5,725 research outputs found
A study of top polarization in single-top production at the LHC
This paper complements the study of single top production at the LHC aiming
to estimate the sensitivity of different observables to the magnitude of the
effective couplings. In a previous paper the dominant -gluon fusion
mechanism was considered, while here we extend the analysis to the subdominant
(10% with our set of experimental cuts) s-channel process. In order to
distinguish left from right effective couplings it is required to consider
polarized cross-sections and/or include effects. The spin of the top is
accessible only indirectly by measuring the angular distribution of its decay
products. We show that the presence of effective right-handed couplings implies
necessarily that the top is not in a pure spin state. We discuss to what extent
quantum interference terms can be neglected in the measurement and therefore
simply multiply production and decay probabilities clasically. The coarsening
involved in the measurement process makes this possible. We determine for each
process the optimal spin basis where theoretical errors are minimized and,
finally, discuss the sensitivity in the s-channel to the effective right-handed
coupling. The results presented here are all analytical and include
corrections. They are derived within the narrow width approximation for the
top.Comment: 30 pages, 14 figure
XMM-Newton observations of the sigma Ori cluster. II. Spatial and spectral analysis of the full EPIC field
We present the results of an XMM-Newton observation of the young (~2-4 Myr)
cluster around the hot star sigma Orionis. In a previous paper we presented the
analysis of the RGS spectrum of the central hot star; here we discuss the
results of the analysis of the full EPIC field. We have detected 175 X-ray
sources, 88 of which have been identified with cluster members, including very
low-mass stars down to the substellar limit. We detected eleven new possible
candidate members from the 2MASS catalogue. We find that late-type stars have a
median log L_X/L_bol ~ -3.3, i.e. very close to the saturation limit. We
detected significant variability in ~40% of late-type members or candidates,
including 10 flaring sources; rotational modulation is detected in one K-type
star and possibly in another 3 or 4 stars. Spectral analysis of the brightest
sources shows typical quiescent temperatures in the range T_1 ~ 0.3-0.8 keV and
T_2 ~ 1-3 keV, with subsolar abundances Z ~ 0.1-0.3 Z_sun, similar to what is
found in other star-forming regions and associations. We find no significant
difference in the spectral properties of classical and weak-lined T Tauri
stars, although classical T Tauri stars tend to be less X-ray luminous than
weak-lined T Tauri stars.Comment: 20 pages, 10 figures, 6 tables. Accepted by A&
XMM-Newton investigations of the Lambda Orionis star-forming region (XILO). I. The young cluster Collinder 69
This is the first paper of a series devoted to the Lambda Orionis
star-forming region, from the X-ray perspective, which will provide a
comprehensive view of this complex region. In this paper we focus in uncovering
the population of the central, young cluster Collinder 69 (C69), and in
particular those diskless members not identified by previous near- and
mid-infrared surveys, and to establish the X-ray luminosity function for the
association. We have combined two exposures taken with the XMM-Newton satellite
with an exhaustive data set of optical, near- and mid-infrared photometry to
assess the membership of the X-ray sources based on color-color and
color-magnitude diagrams, as well as other properties, such as effective
temperatures, masses and bolometric luminosities.
We detected a total of 164 X-ray sources, of which 66 are probable and
possible cluster members. A total of 16 are newly identified probable members.
The two XMM-Newton pointings east and west of the cluster center have allowed
us to verify the heterogeneous spatial distribution of young stars, probably
related to the large scale structure of the region. The disk fraction of the
X-ray detected cluster sample is very low, close to 10%, in remarkable contrast
to the low-mass stellar and substellar population (mostly undetected in X-rays)
where the disk fraction reaches about 50%. The X-ray luminosity function of C69
provides support for an age of several Myr when compared with other well known
young associations. With our improved cluster census we confirm previous
reports on the untypically low disk fraction compared to other clusters of
several Myr age. The different disk fractions of X-ray detected (essentially
solar-like) and undetected (mostly low-mass stars and brown dwarfs) members can
be understood as a consequence of a mass-dependence of the time-scale for disk
evolution.Comment: 38 pages, 16 figure
X-ray Emission near the Substellar Limit: The sigma Orionis and Taurus Star Forming Regions
We have carried out an extensive search for X-ray emission from young, very
low-mass objects near and beyond the substellar limit, making use of archived
ROSAT PSPC and HRI observations pointed at Brown Dwarfs and Brown Dwarf
candidates in the young sigma Orionis and Taurus-Auriga associations. In sigma
Ori we identify three Brown Dwarf candidates with X-ray sources; in
Taurus-Auriga we add one further X-ray detection of a Brown Dwarf to the list
published earlier. We combine this data with all previously X-ray detected
Brown Dwarfs and Brown Dwarf candidates in young stellar associations and star
forming regions to perform a study of stellar activity parameters on the as yet
largest sample of young, very low mass objects. A similar relation between
X-ray and bolometric luminosity, and H-alpha emission, respectively, as is
known for T Tauri stars seems to hold for young objects down to the substellar
limit, too. No signs for a change in X-ray activity are found on the transition
to substellar masses.Comment: 9 pages, 5 (9) figures; fig. 1-4 are available only in the preprint
or in the ps-file via
, accepted for
publication in A&
Viscoelastic response of contractile filament bundles
The actin cytoskeleton of adherent tissue cells often condenses into filament
bundles contracted by myosin motors, so-called stress fibers, which play a
crucial role in the mechanical interaction of cells with their environment.
Stress fibers are usually attached to their environment at the endpoints, but
possibly also along their whole length. We introduce a theoretical model for
such contractile filament bundles which combines passive viscoelasticity with
active contractility. The model equations are solved analytically for two
different types of boundary conditions. A free boundary corresponds to stress
fiber contraction dynamics after laser surgery and results in good agreement
with experimental data. Imposing cyclic varying boundary forces allows us to
calculate the complex modulus of a single stress fiber.Comment: Revtex with 24 pages, 7 Postscript figures included, accepted for
publication in Phys. Rev.
HELAS and MadGraph with spin-3/2 particles
Fortran subroutines to calculate helicity amplitudes with massive spin-3/2
particles, such as massive gravitinos, which couple to the standard model and
supersymmetric particles via the supercurrent, are added to the HELAS (HELicity
Amplitude Subroutines) library. They are coded in such a way that arbitrary
amplitudes with external gravitinos can be generated automatically by MadGraph,
after slight modifications. All the codes have been tested carefully by making
use of the gauge invariance of the helicity amplitudes.Comment: 12 pages, 4 figures; sections rearranged, typos corrected, version to
appear in EPJ
Down Type Isosinglet Quarks in ATLAS
We evaluate the discovery reach of the ATLAS experiment for down type
isosinglet quarks, , using both their neutral and charged decay channels,
namely the process with subsequent decays resulting in
, and final states.
The integrated luminosity required for observation of a heavy quark is
estimated for a mass range between 600 and 1000 GeV using the combination of
results from different search channels.Comment: 12 page
Spectroscopy of Very Low Mass Stars and Brown Dwarfs in the Lambda Orionis Star Forming Region
Context. Most observational studies so far point towards brown dwarfs sharing
a similar formation mechanism as the one accepted for low mass stars. However,
larger databases and more systematic studies are needed before strong
conclusions can be reached. Aims. In this second paper of a series devoted to
the study of the spectroscopic properties of the members of the Lambda Orionis
Star Forming Region, we study accretion, activity and rotation for a wide set
of spectroscopically confirmed members of the central star cluster Collinder 69
to draw analogies and/or differences between the brown dwarf and stellar
populations of this cluster. Moreover, we present comparisons with other star
forming regions of similar and different ages to address environmental effects
on our conclusions. Methods. We study prominent photospheric lines to derive
rotational velocities and emission lines to distinguish between accretion
processes and chromospheric activity. In addition, we include information about
disk presence and X-ray emission. Results. We report very large differences in
the disk fractions of low mass stars and brown dwarfs (~58%) when compared to
higher mass stars (26+4-3%) with 0.6 Msun being the critical mass we find for
this dichotomy. As a byproduct, we address the implications of the spatial
distribution of disk and diskless members in the formation scenario of the
cluster itself. We have used the Halpha emission to discriminate among
accreting and non-accreting sources finding that 38+8-7% of sources harboring
disks undergo active accretion and that his percentage stays similar in the
substellar regime. For those sources we have estimated accretion rates.
Finally, regarding rotational velocities, we find a high dispersion in vsin(i)
which is even larger among the diskless population.Comment: Accepted for publication in A&A. 18 figs including the Appendix and
an online tabl
Theory and simulation of the nematic zenithal anchoring coefficient
Combining molecular simulation, Onsager theory and the elastic description of
nematic liquid crystals, we study the dependence of the nematic liquid crystal
elastic constants and the zenithal surface anchoring coefficient on the value
of the bulk order parameter
Sum rules for helicity amplitudes from BRS invariance
The BRS invariance of the electroweak gauge theory leads to relationships
between amplitudes with external massive gauge bosons and amplitudes where some
of these gauge bosons are replaced with their corresponding Nambu-Goldstone
bosons. Unlike the equivalence theorem, these identities are exact at all
energies. In this paper we discuss such identities which relate the process
to and production. By using
a general form-factor decomposition for , and amplitudes, these identities are
expressed as sum rules among scalar form factors. Because these sum rules may
be applied order by order in perturbation theory, they provide a powerful test
of higher order calculations. By using additional Ward-Takahashi identities we
find that the various contributions are divided into separately gauge-invariant
subsets, the sum rules applying independently to each subset. After a general
discussion of the application of the sum rules we consider the one-loop
contributions of scalar-fermions in the Minimal Supersymmetric Standard Model
as an illustration.Comment: 37 pages, including 16 figure
- …