7,808 research outputs found
A 3+1 covariant suite of Numerical Relativity Evolution Systems
A suite of three evolution systems is presented in the framework of the 3+1
formalism. The first one is of second order in space derivatives and has the
same causal structure of the Baumgarte-Shapiro-Shibata-Nakamura (BSSN) system
for a suitable choice of parameters. The second one is the standard first order
version of the first one and has the same causal structure of the Bona-Masso
system for a given parameter choice. The third one is obtained from the second
one by reducing the space of variables in such a way that the only modes that
propagate with zero characteristic speed are the trivial ones. This last system
has the same structure of the ones recently presented by Kidder, Scheel and
Teukolski: the correspondence between both sets of parameters is explicitly
given. The fact that the suite started with a system in which all the dynamical
variables behave as tensors (contrary to what happens with BSSN system) allows
one to keep the same parametrization when passing from one system to the next
in the suite. The direct relationship between each parameter and a particular
characteristic speed, which is quite evident in the second and the third
systems, is a direct consequence of the manifest 3+1 covariance of the
approach
Internal stresses and breakup of rigid isostatic aggregates in homogeneous and isotropic turbulence
By characterising the hydrodynamic stresses generated by statistically
homogeneous and isotropic turbulence in rigid aggregates, we estimate
theoretically the rate of turbulent breakup of colloidal aggregates and the
size distribution of the formed fragments. The adopted method combines Direct
Numerical Simulation of the turbulent field with a Discrete Element Method
based on Stokesian dynamics. In this way, not only the mechanics of the
aggregate is modelled in detail, but the internal stresses are evaluated while
the aggregate is moving in the turbulent flow. We examine doublets and
cluster-cluster isostatic aggregates, where the failure of a single contact
leads to the rupture of the aggregate and breakup occurs when the tensile force
at a contact exceeds the cohesive strength of the bond. Due to the different
role of the internal stresses, the functional relationship between breakup
frequency and turbulence dissipation rate is very different in the two cases.
In the limit of very small and very large values, the frequency of breakup
scales exponentially with the turbulence dissipation rate for doublets, while
it follows a power law for cluster-cluster aggregates. For the case of large
isostatic aggregates it is confirmed that the proper scaling length for maximum
stress and breakup is the radius of gyration. The cumulative fragment
distribution function is nearly independent of the mean turbulence dissipation
rate and can be approximated by the sum of a small erosive component and a term
that is quadratic with respect to fragment size.Comment: 31 pages, 19 figure
Upper bounds on SUSY contributions to b to s transitions from B_s - B_sbar mixing
We study the constraints on supersymmetric contributions to b to s
transitions from the recent allowed range and measurement of Bs-Bsbar mixing
obtained by the D0 and CDF collaborations at the Tevatron. We compute the upper
bounds on the relevant off-diagonal squark mass terms and compare them with the
bounds coming from Delta F=1 decays. We find that the constraints on
chirality-flipping mass insertions are unaffected. Conversely, the measurement
of Bs-Bsbar mixing is effective in constraining chirality-conserving mass
insertions, and it has striking effects in the case in which left- and
right-handed insertions have similar size. Finally, we discuss the phase of the
Bs-Bsbar mixing amplitude in the presence of SUSY contributions.Comment: 4 pages, 4 figures; v2: CDF measurement include
Physics at SuperB
Flavour will play a crucial role in understanding physics beyond the Standard
Model. Progress in developing a future programme to investigate this central
area of particle physics has recently passed a milestone, with the completion
of the conceptual design report for SuperB, a very high luminosity, asymmetric
e+e- collider. This article summarizes the important role of SuperB in
understanding new physics in the LHC era.Comment: 4 pages, 2 figures. To appear in the proceedings of the International
Europhysics Conference on High Energy Physics (EPS-HEP2007), Manchester,
England, 19-25 July 200
Not all visual symmetry is equal: partially distinct neural bases for vertical and horizontal symmetry
Visual mirror symmetry plays an important role in visual perception in both human and animal vision; its importance is reflected in the fact that it can be extracted automatically during early stages of visual processing. However, how this extraction is implemented at the cortical level remains an open question. Given the importance of symmetry in visual perception, one possibility is that there is a network which extracts all types of symmetry irrespective of axis of orientation; alternatively, symmetry along different axes might be encoded by different brain regions, implying that that there is no single neural mechanism for symmetry processing. Here we used fMRI-guided transcranial magnetic stimulation (TMS) to compare the neural basis of the two main types of symmetry found in the natural world, vertical and horizontal symmetry. TMS was applied over either right Lateral Occipital Cortex (LO), right Occipital Face Area (OFA) or Vertex while participants were asked to detect symmetry in low-level dot configurations. Whereas detection of vertical symmetry was impaired by TMS over both LO and OFA, detection of horizontal symmetry was delayed by stimulation of LO only. Thus, different types of visual symmetry rely on partially distinct cortical networks
TMS over right OFA affects individuation of faces but not of exemplars of objects
In addition to its well-documented role in processing of faces, the occipital face area in the right hemisphere (rOFA) may also play a role in identifying specific individuals within a class of objects. Here we explored this issue by using fMRI-guided TMS. In a first experiment, participants had to judge whether two sequentially presented images of faces or objects represented exactly the same exemplar or two different exemplars of the same class, while receiving online TMS over either the rOFA, the right lateral occipital cortex (rLO) or the Vertex (control). We found that, relative to Vertex, stimulation of rOFA impaired individuation of faces only, with no effect on objects; in contrast, TMS over rLO reduced individuation of objects but not of faces. In a second control experiment participants judged whether a picture representing a fragment of a stimulus belonged or not to the subsequently presented image of a whole stimulus (part-whole matching task). Our results showed that rOFA stimulation selectively disrupted performance with faces, whereas performance with objects (but not with faces) was selectively affected by TMS over rLO. Overall, our findings suggest that rOFA does not contribute to discriminate between exemplars of non-face objects
Hyperbolicity of the Kidder-Scheel-Teukolsky formulation of Einstein's equations coupled to a modified Bona-Masso slicing condition
We show that the Kidder-Scheel-Teukolsky family of hyperbolic formulations of
the 3+1 evolution equations of general relativity remains hyperbolic when
coupled to a recently proposed modified version of the Bona-Masso slicing
condition.Comment: 4 pages. Several changes. Main corrections are in eqs. 4.9 and 4.1
Gambaran Pengetahuan, Sikap, dan Praktik Perilaku Mahasiswa terhadap HIV/AIDS dan ODHA (Orang dengan HIV/AIDS)
Undergraduate students in adolescence and young adulthood period are sexually productive. It is crucial to discover their knowledge, attitude, and practice (KAP) towards HIV/AIDS since the highest HIV infection in Indonesia until 2016 is between those ages. The survey was conducted in Indonesia between November-December 2015. Data was collected using online structured KAP questionnaire to 350 undergraduate students (aged 18-23 years old) in Universitas Indonesia and analyzed with descriptive statistic. The KAP questionnaire used was adapted from Mulu, Abera, and Yinner (2014). Results shown that most participants answered correctly in knowledge items, but most of them still didn't know that HIV and AIDS are two different things, and they think that HIV can be infected through sharing foods, exchanging clothes and toilets with PLWH (People Living with HIV). Most participants have a positive attitude towards HIV/AIDS, but they have a negative attitude regarding on using condom while having sex as an affront to the couple, and disagreeing to live together with PLWH. Regarding on the practice, as much as 76 participants who have had sex ever had a risky sexual behavior but never had VCT (Voluntary Counseling & Testing), while the rest of participants who never had sex mostly have seen a condom but didn't feel comfortable to hold it. Knowledge towards HIV/AIDS was quite sufficient, the attitude was fair enough, but the practice for the participants who have had sex needed attention more. The results also might indicate stigma towards HIV/AIDS in Indonesia still exists because HIV is considered as an easily infected disease
Exploiting gauge and constraint freedom in hyperbolic formulations of Einstein's equations
We present new many-parameter families of strongly and symmetric hyperbolic
formulations of Einstein's equations that include quite general algebraic and
live gauge conditions for the lapse. The first system that we present has 30
variables and incorporates an algebraic relationship between the lapse and the
determinant of the three metric that generalizes the densitized lapse
prescription. The second system has 34 variables and uses a family of live
gauges that generalizes the Bona-Masso slicing conditions. These systems have
free parameters even after imposing hyperbolicity and are expected to be useful
in 3D numerical evolutions. We discuss under what conditions there are no
superluminal characteristic speeds
First order hyperbolic formalism for Numerical Relativity
The causal structure of Einstein's evolution equations is considered. We show
that in general they can be written as a first order system of balance laws for
any choice of slicing or shift. We also show how certain terms in the evolution
equations, that can lead to numerical inaccuracies, can be eliminated by using
the Hamiltonian constraint. Furthermore, we show that the entire system is
hyperbolic when the time coordinate is chosen in an invariant algebraic way,
and for any fixed choice of the shift. This is achieved by using the momentum
constraints in such as way that no additional space or time derivatives of the
equations need to be computed. The slicings that allow hyperbolicity in this
formulation belong to a large class, including harmonic, maximal, and many
others that have been commonly used in numerical relativity. We provide details
of some of the advanced numerical methods that this formulation of the
equations allows, and we also discuss certain advantages that a hyperbolic
formulation provides when treating boundary conditions.Comment: To appear in Phys. Rev.
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