6,077 research outputs found
Thermal hadron production in high energy collisions
It is shown that hadron abundances in high energy e+e-, pp and p{\bar p}
collisions, calculated by assuming that particles originate in hadron gas
fireballs at thermal and partial chemical equilibrium, are in very good
agreement with the data. The freeze-out temperature of the hadron gas fireballs
turns out to be nearly constant over a large center of mass energy range and
not dependent on the initial colliding system. The only deviation from chemical
equilibrium resides in the incomplete strangeness phase space saturation.
Preliminary results of an analysis of hadron abundances in S+S and S+Ag heavy
ion collisions are presented.Comment: 10 pages, 1 .eps figure, talk given at the Strangeness and Quark
Matter 97 conferenc
Relativistic Positioning Systems: The Emission Coordinates
This paper introduces some general properties of the gravitational metric and
the natural basis of vectors and covectors in 4-dimensional emission
coordinates. Emission coordinates are a class of space-time coordinates defined
and generated by 4 emitters (satellites) broadcasting their proper time by
means of electromagnetic signals. They are a constitutive ingredient of the
simplest conceivable relativistic positioning systems. Their study is aimed to
develop a theory of these positioning systems, based on the framework and
concepts of general relativity, as opposed to introducing `relativistic
effects' in a classical framework. In particular, we characterize the causal
character of the coordinate vectors, covectors and 2-planes, which are of an
unusual type. We obtain the inequality conditions for the contravariant metric
to be Lorentzian, and the non-trivial and unexpected identities satisfied by
the angles formed by each pair of natural vectors. We also prove that the
metric can be naturally split in such a way that there appear 2 parameters
(scalar functions) dependent exclusively on the trajectory of the emitters,
hence independent of the time broadcast, and 4 parameters, one for each
emitter, scaling linearly with the time broadcast by the corresponding
satellite, hence independent of the others.Comment: 13 pages, 3 figures. Only format changed for a new submission.
Submitted to Class. Quantum Gra
Two-dimensional approach to relativistic positioning systems
A relativistic positioning system is a physical realization of a coordinate
system consisting in four clocks in arbitrary motion broadcasting their proper
times. The basic elements of the relativistic positioning systems are presented
in the two-dimensional case. This simplified approach allows to explain and to
analyze the properties and interest of these new systems. The positioning
system defined by geodesic emitters in flat metric is developed in detail. The
information that the data generated by a relativistic positioning system give
on the space-time metric interval is analyzed, and the interest of these
results in gravimetry is pointed out.Comment: 11 pages, 5 figures. v2: a brief description of the principal
bibliography has been adde
Relativistic positioning: four-dimensional numerical approach in Minkowski space-time
We simulate the satellite constellations of two Global Navigation Satellite
Systems: Galileo (EU) and GPS (USA). Satellite motions are described in the
Schwarzschild space-time produced by an idealized spherically symmetric non
rotating Earth. The trajectories are then circumferences centered at the same
point as Earth. Photon motions are described in Minkowski space-time, where
there is a well known relation, Coll, Ferrando & Morales-Lladosa (2010),
between the emission and inertial coordinates of any event. Here, this relation
is implemented in a numerical code, which is tested and applied. The first
application is a detailed numerical four-dimensional analysis of the so-called
emission coordinate region and co-region. In a second application, a GPS
(Galileo) satellite is considered as the receiver and its emission coordinates
are given by four Galileo (GPS) satellites. The bifurcation problem (double
localization) in the positioning of the receiver satellite is then pointed out
and discussed in detail.Comment: 16 pages, 9 figures, published (online) in Astrophys. Space Sc
Positioning systems in Minkowski space-time: from emission to inertial coordinates
The coordinate transformation between emission coordinates and inertial
coordinates in Minkowski space-time is obtained for arbitrary configurations of
the emitters. It appears that a positioning system always generates two
different coordinate domains, namely, the front and the back emission
coordinate domains. For both domains, the corresponding covariant expression of
the transformation is explicitly given in terms of the emitter world-lines.
This task requires the notion of orientation of an emitter configuration. The
orientation is shown to be computable from the emission coordinates for the
users of a `central' region of the front emission coordinate domain. Other
space-time regions associated with the emission coordinates are also outlined.Comment: 20 pages; 1 figur
Neutrinos from supernovae: experimental status and perspectives
I discuss the state of the art in the search for neutrinos from galactic
stellar collapses and the future perspectives of this field. The implications
for the neutrino physics of a high statistics supernova neutrino burst
detection by the network of detectors operating around the world are also
reviewed.Comment: 19 pages, 12 figures. Extended version of talk given at IInd
International Workshop on Matter, Anti-Matter and Dark Matter, Trento
(Italy), 29-30 October 2001. A reduced version will appear in Int. J. of Mod.
Phys.
Local thermal equilibrium and ideal gas Stephani universes
The Stephani universes that can be interpreted as an ideal gas evolving in
local thermal equilibrium are determined. Five classes of thermodynamic schemes
are admissible, which give rise to five classes of regular models and three
classes of singular models. No Stephani universes exist representing an exact
solution to a classical ideal gas (one for which the internal energy is
proportional to the temperature). But some Stephani universes may approximate a
classical ideal gas at first order in the temperature: all of them are
obtained. Finally, some features about the physical behavior of the models are
pointed out.Comment: 20 page
Demographic Disparities Using Ranked-Choice Voting? Ranking Difficulty, Under-Voting, and the 2020 Democratic Primary
Ranked choice voting (RCV) has become increasingly popular in recent years, as more jurisdictions in the US adopt the voting system for local, state, and federal elections. Though previous studies have found potential benefits of RCV, some evidence suggests ranking multiple candidates instead of choosing one most preferred candidate may be difficult, with potential demographic disparities linked to age, gender, or racial or ethnic identity. Further, these difficulties have been assumed to cause individuals to improperly fill out RCV ballots, such as ranking too many or not enough candidates. This study seeks to answer three interrelated questions: 1) Which demographic groups find it difficult to rank candidates in RCV elections? 2) Who is more likely to cast under-voted ballots (not ranking all candidates)? 3) Is there a relationship between finding RCV voting difficult and the likelihood of casting an under-voted ballot? Using unique national survey data of 2020 Democratic primary candidate preferences, the results indicate most respondents find ranking candidates easy, but older, less interested, and more ideologically conservative individuals find it more difficult. In a hypothetical ranking of primary candidates, 12% of respondents under-voted (did not rank all options). Despite their perceived increased difficulty, older individuals were less likely to under-vote their ballot. No other demographic groups consistently experienced systematic differences in ranking difficulty or under-voting across a series of model specifications. These findings support previous evidence of older voters having increased difficulty, but challenge research assuming difficulty leads to under-voting, and that racial and ethnic groups are disadvantaged by RCV
On the invariant symmetries of the -metrics
We analyze the symmetries and other invariant qualities of the
-metrics (type D aligned Einstein Maxwell solutions with
cosmological constant whose Debever null principal directions determine
shear-free geodesic null congruences). We recover some properties and deduce
new ones about their isometry group and about their quadratic first integrals
of the geodesic equation, and we analyze when these invariant symmetries
characterize the family of metrics. We show that the subfamily of the Kerr-NUT
solutions are those admitting a Papapetrou field aligned with the Weyl tensor.Comment: 18 pages; v2: minor change
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