2,610 research outputs found
Running neutrino masses and mixing in a SU(4) x SU(2)^2 x U(1)_X model
In this talk, we discuss the implications of the renormalization group
equations for the neutrino masses and mixing angles in a supersymmetric
string-inspired SU(4) x SU(2)_L x SU(2)_R x U(1)_X model with matter in
fundamental and antisymmetric tensor representations only. The quark, charged
lepton and neutrino Yukawa matrices are distinguished by different
Clebsch-Gordan coefficients due to contracting over SU(4) and SU(2)_R indices.
In order to permit for a more realistic, hierarchical light neutrino mass
spectrum with bi-large mixing a second U(1)_X breaking singlet with fractional
charge is introduced. By numerical investigation we find a region in the model
parameter space where the neutrino mass-squared differences and mixing angles
at low energy are consistent with experimental data.Comment: Talk presented at the Corfu Summer Institute, Corfu-Greece, September
4-14, 200
Genetic diversity of the rain tree (Albizia saman) in Colombian seasonally dry tropical forest for informing conservation and restoration interventions
Albizia saman is a multipurpose tree species of seasonally dry tropical forests (SDTFs) of Mesoamerica and northern South America typically cultivated in silvopastoral and other agroforestry systems around the world, a trend that is bound to increase in light of multimillion hectare commitments for forest and landscape restoration. The effective conservation and sustainable use of A. saman requires detailed knowledge of its genetic diversity across its native distribution range of which surprisingly little is known to date. We assessed the genetic diversity and structure of A.saman across twelve representative locations of SDTF in Colombia, and how they may have been shaped by past climatic changes and human influence. We found four different genetic groups which may be the result of differentiation due to isolation of populations in preglacial times. The current distribution and mixture of genetic groups across STDF fragments we observed might be the result of range expansion of SDTFs during the last glacial period followed by range contraction during the Holocene and human‐influenced movement of germplasm associated with cattle ranching. Despite the fragmented state of the presumed natural A. saman stands we sampled, we did not find any signs of inbreeding, suggesting that gene flow is not jeopardized in humanized landscapes. However, further research is needed to assess potential deleterious effects of fragmentation on progeny. Climate change is not expected to seriously threaten the in situ persistence of A. saman populations and might present opportunities for future range expansion. However, the sourcing of germplasm for tree planting activities needs to be aligned with the genetic affinity of reference populations across the distribution of Colombian SDTFs. We identify priority source populations for in situ conservation based on their high genetic diversity, lack or limited signs of admixture, and/or genetic uniqueness
A deep submillimetre survey of the Galactic Centre
We present first results from a submillimetre continuum survey of the
Galactic Centre `Central Molecular Zone' (CMZ), made with SCUBA on the James
Clerk Maxwell Telescope. SCUBA's scan-map mode has allowed us to make extremely
wide-field maps of thermal dust emission with unprecedented speed and
sensitivity. We also discuss some issues related to the elimination of
artefacts in scan-map data. Our simultaneous 850/450 micron maps have a total
size of approximately 2.8 x 0.5 degrees (400 x 75 pc) elongated along the
galactic plane. They cover the Sgr A region-including Sgr A*, the circumnuclear
disc, and the +20 km/s and +50 km/s clouds; the area around the Pistol; Sgr
B2-the brightest feature on the map; and at their Galactic Western and Eastern
edges the Sgr C and Sgr D regions. There are many striking features such as
filaments and shell-like structures, as well as point sources such as Sgr A*
itself. The total mass in the Central Molecular Zone is greater than that
revealed in previous optically-thin molecular line maps by a factor of ~3, and
new details are revealed on scales down to 0.33 pc across this 400 pc wide
region.Comment: 12 pages, 3 figures, (figures now smaller, in paper body), accepted
by ApJ
Spectroscopy of Brown Dwarf Candidates in the rho Ophiuchi Molecular Core
We present an analysis of low resolution infrared spectra for 20 brown dwarf
candidates in the core of the Ophiuchi molecular cloud. Fifteen of the
sources display absorption-line spectra characteristic of late-type stars. By
comparing the depths of water vapor absorption bands in our candidate objects
with a grid of M dwarf standards, we derive spectral types which are
independent of reddening. Optical spectroscopy of one brown dwarf candidate
confirms the spectral type derived from the water bands. Combining their
spectral types with published near-infrared photometry, effective temperatures
and bolometric stellar luminosities are derived enabling us to place our sample
on the Hertzsprung-Russell diagram. We compare the positions of the brown dwarf
candidates in this diagram with two sets of theoretical models in order to
estimate their masses and ages. Considering uncertainties in placing the
candidates in the H-R diagram, six objects consistently lie in the brown dwarf
regime and another five objects lie in the transition region between stellar
and substellar objects. The ages inferred for the sample are consistent with
those derived for higher mass association members. Three of the newly
identified brown dwarfs display infrared excesses at =2.2 m
suggesting that young brown dwarfs can have active accretion disks. Comparing
our mass estimates of the brown dwarf candidates with those derived from
photometric data alone suggests that spectroscopy is an essential component of
investigations of the mass functions of young clusters.Comment: Astronomical Journal, in press: 25 pages, latex, 5 tables and 6
figures (separate
New Debris Disks Around Nearby Main Sequence Stars: Impact on The Direct Detection of Planets
Using the MIPS instrument on the Spitzer telescope, we have searched for
infrared excesses around a sample of 82 stars, mostly F, G, and K main-sequence
field stars, along with a small number of nearby M stars. These stars were
selected for their suitability for future observations by a variety of
planet-finding techniques. These observations provide information on the
asteroidal and cometary material orbiting these stars - data that can be
correlated with any planets that may eventually be found. We have found
significant excess 70um emission toward 12 stars. Combined with an earlier
study, we find an overall 70um excess detection rate of % for mature
cool stars. Unlike the trend for planets to be found preferentially toward
stars with high metallicity, the incidence of debris disks is uncorrelated with
metallicity. By newly identifying 4 of these stars as having weak 24um excesses
(fluxes 10% above the stellar photosphere), we confirm a trend found in
earlier studies wherein a weak 24um excess is associated with a strong 70um
excess. Interestingly, we find no evidence for debris disks around 23 stars
cooler than K1, a result that is bolstered by a lack of excess around any of
the 38 K1-M6 stars in 2 companion surveys. One motivation for this study is the
fact that strong zodiacal emission can make it hard or impossible to detect
planets directly with future observatories like the {\it Terrestrial Planet
Finder (TPF)}. The observations reported here exclude a few stars with very
high levels of emission, 1,000 times the emission of our zodiacal cloud,
from direct planet searches. For the remainder of the sample, we set relatively
high limits on dust emission from asteroid belt counterparts
A Machine-Checked Formalization of the Generic Model and the Random Oracle Model
Most approaches to the formal analyses of cryptographic protocols make the perfect cryptography assumption, i.e. the hypothese that there is no way to obtain knowledge about the plaintext pertaining to a ciphertext without knowing the key. Ideally, one would prefer to rely on a weaker hypothesis on the computational cost of gaining information about the plaintext pertaining to a ciphertext without knowing the key. Such a view is permitted by the Generic Model and the Random Oracle Model which provide non-standard computational models in which one may reason about the computational cost of breaking a cryptographic scheme. Using the proof assistant Coq, we provide a machine-checked account of the Generic Model and the Random Oracle Mode
RATAN-600 7.6-cm Deep Sky Strip Surveys at the Declination of the SS433 Source During the 1980-1999 Period. Data Reduction and the Catalog of Radio Sources in the Right-Ascension Interval 7h < R.A. < 17h
We use two independent methods to reduce the data of the surveys made with
RATAN-600 radio telescope at 7.6 cm in 1988-1999 at the declination of the
SS433 source. We also reprocess the data of the "Cold" survey (1980-1981). The
resulting RCR (RATAN COLD REFINED) catalog contains the right ascensions and
fluxes of objects identified with those of the NVSS catalog in the
right-ascension interval 7h < R.A. < 17h. We obtain the spectra of the radio
sources and determine their spectral indices at 3.94 and 0.5 GHz. The spectra
are based on the data from all known catalogs available from the CATS, Vizier,
and NED databases, and the flux estimates inferred from the maps of the VLSS
and GB6 surveys. For 245 of the 550 objects of the RCR catalog the fluxes are
known at two frequencies only: 3.94 GHz (RCR) and 1.4 GHz (NVSS). These are
mostly sources with fluxes smaller than 30 mJy. About 65% of these sources have
flat or inverse spectra (alpha > -0.5). We analyze the reliability of the
results obtained for the entire list of objects and construct the histograms of
the spectral indices and fluxes of the sources. Our main conclusion is that all
10-15 mJy objects found in the considered right-ascension interval were already
included in the decimeter-wave catalogs.Comment: 26 pages, 18 figure
Relic densities including Sommerfeld enhancements in the MSSM
We have developed a general formalism to compute Sommerfeld enhancement (SE)
factors for a multi-state system of fermions, in all possible spin
configurations and with generic long-range interactions. We show how to include
such SE effects in an accurate calculation of the thermal relic density for
WIMP dark matter candidates. We apply the method to the MSSM and perform a
numerical study of the relic abundance of neutralinos with arbitrary
composition and including the SE due to the exchange of the W and Z bosons,
photons and Higgses. We find that the relic density can be suppressed by a
factor of a few in a seizable region of the parameter space, mostly for
Wino-like neutralino with mass of a few TeV, and up to an order of magnitude
close to a resonance.Comment: 23 pages, 7 figures; table 1 corrected and rearranged, numerical
results practically unchanged, matches published versio
- …