1,749 research outputs found
Axion-photon Couplings in Invisible Axion Models
We reexamine the axion-photon couplings in various invisible axion models
motivated by the recent proposal of using optical interferometry at the ASST
facility in the SSCL to search for axion. We illustrate that the assignment of
charges for the fermion fields plays an important role in
determining the couplings. Several simple non-minimal invisible axion models
with suppressed and enhanced axion-photon couplings are constructed,
respectively. We also discuss the implications of possible new experiments to
detect solar axions by conversion to -rays in a static magnetic apparatus
tracking the sun.Comment: 14 pages, LaTeX fil
Chiral fermion mass and dispersion relations at finite temperature in the presence of hypermagnetic fields
We study the modifications to the real part of the thermal self-energy for
chiral fermions in the presence of a constant external hypermagnetic field. We
compute the dispersion relation for fermions occupying a given Landau level to
first order in g'^2, g^2 and g_phi^2 and to all orders in g'B, where g' and g
are the U(1)_Y and SU(2)_L couplings of the standard model, respectively, g_phi
is the fermion Yukawa coupling, and B is the hypermagnetic field strength. We
show that in the limit where the temperature is large compared to sqrt{g'B},
left- and right-handed modes acquire finite and different B-dependent masses
due to the chiral nature of their coupling with the external field. Given the
current bounds on the strength of primordial magnetic fields, we argue that the
above is the relevant scenario to study the effects of magnetic fields on the
propagation of fermions prior and during the electroweak phase transition.Comment: 11 pages 4 figures, published versio
Relativistic Mass Ejecta from Phase-transition-induced Collapse of Neutron Stars
We study the dynamical evolution of a phase-transition-induced collapse
neutron star to a hybrid star, which consists of a mixture of hadronic matter
and strange quark matter. The collapse is triggered by a sudden change of
equation of state, which result in a large amplitude stellar oscillation. The
evolution of the system is simulated by using a 3D Newtonian hydrodynamic code
with a high resolution shock capture scheme. We find that both the temperature
and the density at the neutrinosphere are oscillating with acoustic frequency.
However, they are nearly 180 out of phase. Consequently, extremely
intense, pulsating neutrino/antineutrino fluxes will be emitted periodically.
Since the energy and density of neutrinos at the peaks of the pulsating fluxes
are much higher than the non-oscillating case, the electron/positron pair
creation rate can be enhanced dramatically. Some mass layers on the stellar
surface can be ejected by absorbing energy of neutrinos and pairs. These mass
ejecta can be further accelerated to relativistic speeds by absorbing
electron/positron pairs, created by the neutrino and antineutrino annihilation
outside the stellar surface. The possible connection between this process and
the cosmological Gamma-ray Bursts is discussed.Comment: 40 pages, 11 figures, accepted for publication in JCA
Is the Sun Embedded in a Typical Interstellar Cloud?
The physical properties and kinematics of the partially ionized interstellar
material near the Sun are typical of warm diffuse clouds in the solar vicinity.
The interstellar magnetic field at the heliosphere and the kinematics of nearby
clouds are naturally explained in terms of the S1 superbubble shell. The
interstellar radiation field at the Sun appears to be harder than the field
ionizing ambient diffuse gas, which may be a consequence of the low opacity of
the tiny cloud surrounding the heliosphere. The spatial context of the Local
Bubble is consistent with our location in the Orion spur.Comment: "From the Outer Heliosphere to the Local Bubble", held at
International Space Sciences Institute, October 200
Large-scale magnetic fields from inflation in dilaton electromagnetism
The generation of large-scale magnetic fields is studied in dilaton
electromagnetism in inflationary cosmology, taking into account the dilaton's
evolution throughout inflation and reheating until it is stabilized with
possible entropy production. It is shown that large-scale magnetic fields with
observationally interesting strength at the present time could be generated if
the conformal invariance of the Maxwell theory is broken through the coupling
between the dilaton and electromagnetic fields in such a way that the resultant
quantum fluctuations in the magnetic field has a nearly scale-invariant
spectrum. If this condition is met, the amplitude of the generated magnetic
field could be sufficiently large even in the case huge amount of entropy is
produced with the dilution factor as the dilaton decays.Comment: 28 pages, 5 figures, the version accepted for publication in Phys.
Rev. D; some references are adde
Development and validation of a radial variable geometry turbine model for transient pulsating flow applications
This paper presents the development and validation of a one-dimensional radial turbine model able to be
used in automotive turbocharger simulations. The model has been validated using results from a numerical
3D CFD simulation of stationary and pulsating flow in a variable geometry radial turbine. As the CFD
analysis showed, the main non-quasi-steady behavior of the turbine is due to the volute geometry, so
special care was taken in order to properly model it while maintaining low computational costs. The flow
in the volute has been decomposed in its radial and azimuthal direction. The azimuthal flow corresponds
to the flow moving along the volute, while the radial flow is computed by coupling its flow with a stator
model. Although the stator caused fewer accumulation effects than the volute, a small accumulation
model has been used for it, which also allows to compute the evolution of the flow inside the turbine with
lower costs. The flow in the moving rotor can be considered quasi-steady, so a zero-dimensional model
for the rotor has been developed. Several losses models where implemented for both the stator and the
rotor. The results show good agreement with the CFD computations.
2014 Elsevier Ltd. All rights reserved.The authors are indebted to the Spanish Ministerio de Economia y Competitividad through Project TRA 2010-16205.Galindo, J.; Tiseira Izaguirre, AO.; Fajardo, P.; García-Cuevas González, LM. (2014). Development and validation of a radial variable geometry turbine model for transient pulsating flow applications. Energy Conversion and Management. 85:190-203. https://doi.org/10.1016/j.enconman.2014.05.072S1902038
SuperWIMP Dark Matter Signals from the Early Universe
Cold dark matter may be made of superweakly-interacting massive particles,
superWIMPs, that naturally inherit the desired relic density from late decays
of metastable WIMPs. Well-motivated examples are weak-scale gravitinos in
supergravity and Kaluza-Klein gravitons from extra dimensions. These particles
are impossible to detect in all dark matter experiments. We find, however, that
superWIMP dark matter may be discovered through cosmological signatures from
the early universe. In particular, superWIMP dark matter has observable
consequences for Big Bang nucleosynthesis and the cosmic microwave background
(CMB), and may explain the observed underabundance of 7Li without upsetting the
concordance between deuterium and CMB baryometers. We discuss implications for
future probes of CMB black body distortions and collider searches for new
particles. In the course of this study, we also present a model-independent
analysis of entropy production from late-decaying particles in light of WMAP
data.Comment: 19 pages, 5 figures, typos correcte
Spin Analysis of Supersymmetric Particles
The spin of supersymmetric particles can be determined at colliders
unambiguously. This is demonstrated for a characteristic set of non-colored
supersymmetric particles -- smuons, selectrons, and charginos/neutralinos. The
analysis is based on the threshold behavior of the excitation curves for pair
production in collisions, the angular distribution in the production
process and decay angular distributions. In the first step we present the
observables in the helicity formalism for the supersymmetric particles.
Subsequently we confront the results with corresponding analyses of
Kaluza-Klein particles in theories of universal extra space dimensions which
behave distinctly different from supersymmetric theories. It is shown in the
third step that a set of observables can be designed which signal the spin of
supersymmetric particles unambiguously without any model assumptions. Finally
in the fourth step it is demonstrated that the determination of the spin of
supersymmetric particles can be performed experimentally in practice at an
collider.Comment: 39 pages, 14 figure
Gravitational collapse of a Hagedorn fluid in Vaidya geometry
The gravitational collapse of a high-density null charged matter fluid,
satisfying the Hagedorn equation of state, is considered in the framework of
the Vaidya geometry. The general solution of the gravitational field equations
can be obtained in an exact parametric form. The conditions for the formation
of a naked singularity, as a result of the collapse of the compact object, are
also investigated. For an appropriate choice of the arbitrary integration
functions the null radial outgoing geodesic, originating from the shell
focussing central singularity, admits one or more positive roots. Hence a
collapsing Hagedorn fluid could end either as a black hole, or as a naked
singularity. A possible astrophysical application of the model, to describe the
energy source of gamma-ray bursts, is also considered.Comment: 14 pages, 2 figures, to appear in Phys. Rev.
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