17,060 research outputs found
Neutralino with the Right Cold Dark Matter Abundance in (Almost) Any Supersymmetric Model
We consider non-standard cosmological models in which the late decay of a
scalar field reheats the Universe to a low reheating temperature,
between 5 MeV and the standard freeze-out temperature of neutralinos of mass
. We point out that in these models all neutralinos with standard
density can have the
density of cold dark matter, provided the right combination of the following
two parameters can be achieved in the high energy theory: the reheating
temperature, and the ratio of the number of neutralinos produced per
decay over the field mass. We present the ranges of these parameters
where a combination of thermal and non-thermal neutralino production leads to
the desired density, as functions of and .Comment: 4 pages, 1 figur
Turbulence-Induced Relative Velocity Of Dust Particles. III. The Probability Distribution
Motivated by its important role in the collisional growth of dust particles in protoplanetary disks, we investigate the probability distribution function (PDF) of the relative velocity of inertial particles suspended in turbulent flows. Using the simulation from our previous work, we compute the relative velocity PDF as a function of the friction timescales, tau(p1) and tau(p2), of two particles of arbitrary sizes. The friction time of the particles included in the simulation ranges from 0.1 tau(eta) to 54T(L), where tau(eta) and T-L are the Kolmogorov time and the Lagrangian correlation time of the flow, respectively. The relative velocity PDF is generically non-Gaussian, exhibiting fat tails. For a fixed value of tau(p1), the PDF shape is the fattest for equal-size particles (tau(p2) = tau(p1)), and becomes thinner at both tau(p2) tau(p1). Defining f as the friction time ratio of the smaller particle to the larger one, we find that, at a given f in (1/2) less than or similar to f less than or similar to 1, the PDF fatness first increases with the friction time tau(p,h) of the larger particle, peaks at tau(p,h) similar or equal to tau(eta), and then decreases as tp, h increases further. For 0 > T-L). These features are successfully explained by the Pan & Padoan model. Using our simulation data and some simplifying assumptions, we estimated the fractions of collisions resulting in sticking, bouncing, and fragmentation as a function of the dust size in protoplanetary disks, and argued that accounting for non-Gaussianity of the collision velocity may help further alleviate the bouncing barrier problem.Astronom
The Hatsopoulos-Gyftopoulos resolution of the Schroedinger-Park paradox about the concept of "state" in quantum statistical mechanics
A seldom recognized fundamental difficulty undermines the concept of
individual ``state'' in the present formulations of quantum statistical
mechanics (and in its quantum information theory interpretation as well). The
difficulty is an unavoidable consequence of an almost forgotten corollary
proved by E. Schroedinger in 1936 and perused by J.L. Park, Am. J. Phys., Vol.
36, 211 (1968). To resolve it, we must either reject as unsound the concept of
state, or else undertake a serious reformulation of quantum theory and the role
of statistics. We restate the difficulty and discuss a possible resolution
proposed in 1976 by G.N. Hatsopoulos and E.P. Gyftopoulos, Found. Phys., Vol.
6, 15, 127, 439, 561 (1976).Comment: RevTeX4, 7 pages, corrected a paragraph and added an example at page
3, to appear in Mod. Phys. Lett.
Aberration features in directional dark matter detection
The motion of the Earth around the Sun causes an annual change in the
magnitude and direction of the arrival velocity of dark matter particles on
Earth, in a way analogous to aberration of stellar light. In directional
detectors, aberration of weakly interacting massive particles (WIMPs) modulates
the pattern of nuclear recoil directions in a way that depends on the orbital
velocity of the Earth and the local galactic distribution of WIMP velocities.
Knowing the former, WIMP aberration can give information on the latter, besides
being a curious way of confirming the revolution of the Earth and the
extraterrestrial provenance of WIMPs. While observing the full aberration
pattern requires extremely large exposures, we claim that the annual variation
of the mean recoil direction or of the event counts over specific solid angles
may be detectable with moderately large exposures. For example, integrated
counts over Galactic hemispheres separated by planes perpendicular to Earth's
orbit would modulate annually, resulting in Galactic Hemisphere Annual
Modulations (GHAM) with amplitudes larger than the usual non-directional annual
modulation.Comment: 24 pages, 46 figures, addition of new section 7 on anisotropic models
and Fig. 1
Seismic Vulnerability of the Italian Roadway Bridge Stock
This study focuses on the seismic vulnerability evaluation of the Italian roadway bridge stock, within the framework of a Civil Protection sponsored project. A comprehensive database of existing bridges (17,000 bridges with different level of knowledge) was implemented. At the core of the study stands a procedure for automatically carrying out state-of-the-art analytical evaluation of fragility curves for two performance levels – damage and collapse – on an individual bridge basis. A webGIS was developed to handle data and results. The main outputs are maps of bridge seismic risk (from the fragilities and the hazard maps) at the national level and real-time scenario damage-probability maps (from the fragilities and the scenario shake maps). In the latter case the webGIS also performs network analysis to identify routes to be followed by rescue teams. Consistency of the fragility derivation over the entire bridge stock is regarded as a major advantage of the adopted approach
Kosmotropes and chaotropes: modelling preferential exclusion, binding and aggregate stability
Kosmotropic cosolvents added to an aqueous solution promote the aggregation
of hydrophobic solute particles, while chaotropic cosolvents act to destabilise
such aggregates. We discuss the mechanism for these phenomena within an adapted
version of the two-state Muller-Lee-Graziano model for water, which provides a
complete description of the ternary water/cosolvent/solute system for small
solute particles. This model contains the dominant effect of a kosmotropic
substance, which is to enhance the formation of water structure. The consequent
preferential exclusion both of cosolvent molecules from the solvation shell of
hydrophobic particles and of these particles from the solution leads to a
stabilisation of aggregates. By contrast, chaotropic substances disrupt the
formation of water structure, are themselves preferentially excluded from the
solution, and thereby contribute to solvation of hydrophobic particles. We use
Monte Carlo simulations to demonstrate at the molecular level the preferential
exclusion or binding of cosolvent molecules in the solvation shell of
hydrophobic particles, and the consequent enhancement or suppression of
aggregate formation. We illustrate the influence of structure-changing
cosolvents on effective hydrophobic interactions by modelling qualitatively the
kosmotropic effect of sodium chloride and the chaotropic effect of urea.Comment: 13 pages, 12 figures; inclusion of review material, parameter
analysis and comparison of kosmotropic and chaotropic effect
Astrophysical signatures of boson stars: quasinormal modes and inspiral resonances
Compact bosonic field configurations, or boson stars, are promising dark
matter candidates which have been invoked as an alternative description for the
supermassive compact objects in active galactic nuclei. Boson stars can be
comparable in size and mass to supermassive black holes and they are hard to
distinguish by electromagnetic observations. However, boson stars do not
possess an event horizon and their global spacetime structure is different from
that of a black hole. This leaves a characteristic imprint in the
gravitational-wave emission, which can be used as a discriminant between black
holes and other horizonless compact objects. Here we perform a detailed study
of boson stars and their gravitational-wave signatures in a fully relativistic
setting, a study which was lacking in the existing literature in many respects.
We construct several fully relativistic boson star configurations, and we
analyze their geodesic structure and free oscillation spectra, or quasinormal
modes. We explore the gravitational and scalar response of boson star
spacetimes to an inspiralling stellar-mass object and compare it to its black
hole counterpart. We find that a generic signature of compact boson stars is
the resonant-mode excitation by a small compact object on stable quasi-circular
geodesic motion.Comment: 20 pages, 8 figures. v2: minor corrections, version to be published
in Phys. Rev. D. v3: final versio
The dynamics of Italian public debt: Alternative paths for fiscal consolidation
This paper analyses possible targets for the Italian debt-to-GDP ratio with a small macroeconomic model. The role of international macroeconomic variables such as the US GDP growth, prices of raw materials, EUR/USD exchange rate, and ECB monetary policy stance and domestic policy instruments is analyzed in the debt dynamics. We find that external conditions play a fundamental role for the Italian fiscal consolidation. To reach a target of 100% of debt-to-GDP ratio by 2020, a further growth sustaining policy has to be implemented.Debt to GDP Ratio, Italian Economy, International Factors, SUR.
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