limit to the density of stable compact stars. However, if there is a
deeper layer of constituents, below that of quarks and leptons, stability may
be re-established far beyond this limiting density and a new class of compact
stars could exist. These objects would cause gravitational lensing of white
dwarfs and gamma-ray bursts, which might be observable as a diffraction pattern
in the spectrum. Such observations could provide means for obtaining new clues
about the fundamental particles and the origin of cold dark matter.Comment: 16 pages, 6 figures, contribution to the 42nd course of the
international school of subnuclear physics, 'How and where to go beyond the
standard model', Erice, Aug. 29 - Sep. 7, 200
Mass Loss Evolution and the Formation of Detached Shells around TP-AGB Stars
The origin of the so called 'detached shells' around AGB stars is not fully
understood, but two common hypotheses state that these shells form either
through the interaction of distinct wind phases or an eruptive mass loss
associated with a He-shell flash. We present a model of the formation of
detached shells around thermal pulse asymptotic giant branch (TP-AGB) stars,
based on detailed modelling of mass loss and stellar evolution, leading to a
combination of eruptive mass loss and wind interaction.
The purpose of this paper is first of all to connect stellar evolution with
wind and mass loss evolution and demonstrate its consistency with observations,
but also to show how thin detached shells around TP-AGB stars can be formed.
Previous attempts to link mass loss evolution with the formation of detached
shells were based on approximate prescriptions for the mass loss and have not
included detailed modelling of the wind formation as we do here. (abridged)Comment: 16 pages, 15 figures. Accepted for publication in Astronomy &
Astrophysic
Neutron Stars and the High Density Equation of State
One of the key ingredients to understand the properties of neutrons stars is
the equation of state at finite densities far beyond nuclear saturation.
Investigating the phase structure of quark matter that might be realized in the
core of NS inspires theory and observation. We discuss recent results of our
work to point out our view on challenges and possibilities in this evolving
field by means of a few examples.Comment: 10 pages, 6 figures, Proc. 5th ANL/MSU/JINA/INT FRIB Workshop on Bulk
Nuclear Properties, E. Lansing, Nov. 19-22, 200
WISER Deliverable D3.3-2: The importance of invertebrate spatial and temporal variation for ecological status classification for European lakes
European lakes are affected by many human induced disturbances. In principle, ecological
theories predict that the structure and functioning of benthic invertebrate assemblage (one of
the Biological Quality Elements following the Water Framework Directive, WFD
terminology) change in response to the level of disturbances, making this biological element
suitable for assessing the status and management of lake ecosystems. In practice, to set up
assessment systems based on invertebrates, we need to distiguish community changes that are
related to human pressures from those that are inherent natural variability. This task is
complicated by the fact that invertebrate communities inhabiting the littoral and the profundal
zones of lakes are constrained by different factors and respond unevenly to distinct human
disturbances. For example it is not clear yet how the invertebrates assemblages respond to
watershed and shoreline alterations, nor the relative importance of spatial and temporal
factors on assemblage dynamics and relative bioindicator values of taxa, the habitat
constraints on species traits and other taxonomic and methodological limitations.
The current lack of knowledge of basic features of invertebrate temporal and spatial variations
is limiting the fulfillment of the EU-wide intercalibration of lake ecological quality
assessment systems in Europe, and thus compromising the basis for setting the environmental
objectives as required by the WFD. The aim of this deliverable is to provide a contribution
towards the understanding of basic sources of spatial and temporal variation of lake
invertebrate assemblages. The report is structured around selected case studies, manly
involving the analysis of existing datasets collated within WISER. The case studies come
from different European lake types in the Northern, Central, Alpine and Mediterranean
regions. All chapters have an obvious applied objective and our aim is to provide to those
dealing with WFD implementation at various levels useful information to consider when
designing monitoring programs and / or invertebrate-based classification systems
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