7,424 research outputs found
Finite temperature corrections and embedded strings in noncommutative geometry and the standard model with neutrino mixing
The recent extension of the standard model to include massive neutrinos in
the framework of noncommutative geometry and the spectral action principle
involves new scalar fields and their interactions with the usual complex scalar
doublet. After ensuring that they bring no unphysical consequences, we address
the question of how these fields affect the physics predicted in Weinberg-Salam
theory, particularly in the context of the Electroweak phase transition.
Applying the Dolan-Jackiw procedure, we calculate the finite temperature
corrections, and find that the phase transition is first order. The new scalar
interactions significantly improve the stability of the Electroweak Z string,
through the ``bag'' phenomenon described by Watkins and Vachaspati. (Recently
cosmic strings have climbed back into interest due to new evidence). Sourced by
static embedded strings, an internal space analogy of Cartan's torsion is
drawn, and a possible Higgs-force-like `gravitational' effect of this
non-propagating torsion on the fermion masses is described. We also check that
the field generating the Majorana mass for the is non-zero in the
physical vacuum.Comment: 42 page
The contrast of magnetic elements in synthetic CH- and CN-band images of solar magnetoconvection
We present a comparative study of the intensity contrast in synthetic CH-band
and violet CN-band filtergrams computed from a high-resolution simulation of
solar magnetoconvection. The underlying simulation has an average vertical
magnetic field of 250 G with kG fields concentrated in its intergranular lanes,
and is representative of a plage region. To simulate filtergrams typically
obtained in CH- and CN-band observations we computed spatially resolved spectra
in both bands and integrated these spectra over 1 nm FWHM filter functions
centred at 430.5 nm and 388.3 nm, respectively. We find that the average
contrast of magnetic bright points in the simulated filtergrams is lower in the
CN-band by a factor of 0.96. This result strongly contradicts earlier
semi-empirical modeling and recent observations, which both etimated that the
bright-point contrast in the CN-band is \emph{higher} by a factor of 1.4. We
argue that the near equality of the bright-point contrast in the two bands in
the present simulation is a natural consequence of the mechanism that causes
magnetic flux elements to be particularly bright in the CN and CH filtergrams,
namely the partial evacuation of these elements and the concomitant weakening
of molecular spectral lines in the filter passbands. We find that the RMS
intensity contrast in the whole field-of-view of the filtergrams is 20.5% in
the G band and 22.0% in the CN band and conclude that this slight difference in
contrast is caused by the shorter wavelength of the latter. Both the
bright-point and RMS intensity contrast in the CN band are sensitive to the
precise choice of the central wavelength of the filter.Comment: 24 pages, 9 figures, submitted to Ap
Multimodal Differential Emission Measure in the Solar Corona
The Atmospheric Imaging Assembly (AIA) telescope on board the Solar Dynamics
Observatory (SDO) provides coronal EUV imaging over a broader temperature
sensitivity range than the previous generations of instruments (EUVI, EIT, and
TRACE). Differential emission measure tomography (DEMT) of the solar corona
based on AIA data is presented here for the first time. The main product of
DEMT is the three-dimensional (3D) distribution of the local differential
emission measure (LDEM). While in previous studies, based on EIT or EUVI data,
there were 3 available EUV bands, with a sensitivity range
MK, the present study is based on the 4 cooler AIA bands (aimed at studying the
quiet sun), sensitive to the range MK. The AIA filters allow
exploration of new parametric LDEM models. Since DEMT is better suited for
lower activity periods, we use data from Carrington Rotation 2099, when the Sun
was in its most quiescent state during the AIA mission. Also, we validate the
parametric LDEM inversion technique by applying it to standard bi-dimensional
(2D) differential emission measure (DEM) analysis on sets of simultaneous AIA
images, and comparing the results with DEM curves obtained using other methods.
Our study reveals a ubiquitous bimodal LDEM distribution in the quiet diffuse
corona, which is stronger for denser regions. We argue that the nanoflare
heating scenario is less likely to explain these results, and that alternative
mechanisms, such as wave dissipation appear better supported by our results.Comment: 52 pages, 18 figure
Gravity from Dirac Eigenvalues
We study a formulation of euclidean general relativity in which the dynamical
variables are given by a sequence of real numbers , representing
the eigenvalues of the Dirac operator on the curved spacetime. These quantities
are diffeomorphism-invariant functions of the metric and they form an infinite
set of ``physical observables'' for general relativity. Recent work of Connes
and Chamseddine suggests that they can be taken as natural variables for an
invariant description of the dynamics of gravity. We compute the Poisson
brackets of the 's, and find that these can be expressed in terms
of the propagator of the linearized Einstein equations and the energy-momentum
of the eigenspinors. We show that the eigenspinors' energy-momentum is the
Jacobian matrix of the change of coordinates from the metric to the
's. We study a variant of the Connes-Chamseddine spectral action
which eliminates a disturbing large cosmological term. We analyze the
corresponding equations of motion and find that these are solved if the energy
momenta of the eigenspinors scale linearly with the mass. Surprisingly, this
scaling law codes Einstein's equations. Finally we study the coupling to a
physical fermion field.Comment: An enlarged and improved version which will be pubblished in Mod.
Phys. Lett.
ADAS analysis of the differential emission measure structure of the inner solar corona. II. A study of the `quiet Sun' inhomogeneities from SOHO CDS-NIS spectra
We present a study of the differential emission measure (DEM) of a `quiet
Sun' area observed in the extreme ultraviolet at normal incidence by the
Coronal Diagnostic Spectrometer (CDS) on the SOHO spacecraft. The data used for
this work were taken using the NISAT_S observing sequence. This takes the full
wavelength ranges from both the NIS channels (308-381 Angtr. and 513-633
Angst.) with the 2 arcsec by 240 arcsec slit, which is the narrowest slit
available, yielding the best spectral resolution. In this work we contrast the
DEM from subregions of 2 by 80 arcsec with that obtained from the mean
spectrum of the whole raster (20 by 240 arcsec). We find that the DEM
maintains essentially the same shape in the subregions, differing by a constant
factor between 0.5 and 2 from the mean DEM, except in areas were the electron
density is below cm and downflow velocities of 50 km/s
are found in the transition region. Such areas are likely to contain plasma
departing from ionisation equilibrium, violating the basic assumptions
underlying the DEM method. The comparison between lines of Li-like and Be-like
ions may provide further evidence of departure from ionisation equilibrium. We
find also that line intensities tend to be lower where velocities of the order
of 30 km/s or higher are measured in transition region lines. The DEM analysis
is also exploited to improve the line identification performed by Brooks et al
(1999) and to investigate possible elemental abundance variations from region
to region. We find that the plasma has composition close to photospheric in all
the subregions examined.Comment: 18 pages, 9 figures, 7 tables. Table 5 is available only online. A
copy of Table 5 can be found at http://webusers.ct.astro.it/acl/table5.dat.
The ReadMe file is at http://webusers.ct.astro.it/acl/ReadMe. Accepted by
Astronomy and Astrophysic
Chasing extreme blazars with INTEGRAL
Within the blazar population, hard X-ray selected objects are of particular
interest as they tend to lie at each end of the blazar sequence. In particular,
flat spectrum radio quasars located at high redshifts display the most powerful
jets, the largest black hole masses and the most luminous accretion disks:
their spectral energy distribution has a Compton peak in the sub-MeV region
which favours their detection by instruments like INTEGRAL/IBIS and Swift/BAT.
These sources are even more extreme than blazars selected in other wavebands,
like, for example, the gamma-ray range explored by Fermi. Here we report on a
sample of 12 high redshift blazars detected so far by INTEGRAL, including 3
newly identified objects. Some properties of the combined IBIS/BAT sample of
high redshift blazars (z>2) are also compared to those of a similar similar
sample obtained by Fermi.Comment: 4 pages, 2 figures. Proceedings of "An INTEGRAL view of the
high-energy sky (the first 10 years)" the 9th INTEGRAL Workshop, October
15-19, 2012, Paris, France, in Proceedings of Science (INTEGRAL 2012), Eds.
A. Goldwurm, F. Lebrun and C. Winkler,
(http://pos.sissa.it/cgi-bin/reader/conf.cgi?confid=176), id=05
Advanced Forward Modeling and Inversion of Stokes Profiles Resulting from the Joint Action of the Hanle and Zeeman Effects
A big challenge in solar and stellar physics in the coming years will be to
decipher the magnetism of the solar outer atmosphere (chromosphere and corona)
along with its dynamic coupling with the magnetic fields of the underlying
photosphere. To this end, it is important to develop rigorous diagnostic tools
for the physical interpretation of spectropolarimetric observations in suitably
chosen spectral lines. Here we present a computer program for the synthesis and
inversion of Stokes profiles caused by the joint action of atomic level
polarization and the Hanle and Zeeman effects in some spectral lines of
diagnostic interest, such as those of the He I 10830 A and D_3 multiplets. It
is based on the quantum theory of spectral line polarization, which takes into
account all the relevant physical mechanisms and ingredients (optical pumping,
atomic level polarization, Zeeman, Paschen-Back and Hanle effects). The
influence of radiative transfer on the emergent spectral line radiation is
taken into account through a suitable slab model. The user can either calculate
the emergent intensity and polarization for any given magnetic field vector or
infer the dynamical and magnetic properties from the observed Stokes profiles
via an efficient inversion algorithm based on global optimization methods. The
reliability of the forward modeling and inversion code presented here is
demonstrated through several applications, which range from the inference of
the magnetic field vector in solar active regions to determining whether or not
it is canopy-like in quiet chromospheric regions. This user-friendly diagnostic
tool called "HAZEL" (from HAnle and ZEeman Light) is offered to the
astrophysical community, with the hope that it will facilitate new advances in
solar and stellar physics.Comment: 62 pages, 19 figures, 3 tables. Accepted for publication in Ap
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