15,862 research outputs found
An Model for Lepton Mass Matrices with Nearly Minimal Texture
We propose a simple extension of the electroweak standard model based on the
discrete symmetry that is capable of realizing a nearly minimal
Fritzsch-type texture for the Dirac mass matrices of both charged leptons and
neutrinos. This is achieved with the aid of additional and
symmetries, one of which can be embedded in . Five complex scalar
singlet fields are introduced in addition to the SM with right-handed
neutrinos. Although more general, the modified texture of the model retains the
successful features of the minimal texture without fine-tuning; namely, it
accommodates the masses and mixing of the leptonic sector and relates the
emergence of large leptonic mixing angles with the seesaw mechanism. For large
deviations of the minimal texture, both quasidegenerate spectrum or inverted
hierarchy are allowed for neutrino masses.Comment: 11pp, 2 figures. v2: vev alignment addressed, additional analysis
performed; to appear in PR
Bar formation and evolution in disc galaxies with gas and a triaxial halo: Morphology, bar strength and halo properties
We follow the formation and evolution of bars in N-body simulations of disc
galaxies with gas and/or a triaxial halo. We find that both the relative gas
fraction and the halo shape play a major role in the formation and evolution of
the bar. In gas-rich simulations, the disc stays near-axisymmetric much longer
than in gas-poor ones, and, when the bar starts growing, it does so at a much
slower rate. Due to these two effects combined, large-scale bars form much
later in gas-rich than in gas-poor discs. This can explain the observation that
bars are in place earlier in massive red disc galaxies than in blue spirals. We
also find that the morphological characteristics in the bar region are strongly
influenced by the gas fraction. In particular, the bar at the end of the
simulation is much weaker in gas-rich cases. In no case did we witness bar
destruction.
Halo triaxiality has a dual influence on bar strength. In the very early
stages of the simulation it induces bar formation to start earlier. On the
other hand, during the later, secular evolution phase, triaxial haloes lead to
considerably less increase of the bar strength than spherical ones. The shape
of the halo evolves considerably with time. The inner halo parts may become
more elongated, or more spherical, depending on the bar strength. The main body
of initially triaxial haloes evolves towards sphericity, but in initially
strongly triaxial cases it stops well short of becoming spherical. Part of the
angular momentum absorbed by the halo generates considerable rotation of the
halo particles that stay located relatively near the disc for long periods of
time. Another part generates halo bulk rotation, which, contrary to that of the
bar, increases with time but stays small.Comment: 21 pages, 16 figures, accepted for publication in MNRAS. A high
resolution version is at
http://195.221.212.246:4780/dynam/paper/amr12/rm_3axhalo_gas.pd
The Quest for an Intermediate-Scale Accidental Axion and Further ALPs
The recent detection of the cosmic microwave background polarimeter
experiment BICEP2 of tensor fluctuations in the B-mode power spectrum basically
excludes all plausible axion models where its decay constant is above
GeV. Moreover, there are strong theoretical, astrophysical, and cosmological
motivations for models involving, in addition to the axion, also axion-like
particles (ALPs), with decay constants in the intermediate scale range, between
GeV and GeV. Here, we present a general analysis of models
with an axion and further ALPs and derive bounds on the relative size of the
axion and ALP photon (and electron) coupling. We discuss what we can learn from
measurements of the axion and ALP photon couplings about the fundamental
parameters of the underlying ultraviolet completion of the theory. For the
latter we consider extensions of the Standard Model in which the axion and the
ALP(s) appear as pseudo Nambu-Goldstone bosons from the breaking of global
chiral (Peccei-Quinn (PQ)) symmetries, occuring accidentally as low
energy remnants from exact discrete symmetries. In such models, the axion and
the further ALP are protected from disastrous explicit symmetry breaking
effects due to Planck-scale suppressed operators. The scenarios considered
exploit heavy right handed neutrinos getting their mass via PQ symmetry
breaking and thus explain the small mass of the active neutrinos via a seesaw
relation between the electroweak and an intermediate PQ symmetry breaking
scale. We show some models that can accommodate simultaneously an axion dark
matter candidate, an ALP explaining the anomalous transparency of the universe
for -rays, and an ALP explaining the recently reported 3.55 keV gamma
line from galaxies and clusters of galaxies, if the respective decay constants
are of intermediate scale.Comment: 43pp, 4 figures. v2: version accepted for publication in JHE
Discrepancies between empirical and theoretical models of the flaring solar chromosphere and their possible resolution
Models of the solar chromosphere during flaring deduced theoretically or empirically are compared. Marked discrepancies are noted and various reasons are offered to explain their existence. A means is presented for testing theoretical heating models (electron heating) by analyzing the net energy loss rates in (observed) empirical atmospheres and inverting the flare energy equation to deduce the parameters of the supposed heating mechanism
Type O pure radiation metrics with a cosmological constant
In this paper we complete the integration of the conformally flat pure
radiation spacetimes with a non-zero cosmological constant , and , by considering the case . This is a
further demonstration of the power and suitability of the generalised invariant
formalism (GIF) for spacetimes where only one null direction is picked out by
the Riemann tensor. For these spacetimes, the GIF picks out a second null
direction, (from the second derivative of the Riemann tensor) and once this
spinor has been identified the calculations are transferred to the simpler GHP
formalism, where the tetrad and metric are determined. The whole class of
conformally flat pure radiation spacetimes with a non-zero cosmological
constant (those found in this paper, together with those found earlier for the
case ) have a rich variety of subclasses with zero,
one, two, three, four or five Killing vectors
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