602 research outputs found
Modified Gravitational Waves Across Galaxies from Macroscopic Gravity
We analyze the propagation of gravitational waves in a medium containing
bounded subsystems ("molecules"), able to induce significant Macroscopic
Gravity effects. We establish a precise constitutive relation between the
average quadrupole and the amplitudes of a vacuum gravitational wave, via the
geodesic deviation equation. Then we determine the modified equation for the
wave inside the medium and the associated dispersion relation. A
phenomenological analysis shows that anomalous polarizations of the wave emerge
with an appreciable experimental detectability if the medium is identified with
a typical galaxy. Both the modified dispersion relation (wave velocity less
than the speed of light) and anomalous oscillations modes could be detectable
by the incoming LISA or pulsar timing arrays experiments, having the
appropriate size to see the concerned wavelengths (larger than the molecular
size) and the appropriate sensitivity to detect the expected deviation from
vacuum General Relativity.Comment: 10 pages, comments are welcom
Optimally chosen small portfolios are better than large ones
One of the fundamental principles in portfolio selection models is minimization of risk through diversification of the investment. However, this principle does not necessarily translate into a request for investing in all the assets of the investment universe. Indeed, following a line of research started by Evans and Archer almost fifty years ago, we provide here further evidence that small portfolios are sufficient to achieve almost optimal in-sample risk reduction with respect to variance and to some other popular risk measures, and very good out-of-sample performances. While leading to similar results, our approach is significantly different from the classical one pioneered by Evans and Archer. Indeed, we describe models for choosing the portfolio of a prescribed size with the smallest possible risk, as opposed to the random portfolio choice investigated in most of the previous works. We find that the smallest risk portfolios generally require no more than 15 assets. Furthermore, it is almost always possible to find portfolios that are just 1% more risky than the smallest risk portfolios and contain no more than 10 assets. Furthermore, the optimal small portfolios generally show a better performance than the optimal large ones. Our empirical analysis is based on some new and on some publicly available benchmark data sets often used in the literature
Low Complexity WMMSE Power Allocation In NOMA-FD Systems
In this paper we study the problem of power and channel allocation with the
objective of maximizing the system sum-rate for multicarrier non-orthogonal
multiple access (NOMA) full duplex (FD) systems. Such an allocation problem is
non-convex and, thus, with the goal of designing a low complexity solution, we
propose a scheme based on the minimization of the weighted mean square error,
which achieves performance reasonably close to the optimum and allows to
clearly outperforms a conventional orthogonal multiple access approach.
Numerical results assess the effectiveness of our algorithm.Comment: 5 pages conference paper, 3 figures. Submitted on ICASSP 202
NLO matching for ttbb production with massive b-quarks
Theoretical uncertainties in the simulation of ttbb production represent one
of the main obstacles that still hamper the observation of Higgs-boson
production in association with top-quark pairs in the H->bb channel. In this
letter we present a next-to-leading order (NLO) simulation of ttbb production
with massive b-quarks matched to the Sherpa parton shower. This allows one to
extend NLO predictions to arbitrary ttbb kinematics, including the case where
one or both b-jets arise from collinear g->bb splittings. We find that this
splitting mechanism plays an important role for the ttH(bb) analysis.Comment: 4 pages, 3 figures. v2: b-jet momenta replaced by b-quark momenta in
the dynamical QCD scale; related technical cut removed; numerical results
updated accordingly and conclusions unchanged. References and various
comments on the resummation of b-mass logarithms and on the consistency of
double-splitting contributions added. Version to appear in Phys. Lett
Higgs Boson Production in Association with a Photon in Vector Boson Fusion at the LHC
Higgs boson production in association with two forward jets and a central
photon at the CERN Large Hadron Collider is analyzed, for the Higgs boson
decaying into a b bbar pair in the m_H <= 140 GeV mass region. We study both
irreducible and main reducible backgrounds at parton level. Compared to the
Higgs production via vector-boson fusion, the request of a further photon at
moderate rapidities dramatically enhances the signal/background ratio.
Inclusive cross sections for p_T^\gamma >= 20 GeV can reach a few tens of fb's.
After a suitable choice of kinematical cuts, the cross-section ratio for signal
and irreducible-background can be enhanced up to >= ~1/10, with a signal cross
section of the order of a few fb's, for m_H ~ 120 GeV. The request of a central
photon radiation also enhances the relative signal sensitivity to the WWH
coupling with respect to the ZZH coupling. Hence, a determination of the cross
section for the associated production of a Higgs boson decaying into a b bbar
pair plus a central photon in vector-boson fusion could help in constraining
the b bbar H coupling, and the WWH coupling as well. A preliminary study of QCD
showering effects points to a further significant improvement of the signal
detectability over the background.Comment: 30 pages, 8 figures, 8 tables; minor corrections to the text; version
appeared in Nuclear Physics
Gravitational Landau Damping for massive scalar modes
We establish the possibility of Landau damping for gravitational scalar waves
which propagate in a non-collisional gas of particles. In particular, under the
hypothesis of homogeneity and isotropy, we describe the medium at the
equilibrium with a J\"uttner-Maxwell distribution, and we analytically
determine the damping rate from the Vlasov equation. We find that damping
occurs only if the phase velocity of the wave is subluminal throughout the
propagation within the medium. Finally, we investigate relativistic media in
cosmological settings by adopting numerical techniques.Comment: 11 pages, 4 figures. Comments are welcom
Operating times and users' behavior at urban road intersections
The safety of at grade road intersections is a relevant issue with social, economic, and environmental implications. It is related to the behavior of a driver approaching an intersection that, in its turn, is affected by kinematic and physiological variables. This study proposes a model to calculate the intersection operation time (IOT) for typical non-signalized 4-leg and 3-leg (or T-leg) urban intersections. Data available in the literature have been considered in order to identify the points of interest and assess the number and the time of a driver's eye fixation on them. When approaching an intersection, the probability of glancing in a particular area changes with the distance to the yield or stop line; for this reason, a probabilistic approach was used to model the phenomenon. All possible maneuvers have been considered: left turning, right turning, and through-movement. The proposed model allowed an objective comparison between time spent by drivers for various maneuvers and layout conditions, and identification of the critical conditions. Indeed, significant differences in terms of IOT were found: they could lead to modification of the traffic management considering different needs of road users, traffic demand, and geometrical and functional constraints
Contribution to the assessment of CFD codes for in-vessel flow investigation
The present research aims at contributing to the CFD code assessment process for
nuclear reactor applications, and particularly for the predictive analysis of the fluid dynamic
phenomena occurring inside the reactor pressure vessel of a pressurized
water reactor. The importance of such phenomena relies, for instance, on the
influence that they can have on the spatial and temporal distribution of coolant
properties (such as temperature or boron concentration) at the core inlet during
certain accident transients involving perturbations of such properties with respect to
nominal conditions; furthermore, in-vessel mixing phenomena can also affect the
thermal interaction between coolant and pressure vessel during pressurized
thermal shock scenarios.
The contribution provided by this work consists in the proposal of a general and
systematic methodology to be applied in the CFD code assessment for in-vessel
flow investigations. Within the proposed approach, the relevant modelling issues
are identified and discussed, so as to point out the main capabilities and limitations
in the present state-of-the-art tools and methods. Then, the main steps of the code
application procedure are described and discussed analytically, thus providing
guidance for a quality-oriented use of the codes, and complementing the existing
best practice guidelines for this specific problem.
Furthermore, the research addresses the problem of the quantification of the
accuracy for numerical predictions (both from CFD and integral codes) about
coolant properties perturbations at the core inlet. As a result, a methodology is
proposed based on a set of accuracy indicators, which can represent a means for
judging whether the code results are sufficiently close to experimental data, once
acceptance thresholds have been defined and the method has been completely
assessed.
The work is supported by extensive CFD code validation and application results
obtained in the frame of several international research projects and co-operations,
and by a continuous interaction with the involved International scientific community
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