2,595 research outputs found
Neutralino Phenomenology at LEP2 in Supersymmetry with Bilinear Breaking of R-parity
We discuss the phenomenology of the lightest neutralino in models where an
effective bilinear term in the superpotential parametrizes the explicit
breaking of R-parity. We consider supergravity scenarios where the lightest
supersymmetric particle (LSP) is the lightest neutralino and which can be
explored at LEP2. We present a detailed study of the LSP decay properties and
general features of the corresponding signals expected at LEP2. We also
contrast our model with gauge mediated supersymmetry breaking.Comment: 21 pages, Latex, uses axodraw.sty (included), 13 figures included as
ps- and eps-files, figures slightly changed after bug-fixing, comparison with
GMSB and a few references added, version to appear in NP
Negative-energy perturbations in cylindrical equilibria with a radial electric field
The impact of an equilibrium radial electric field on negative-energy
perturbations (NEPs) (which are potentially dangerous because they can lead to
either linear or nonlinear explosive instabilities) in cylindrical equilibria
of magnetically confined plasmas is investigated within the framework of
Maxwell-drift kinetic theory. It turns out that for wave vectors with a
non-vanishing component parallel to the magnetic field the conditions for the
existence of NEPs in equilibria with E=0 [G. N. Throumoulopoulos and D.
Pfirsch, Phys. Rev. E 53, 2767 (1996)] remain valid, while the condition for
the existence of perpendicular NEPs, which are found to be the most important
perturbations, is modified. For ( is the
electrostatic potential) and ( is
the total plasma pressure), a case which is of operational interest in magnetic
confinement systems, the existence of perpendicular NEPs depends on ,
where is the charge of the particle species . In this case the
electric field can reduce the NEPs activity in the edge region of tokamaklike
and stellaratorlike equilibria with identical parabolic pressure profiles, the
reduction of electron NEPs being more pronounced than that of ion NEPs.Comment: 30 pages, late
Critical behavior of ferromagnetic pure and random diluted nanoparticles with competing interactions: variational and Monte Carlo approaches
The magnetic properties and critical behavior of both ferromagnetic pure and
metallic nanoparticles having concurrently atomic disorder, dilution and
competing interactions, are studied in the framework of an Ising model. We have
used both the free energy variational principle based on the Bogoliubov
inequality and Monte Carlo simulation. As a case of study for random diluted
nanoparticles we have considered the FeMnAl alloy
characterized for exhibiting, under bulk conditions, low temperature reentrant
spin glass (RSG) behavior and for which experimental and simulation results are
available. Our results allow concluding that the variational model is
successful in reproducing features of the particle size dependence of the Curie
temperature for both pure and random diluted particles. In this last case, low
temperature magnetization reduction was consistent with the same type of RSG
behavior observed in bulk in accordance with the Almeida-Thouless line at low
fields and a linear dependence of the freezing temperature with the reciprocal
of the particle diameter was also obtained. Computation of the correlation
length critical exponent yielded the values via Bogoliubov
and via Monte Carlo. This fact indicates that even though
thermodynamical models can be indeed used in the study of nanostructures and
they can reproduce experimental features, special attention must be paid
regarding critical behavior. From both approaches, differences in the
exponent with respect to the pure Ising model agree with Harris and Fisher
arguments.Comment: 11 pages, 11 figures. Submitted to Phys. Rev.
Negative-Energy Perturbations in Circularly Cylindrical Equilibria within the Framework of Maxwell-Drift Kinetic Theory
The conditions for the existence of negative-energy perturbations (which
could be nonlinearly unstable and cause anomalous transport) are investigated
in the framework of linearized collisionless Maxwell-drift kinetic theory for
the case of equilibria of magnetically confined, circularly cylindrical plasmas
and vanishing initial field perturbations. For wave vectors with a
non-vanishing component parallel to the magnetic field, the plane equilibrium
conditions (derived by Throumoulopoulos and Pfirsch [Phys Rev. E {\bf 49}, 3290
(1994)]) are shown to remain valid, while the condition for perpendicular
perturbations (which are found to be the most important modes) is modified.
Consequently, besides the tokamak equilibrium regime in which the existence of
negative-energy perturbations is related to the threshold value of 2/3 of the
quantity , a new
regime appears, not present in plane equilibria, in which negative-energy
perturbations exist for {\em any} value of . For various analytic
cold-ion tokamak equilibria a substantial fraction of thermal electrons are
associated with negative-energy perturbations (active particles). In
particular, for linearly stable equilibria of a paramagnetic plasma with flat
electron temperature profile (), the entire velocity space is
occupied by active electrons. The part of the velocity space occupied by active
particles increases from the center to the plasma edge and is larger in a
paramagnetic plasma than in a diamagnetic plasma with the same pressure
profile. It is also shown that, unlike in plane equilibria, negative-energy
perturbations exist in force-free reversed-field pinch equilibria with a
substantial fraction of active particles.Comment: 31 pages, late
Microsolvation of Mg2+, Ca2+: Strong influence of formal charges in hydrogen bond networks
A stochastic exploration of the quantum conformational spaces in the microsolvation of divalent cations with explicit consideration of up to six solvent molecules [Mg (H 2 O) n )]2+, (n = 3, 4, 5, 6) at the B3LYP, MP2, CCSD(T) levels is presented. We find several cases in which the formal charge in Mg2+ causes dissociation of water molecules in the first solvation shell, leaving a hydroxide ion available to interact with the central cation, the released proton being transferred to outer solvation shells in a Grotthus type mechanism; this particular finding sheds light on the capacity of Mg2+ to promote formation of hydroxide anions, a process necessary to regulate proton transfer in enzymes with exonuclease activity. Two distinct types of hydrogen bonds, scattered over a wide range of distances (1.35–2.15 Å) were identified. We find that in inner solvation shells, where hydrogen bond networks are severely disturbed, most of the interaction energies come from electrostatic and polarization+charge transfer, while in outer solvation shells the situation approximates that of pure water clusters
Location, orbit and energy of a meteoroid impacting the moon during the Lunar Eclipse of January 21, 2019
During lunar eclipse of January 21, 2019 a meteoroid impacted the Moon
producing a visible light flash. The impact was witnessed by casual observers
offering an opportunity to study the phenomenon from multiple geographical
locations. We use images and videos collected by observers in 7 countries to
estimate the location, impact parameters (speed and incoming direction) and
energy of the meteoroid. Using parallax, we achieve determining the impact
location at lat. , lon. and
geocentric distance as 356553 km. After devising and applying a photo-metric
procedure for measuring flash standard magnitudes in multiple RGB images having
different exposure times, we found that the flash, had an average G-magnitude
. We use gravitational ray tracing (GRT) to
estimate the orbital properties and likely radiant of the impactor. We find
that the meteoroid impacted the moon with a speed of km/s (70%
C.L.) and at a shallow angle, degrees. Assuming a normal error
for our estimated flash brightness, educated priors for the luminous efficiency
and object density, and using the GRT-computed probability distributions of
impact speed and incoming directions, we calculate posterior probability
distributions for the kinetic energy (median = 0.8 kton), body
mass ( = 27 kg) and diameter ( = 29 cm), and crater
size ( = 9 m). If our assumptions are correct, the crater left by
the impact could be detectable by prospecting lunar probes. These results arose
from a timely collaboration between professional and amateur astronomers which
highlight the potential importance of citizen science in astronomy.Comment: 19 pages, 11 figures, 4 tables. Data and scripts available in
https://github.com/seap-udea/MoonFlashes. Accepted for publication in MNRA
On the connection of Gamma-rays, Dark Matter and Higgs searches at LHC
Motivated by the upcoming Higgs analyzes we investigate the importance of the
complementarity of the Higgs boson chase on the low mass WIMP search in direct
detection experiments and the gamma-ray emission from the Galactic Center
measured by the Fermi-LAT telescope in the context of the . We obtain the relic abundance, thermal cross section,
the WIMP-nucleon cross section in the low mass regime and network them with the
branching ratios of the Higgs boson in the model. We conclude that the Higgs
boson search has a profound connection to the dark matter problem in our model,
in particular for the case that ( GeV) the BR(
WIMPs) . This scenario could explain this plateau of any mild
excess regarding the Higgs search as well as explain the gamma-ray emission
from the galactic center through the channel with a WIMP in the mass
range of 25-45 GeV, while still being consistent with the current limits from
XENON100 and CDMSII. However, if the recent modest excesses measured at LHC and
TEVATRON are confirmed and consistent with a standard model Higgs boson this
would imply that GeV, consequently ruling out any attempt to
explain the Fermi-LAT observations.Comment: 8 pages, 9 figure
FPTAS for Weighted Fibonacci Gates and Its Applications
Fibonacci gate problems have severed as computation primitives to solve other
problems by holographic algorithm and play an important role in the dichotomy
of exact counting for Holant and CSP frameworks. We generalize them to weighted
cases and allow each vertex function to have different parameters, which is a
much boarder family and #P-hard for exactly counting. We design a fully
polynomial-time approximation scheme (FPTAS) for this generalization by
correlation decay technique. This is the first deterministic FPTAS for
approximate counting in the general Holant framework without a degree bound. We
also formally introduce holographic reduction in the study of approximate
counting and these weighted Fibonacci gate problems serve as computation
primitives for approximate counting. Under holographic reduction, we obtain
FPTAS for other Holant problems and spin problems. One important application is
developing an FPTAS for a large range of ferromagnetic two-state spin systems.
This is the first deterministic FPTAS in the ferromagnetic range for two-state
spin systems without a degree bound. Besides these algorithms, we also develop
several new tools and techniques to establish the correlation decay property,
which are applicable in other problems
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