369 research outputs found
A Motivating Exploration on Lunar Craters and Low-Energy Dynamics in the Earth -- Moon System
It is known that most of the craters on the surface of the Moon were created
by the collision of minor bodies of the Solar System. Main Belt Asteroids,
which can approach the terrestrial planets as a consequence of different types
of resonance, are actually the main responsible for this phenomenon. Our aim is
to investigate the impact distributions on the lunar surface that low-energy
dynamics can provide. As a first approximation, we exploit the hyberbolic
invariant manifolds associated with the central invariant manifold around the
equilibrium point L_2 of the Earth - Moon system within the framework of the
Circular Restricted Three - Body Problem. Taking transit trajectories at
several energy levels, we look for orbits intersecting the surface of the Moon
and we attempt to define a relationship between longitude and latitude of
arrival and lunar craters density. Then, we add the gravitational effect of the
Sun by considering the Bicircular Restricted Four - Body Problem. As further
exploration, we assume an uniform density of impact on the lunar surface,
looking for the regions in the Earth - Moon neighbourhood these colliding
trajectories have to come from. It turns out that low-energy ejecta originated
from high-energy impacts are also responsible of the phenomenon we are
considering.Comment: The paper is being published in Celestial Mechanics and Dynamical
Astronomy, vol. 107 (2010
Ultra-High Energy Cosmic Rays from Neutrino Emitting Acceleration Sources?
We demonstrate by numerical flux calculations that neutrino beams producing
the observed highest energy cosmic rays by weak interactions with the relic
neutrino background require a non-uniform distribution of sources. Such sources
have to accelerate protons at least up to 10^{23} eV, have to be opaque to
their primary protons, and should emit the secondary photons unavoidably
produced together with the neutrinos only in the sub-MeV region to avoid
conflict with the diffuse gamma-ray background measured by the EGRET
experiment. Even if such a source class exists, the resulting large
uncertainties in the parameters involved in this scenario does currently not
allow to extract any meaningful information on absolute neutrino masses.Comment: 6 pages, 4 figures, RevTeX styl
Heavy quarkonium 2S states in light-front quark model
We study the charmonium 2S states and , and the bottomonium
2S states and , using the light-front quark model and the
2S state wave function of harmonic oscillator as the approximation of the 2S
quarkonium wave function. The decay constants, transition form factors and
masses of these mesons are calculated and compared with experimental data.
Predictions of quantities such as Br are made. The
2S wave function may help us learn more about the structure of these heavy
quarkonia.Comment: 5 latex pages, final version for journal publicatio
The New Physics at RHIC. From Transparency to High p Suppression
Heavy ion collisions at RHIC energies (Au+Au collisions at
GeV) exhibit significant new features as compared to
earlier experiments at lower energies. The reaction is characterized by a high
degree of transparency of the collisions partners leading to the formation of a
baryon-poor central region. In this zone, particle production occurs mainly
from the stretching of the color field. The initial energy density is well
above the one considered necessary for the formation of the Quark Gluon Plasma,
QGP. The production of charged particles of various masses is consistent with
chemical and thermal equilibrium. Recently, a suppression of the high
transverse momentum component of hadron spectra has been observed in central
Au+Au collisions. This can be explained by the energy loss experienced by
leading partons in a medium with a high density of unscreened color charges. In
contrast, such high jets are not suppressed in d+Au collisions suggesting
that the high suppression is not due to initial state effects in the
ultrarelativistic colliding nuclei.Comment: 15 pages, 11 figures. to appear in Nucl. Physics A. Invited talk at
'Nucleus-Nucleus Collisions 2003' conference, Mosco
Ultra-High Energy Neutrino Fluxes and Their Constraints
Applying our recently developed propagation code we review extragalactic
neutrino fluxes above 10^{14} eV in various scenarios and how they are
constrained by current data. We specifically identify scenarios in which the
cosmogenic neutrino flux, produced by pion production of ultra high energy
cosmic rays outside their sources, is considerably higher than the
"Waxman-Bahcall bound". This is easy to achieve for sources with hard injection
spectra and luminosities that were higher in the past. Such fluxes would
significantly increase the chances to detect ultra-high energy neutrinos with
experiments currently under construction or in the proposal stage.Comment: 11 pages, 15 figures, version published in Phys.Rev.
From chemical gardens to chemobrionics
Chemical gardens in laboratory chemistries ranging from silicates to polyoxometalates, in applications ranging from corrosion products to the hydration of Portland cement, and in natural settings ranging from hydrothermal vents in the ocean depths to brinicles beneath sea ice. In many chemical-garden experiments, the structure forms as a solid seed of a soluble ionic compound dissolves in a solution containing another reactive ion. In general any alkali silicate solution can be used due to their high solubility at high pH. The cation should not precipitate with the counterion of the metal salt used as seed. A main property of seed chemical-garden experiments is that initially, when the fluid is not moving under buoyancy or osmosis, the delivery of the inner reactant is diffusion controlled. Another experimental technique that isolates one aspect of chemical-garden formation is to produce precipitation membranes between different aqueous solutions by introducing the two solutions on either side of an inert carrier matrix. Chemical gardens may be grown upon injection of solutions into a so-called Hele-Shaw cell, a quasi-two-dimensional reactor consisting in two parallel plates separated by a small gap
Which blazars are neutrino loud?
Protons accelerated in the cores of active galactic nuclei can effectively
produce neutrinos only if the soft radiation background in the core is
sufficiently high. We find restrictions on the spectral properties and
luminosity of blazars under which they can be strong neutrino sources. We
analyze the possibility that neutrino flux is highly beamed along the rotation
axis of the central black hole. The enhancement of neutrino flux compared to
GeV gamma-ray flux from a given source makes the detection of neutrino point
sources more probable. At the same time the smaller open angle reduces the
number of possible neutrino-loud blazars compared to the number of gamma-ray
loud ones. We present the table of 15 blazars which are the most likely
candidates for the detection by future neutrino telescopes.Comment: 9 pages, 5 figures, version to be published in PR
New hadrons as ultra-high energy cosmic rays
Ultra-high energy cosmic ray (UHECR) protons produced by uniformly
distributed astrophysical sources contradict the energy spectrum measured by
both the AGASA and HiRes experiments, assuming the small scale clustering of
UHECR observed by AGASA is caused by point-like sources. In that case, the
small number of sources leads to a sharp exponential cutoff at the energy
E<10^{20} eV in the UHECR spectrum. New hadrons with mass 1.5-3 GeV can solve
this cutoff problem. For the first time we discuss the production of such
hadrons in proton collisions with infrared/optical photons in astrophysical
sources. This production mechanism, in contrast to proton-proton collisions,
requires the acceleration of protons only to energies E<10^{21} eV. The diffuse
gamma-ray and neutrino fluxes in this model obey all existing experimental
limits. We predict large UHE neutrino fluxes well above the sensitivity of the
next generation of high-energy neutrino experiments. As an example we study
hadrons containing a light bottom squark. These models can be tested by
accelerator experiments, UHECR observatories and neutrino telescopes.Comment: 17 pages, revtex style; v2: shortened, as to appear in PR
Gamma-Ray Bursts: The Underlying Model
A pedagogical derivation is presented of the ``fireball'' model of gamma-ray
bursts, according to which the observable effects are due to the dissipation of
the kinetic energy of a relativistically expanding wind, a ``fireball.'' The
main open questions are emphasized, and key afterglow observations, that
provide support for this model, are briefly discussed. The relativistic outflow
is, most likely, driven by the accretion of a fraction of a solar mass onto a
newly born (few) solar mass black hole. The observed radiation is produced once
the plasma has expanded to a scale much larger than that of the underlying
``engine,'' and is therefore largely independent of the details of the
progenitor, whose gravitational collapse leads to fireball formation. Several
progenitor scenarios, and the prospects for discrimination among them using
future observations, are discussed. The production in gamma- ray burst
fireballs of high energy protons and neutrinos, and the implications of burst
neutrino detection by kilometer-scale telescopes under construction, are
briefly discussed.Comment: In "Supernovae and Gamma Ray Bursters", ed. K. W. Weiler, Lecture
Notes in Physics, Springer-Verlag (in press); 26 pages, 2 figure
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