7,349 research outputs found
Aspects of a supersymmetric Brans-Dicke theory
We consider a locally supersymmetric theory where the Planck mass is replaced
by a dynamical superfield. This model can be thought of as the Minimal
Supersymmetric extension of the Brans-Dicke theory (MSBD). The motivation that
underlies this analysis is the research of possible connections between Dark
Energy models based on Brans-Dicke-like theories and supersymmetric Dark Matter
scenarios. We find that the phenomenology associated with the MSBD model is
very different compared to the one of the original Brans-Dicke theory: the
gravitational sector does not couple to the matter sector in a universal metric
way. This feature could make the minimal supersymmetric extension of the BD
idea phenomenologically inconsistent.Comment: 6 pages, one section is adde
Privacy-Aware Processing of Biometric Templates by Means of Secure Two-Party Computation
The use of biometric data for person identification and access control is gaining more and more popularity. Handling biometric data, however, requires particular care, since biometric data is indissolubly tied to the identity of the owner hence raising important security and privacy issues. This chapter focuses on the latter, presenting an innovative approach that, by relying on tools borrowed from Secure Two Party Computation (STPC) theory, permits to process the biometric data in encrypted form, thus eliminating any risk that private biometric information is leaked during an identification process. The basic concepts behind STPC are reviewed together with the basic cryptographic primitives needed to achieve privacy-aware processing of biometric data in a STPC context. The two main approaches proposed so far, namely homomorphic encryption and garbled circuits, are discussed and the way such techniques can be used to develop a full biometric matching protocol described. Some general guidelines to be used in the design of a privacy-aware biometric system are given, so as to allow the reader to choose the most appropriate tools depending on the application at hand
The Impact of Distance Uncertainties on Local Luminosity and Mass Functions
In order to investigate discrepancies between recent published estimates of
the the HI mass function (HIMF), we explore the impact of distance
uncertainties on the derivation of the faint end slope of mass and luminosity
functions of galaxies in the local volume by deriving HIMFs from mock HI
surveys. We consider various survey geometries and depths and compare the HIMFs
measured when using ``real'' distances, distances derived by assuming pure
Hubble flow and distances assigned from parametric models of the local velocity
field. The effect is variable and dependent on the exact survey geometry, but
can easily lead to incorrect estimates of the HIMF, particularly at the low
mass end. We show that at least part of the discrepancies among recent
derivations of the HIMF can be accounted for by the use of different methods to
assign distances. We conclude that a better understanding of the local velocity
field will be necessary for accurate determinations of the local galaxy
luminosity and mass functions.Comment: 4 pages, accepted to ApJ
Circum-galactic medium in the halo of quasars
The properties of circum-galactic gas in the halo of quasar host galaxies are
investigated analyzing Mg II 2800 and C IV 1540 absorption-line systems along
the line of sight close to quasars. We used optical spectroscopy of closely
aligned pairs of quasars (projected distance 200 kpc, but at very
different redshift) obtained at the VLT and Gran Telescopio Canarias to
investigate the distribution of the absorbing gas for a sample of quasars at
z1. Absorption systems of EW 0.3 associated with the
foreground quasars are revealed up to 200 kpc from the centre of the host
galaxy, showing that the structure of the absorbing gas is patchy with a
covering fraction quickly decreasing beyond 100 kpc. In this contribution we
use optical and near-IR images obtained at VLT to investigate the relations
between the properties of the circum-galactic medium of the host galaxies and
of the large scale galaxy environments of the foreground quasars.Comment: 6 pages, 3 figures, proceedings of the conference "QUASARS at all
cosmic epochs", accepted for publication on Frontiers in Astronomy and Space
Scienc
Revenue maximizing envy-free fixed-price auctions with budgets
Traditional incentive-compatible auctions [6,16] for selling multiple goods to unconstrained and budgeted bidders can discriminate between bidders by selling identical goods at different prices. For this reason, Feldman et al. [7] dropped incentive compatibility and turned the attention to revenue maximizing envy-free item-pricing allocations for budgeted bidders. Envy-free allocations were suggested by classical papers [9,15]. The key property of such allocations is that no one envies the allocation and the price charged to anyone else. In this paper we consider this classical notion of envy-freeness and study fixed-price mechanisms which use nondiscriminatory uniform prices for all goods. Feldman et al. [7] gave an item-pricing mechanism that obtains 1/2 of the revenue obtained from any envy-free fixed-price mechanism for identical goods. We improve over this result by presenting an FPTAS for the problem that returns an (1 − ε)-approximation of the revenue obtained by any envy-free fixed-price mechanism for any ε > 0 and runs in polynomial time in the number of bidders n and 1/ ε even for exponential supply of goods m. Next, we consider the case of budgeted bidders with matching-type preferences on the set of goods, i.e., the valuation of each bidder for each item is either v i or 0. In this more general case, we prove that it is impossible to approximate the optimum revenue within O( min (n,m)1/2 − ε ) for any ε > 0 unless P = NP. On the positive side, we are able to extend the FPTAS for identical goods to budgeted bidders in the case of constant number of different types of goods. Our FPTAS gives also a constant approximation with respect to the general envy-free auction
Is there a "too big to fail" problem in the field?
We use the Arecibo legacy fast ALFA (ALFALFA) 21cm survey to measure the
number density of galaxies as a function of their rotational velocity,
(as inferred from the width of their 21cm emission line).
Based on the measured velocity function we statistically connect galaxies with
their host halo, via abundance matching. In a lambda cold dark matter
(CDM) cosmology, dwarf galaxies are expected to be hosted by halos
that are significantly more massive than indicated by the measured galactic
velocity; if smaller halos were allowed to host galaxies, then ALFALFA would
measure a much higher galactic number density. We then seek observational
verification of this predicted trend by analyzing the kinematics of a
literature sample of gas-rich dwarf galaxies. We find that galaxies with
are
kinematically incompatible with their predicted CDM host halos, in the
sense that hosts are too massive to be accommodated within the measured
galactic rotation curves. This issue is analogous to the "too big to fail"
problem faced by the bright satellites of the Milky Way, but here it concerns
extreme dwarf galaxies in the field. Consequently, solutions based on
satellite-specific processes are not applicable in this context. Our result
confirms the findings of previous studies based on optical survey data and
addresses a number of observational systematics present in these works.
Furthermore, we point out the assumptions and uncertainties that could strongly
affect our conclusions. We show that the two most important among them -namely
baryonic effects on the abundances of halos and on the rotation curves of
halos- do not seem capable of resolving the reported discrepancy.Comment: v3 matches the version published in A&A. Main differences with v2 are
in Secs 3.2 & 4.4 and the addition of Appendix B. 11 figures, 14 pages (+2
appendices
Effects of Ionospheric Asymmetry on Electron Density Standard Inversion Algorithm Applicable to Radio Occultation (RO) Data Using Best-suited Ionospheric Model
The "Onion-peeling" algorithm is a very common technique used to invert Radio Occultation (RO) data in the ionosphere. Because of the implicit assumption of spherical symmetry for the electron density (Ne) distribution in the ionosphere, the standard Onion-peeling algorithm could give erroneous concentration values in the retrieved electron density vertical profile Ne(h). In particular, this happens when strong horizontal ionospheric electron density gradients are present, like for example in the Equatorial Ionization Anomaly (EIA) region during high solar activity periods. Using simulated RO Total Electron Content (TEC) data computed by means of the best-suited ionospheric model and ideal RO geometries, we evaluated the asymmetry level index for quasi-horizontal TEC observations. This asymmetry index is based on the Ne variations that a signal may experience along its ray-path (satellite to satellite link) during a RO event. The index is strictly dependent on RO geometry and azimuth of the occultation plane and is able to provide us indication of the errors (in particular those concerning the peak electron density NmF2 and the vertical TEC) expected in the retrieval of Ne(h) using standard Onion-peeling algorithm. On the basis of the outcomes of our work, and using best-suited ionospheric model, we will try to investigate the possibility to predict the ionospheric asymmetry expected for the particular RO geometry considered. We could also try to evaluate, in advance, its impact on the inverted electron density profile, providing an indication of the product qualit
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