845 research outputs found
Single top-quark production by strong and electroweak supersymmetric flavor-changing interactions at the LHC
(Abridged) We report on a complete study of the single top-quark production
by direct supersymmetric flavor-changing neutral-current (FCNC) processes at
the LHC. The total cross section for pp(gg)->t\bar{c}+\bar{t}c is computed at
the 1-loop order within the unconstrained Minimal Supersymmetric Standard Model
(MSSM). The present study extends the results of the supersymmetric strong
effects (SUSY-QCD), which were advanced by some of us in a previous work, and
includes the computation of the full supersymmetric electroweak corrections
(SUSY-EW). Our analysis of pp(gg)->t\bar{c}+\bar{t}c in the MSSM has been
performed in correspondence with the stringent low-energy constraints from b->s
gamma. In the most favorable scenarios, the SUSY-QCD contribution can give rise
to production rates of around 10^5 events per 100 fb^{-1} of integrated
luminosity. Furthermore, we show that there exist regions of the MSSM parameter
space where the SUSY-EW correction becomes sizeable. In the SUSY-EW favored
regions, one obtains lower, but still appreciable, event production rates that
can reach the 10^3 level for the same range of integrated luminosity. We study
also the possible reduction in the maximum event rate obtained from the full
MSSM contribution if we additionally include the constraints from
B^0_s-\bar{B}^0_s. In view of the fact that the FCNC production of heavy quark
pairs of different flavors is extremely suppressed in the SM, the detection of
a significant number of these events could lead to evidence of new physics --
of likely supersymmetric origin.Comment: LaTex, 35 pages, typos corrected. Version accepted in JHE
Discovery of a Low-Mass Brown Dwarf Companion of the Young Nearby Star G196-3
A substellar-mass object in orbit at about 300 astronomical units (AU) from
the young low-mass star G196-3 was detected by direct imaging. Optical and
infrared photometry and low- and intermediate-resolution spectroscopy of the
faint companion, hereafter referred to as G196-3B, confirms its cool atmosphere
and allows its mass to be estimated at 25^{+15}_{-10} Jupiter masses. The
separation between both objects and their mass ratio suggest the fragmentation
of a collapsing cloud as the most likely origin for G196-3B, but alternatively
it could have originated from a proto-planetary disc which has been dissipated.
Whatever the formation process was, the young age of the primary star (about
100 Myr) demonstrates that substellar companions can form in short time scales.Comment: Published in Science (13 Nov). One color figur
Spectroscopy of Hyades L dwarf candidates
We present the results of photometric, astrometric, and spectroscopic
follow-up of L dwarf candidates identified in the Hyades cluster by Hogan et
al. (2008). We obtained low-resolution optical spectroscopy with the OSIRIS
spectrograph on the Gran Telescopio de Canarias for all 12 L dwarf candidates
as well as new J-band imaging for a subsample of eight to confirm their proper
motion. We also present mid-infrared photometry from the Wise Field Infrared
Survey Explorer (WISE) for the Hyades L and T dwarf candidates and estimate
their spectroscopic distances, effective temperatures, and masses. We confirm
the cool nature of several L dwarf candidates and confirm astrometrically their
membership, bridging the gap between the coolest M dwarfs and the two T dwarfs
previously reported in the Hyades cluster. These members represent valuable
spectral templates at an age of 625 Myr and slightly super solar metallicity
(Fe/H=+0.13). We update the Hyades mass function across the hydrogen-burning
limit and in the substellar regime. We confirm a small number numbers of
very-low-mass members below ~0.1 Msun belonging to the Hyades cluster.Comment: 12 pages, 7 figures, 3 tables; accepted for publication in MNRA
VLT X-shooter spectroscopy of the nearest brown dwarf binary
The aim of the project is to characterise both components of the nearest
brown dwarf sytem to the Sun, WISE J104915.57-531906.1 (=Luhman16AB) at optical
and near-infrared wavelengths. We obtained high signal-to-noise
intermediate-resolution (R~6000-11000) optical (600-1000 nm) and near-infrared
(1000-2480nm) spectra of each component of Luhman16AB, the closest brown dwarf
binary to the Sun, with the X-Shooter instrument on the Very Large Telescope.
We classify the primary and secondary of the Luhman16 system as L6-L7.5 and
T0+/-1, respectively, in agreement with previous measurements published in the
literature. We present measurements of the lithium pseudo-equivalent widths,
which appears of similar strength on both components (8.2+/-1.0 Angstroms and
8.4+/-1.5 Angstroms for the L and T components, respectively). The presence of
lithium (Lithium 7) in both components imply masses below 0.06 Msun while
comparison with models suggests lower limits of 0.04 Msun. The detection of
lithium in the T component is the first of its kind. Similarly, we assess the
strength of other alkali lines (e.g. pseudo-equivalent widths of 6-7 Angstroms
for RbI and 4-7 Angstroms for CsI) present in the optical and near-infrared
regions and compare with estimates for L and T dwarfs. We also derive effective
temperatures and luminosities of each component of the binary: -4.66+/-0.08 dex
and 1305(+180)(-135) for the L dwarf and -4.68+/-0.13 dex and 1320(+185)(-135)
for the T dwarf, respectively. Using our radial velocity determinations, the
binary does not appear to belong to any of the well-known moving group. Our
preliminary theoretical analysis of the optical and J-band spectra indicates
that the L- and T-type spectra can be reproduced with a single temperature and
gravity but different relative chemical abundances which impact strongly the
spectral energy distribution of L/T transition objects.Comment: 12 pages, 9 figure, 3 tables, accepted to A&
2MASS J154043.42-510135.7: a new addition to the 5 pc population
The aim of the project is to find the stars nearest to the Sun and to
contribute to the completion of the stellar and substellar census of the solar
neighbourhood. We identified a new late-M dwarf within 5 pc, looking for high
proper motion sources in the 2MASS-WISE cross-match. We collected astrometric
and photometric data available from public large-scale surveys. We complemented
this information with low-resolution optical and near-infrared spectroscopy
with instrumentation on the ESO NTT to confirm the nature of our candidate. We
also present a high-quality medium-resolution VLT/X-shooter spectrum covering
the 400 to 2500 nm wavelength range. We classify this new neighbour as an
M7.00.5 dwarf using spectral templates from the Sloan Digital Sky Survey
and spectral indices. Lithium absorption at 670.8 nm is not detected in the
X-shooter spectrum, indicating that the M7 dwarf is older than 600 Myr and more
massive than 0.06 M. We also derive a trigonometric distance of 4.4
pc, in agreement with the spectroscopic distance estimate, making
2MASS\,J154043.42510135.7 the nearest M7 dwarf to the Sun. This
trigonometric distance is somewhat closer than the 6 pc distance reported
by the ALLWISE team, who independently identified this object recently. This
discovery represents an increase of 25\% in the number of M7--M8 dwarfs already
known at distances closer than 8\,pc from our Sun. We derive a density of
\,=\,1.90.910\,pc for M7 dwarfs in the 8 pc
volume, a value similar to those quoted in the literature. This new ultracool
dwarf is among the 50 nearest systems to the Sun, demonstrating that our
current knowledge of the stellar census within the 5 pc sample remains
incomplete. 2M1540 represents a unique opportunity to search for extrasolar
planets around ultracool dwarfs due to its proximity and brightness.Comment: 8 pages, 5 figures. Acepted in Astronomy & Astrophysics (15/05/2005
Binary frequency of planet-host stars at wide separations: A new brown dwarf companion to a planet-host star
The aim of the project is to improve our knowledge on the multiplicity of
planet-host stars at wide physical separations.
We cross-matched approximately 6200 square degree area of the Southern sky
imaged by the Visible Infrared Survey Telescope for Astronomy (VISTA)
Hemisphere Survey (VHS) with the Two Micron All Sky Survey (2MASS) to look for
wide common proper motion companions to known planet-host stars. We
complemented our astrometric search with photometric criteria.
We confirmed spectroscopically the co-moving nature of seven sources out of
16 companion candidates and discarded eight, while the remaining one stays as a
candidate. Among these new wide companions to planet-host stars, we discovered
a T4.5 dwarf companion at 6.3 arcmin (~9000 au) from HIP70849, a K7V star which
hosts a 9 Jupiter mass planet with an eccentric orbit. We also report two new
stellar M dwarf companions to one G and one metal-rich K star. We infer stellar
and substellar binary frequencies for our complete sample of 37 targets of
5.4+/-3.8% and 2.7+/-2.7% (1 sigma confidence level), respectively, for
projected physical separations larger than ~60-160 au assuming the range of
distances of planet-host stars (24-75 pc). These values are comparable to the
frequencies of non planet-host stars. We find that the period-eccentricity
trend holds with a lack of multiple systems with planets at large
eccentricities (e > 0.2) for periods less than 40 days. However, the lack of
planets more massive than 2.5 Jupiter masses and short periods (<40 days)
orbiting single stars is not so obvious due to recent discoveries by
ground-based transit surveys and space missions.Comment: Accepted for publication in A&A, 13 pages, 5 figures, 3 tables,
optical spectra will be available at CDS Strasbour
The single t-quark productions via the flavor-changing processes in the topcolor-assisted technicolor model at the hadron colliders
In the framework of topcolor-assisted technicolor(TC2) model, there exist
tree-level flavor-changing (FC) couplings which can result in the loop-level FC
coupling . Such coupling can contribute significant clues at the
forthcoming Large Hadron Collider (LHC) experiments. In this paper, based on
the TC2 model, we study some single t-quark production processes involving
coupling at the Tevatron and LHC: .
We calculate the cross sections of these processes. The results show that the
cross sections at the Tevatron are too small to observe the signal, but at the
LHC it can reach a few pb. With the high luminosity, the LHC has considerable
capability to find the single t-quark signal produced via some FC processes
involving coupling . On the other hand, these processes can also provide
some valuable information of the coupling with detailed study of the
processes and furthermore provide the reliable evidence to test the TC2 model.Comment: 15 pages, 10 figure
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