5,758 research outputs found
Increased Yield of ttbb at Hadron Colliders in Low-Energy Supersymmetry
Light bottom squarks and gluinos have been invoked to explain the b quark
pair production excess at the Tevatron. We investigate the associated
production of ttbb at hadron colliders in this scenario, and find that the
rates for this process are enhanced over the Standard Model prediction. If
light gluinos exist, it may be possible to detect them at the Tevatron, and
they could easily be observed at the LHC.Comment: 5p, references added, version accepted to PR
The ultracool dwarf DENIS-P J104814.7-395606. Chromospheres and coronae at the low-mass end of the main-sequence
We have obtained an XMM-Newton observation and a broad-band spectrum from the
ultraviolet to the near infrared with X-Shooter for one of the nearest M9
dwarfs, DENIS-P J1048-3956 (4pc). We integrate these data by a compilation of
activity parameters for ultracool dwarfs from the literature with the aim to
advance our understanding of these objects by comparing them to early-M type
dwarf stars and the Sun.
Our deep XMM-Newton observation has led to the first X-ray detection of
DENIS-P J1048-3956 (log Lx = 25.1) as well as the first measurement of its V
band brightness (V = 17.35mag). Flux-flux relations between X-ray and
chromospheric activity indicators are here for the first time extended into the
regime of the ultracool dwarfs. The approximate agreement of DENIS-P J1048-3956
and other ultracool dwarfs with flux-flux relations for early-M dwarfs suggests
that the same heating mechanisms work in the atmospheres of ultracool dwarfs,
albeit weaker as judged from their lower fluxes. The observed Balmer decrements
of DENIS-P J1048-3956 are compatible with optically thick plasma in LTE at low,
nearly photospheric temperature or optically thin LTE plasma at 20000K.
Describing the decrements with CaseB recombination requires different emitting
regions for Halpha and the higher Balmer lines. The high observed Halpha/Hbeta
flux ratio is also poorly fitted by the optically thin models. We derive a
similarly high value for the Halpha/Hbeta ratio of vB10 and LHS2065 and
conclude that this may be a characteristic of ultracool dwarfs. We add DENIS-P
J1048-3956 to the list of ultracool dwarfs detected in both the radio and the
X-ray band. The Benz-Guedel relation between radio and X-ray luminosity of
late-type stars is well-known to be violated by ultracool dwarfs. We speculate
on the presence of two types of ultracool dwarfs with distinct radio and X-ray
behavior.Comment: accepted for publication in Astronomy & Astrophysic
Finite-Width Effects in Top Quark Production at Hadron Colliders
Production cross sections for t\bar{t} and t\bar{t}j events at hadron
colliders are calculated, including finite width effects and off resonance
contributions for the entire decay chain, t --> bW --> b\ell\nu, for both top
quarks. Resulting background rates to Higgs search at the CERN LHC are updated
for inclusive H --> WW studies and for H --> \tau\tau and H --> WW decays in
weak boson fusion events. Finite width effects are large, increasing
t\bar{t}(j) rates by 20% or more, after typical cuts which are employed for
top-background rejection.Comment: 32 pages, 11 figures, 7 tables; minor changes, reference added, to be
published in Phys. Rev.
Bright X-ray flares in Orion young stars from COUP: evidence for star-disk magnetic fields?
We have analyzed a number of intense X-ray flares observed in the Chandra
Orion Ultradeep Project (COUP), a 13 days observation of the Orion Nebula
Cluster (ONC). Analysis of the flare decay allows to determine the size, peak
density and magnetic field of the flaring structure. A total of 32 events (the
most powerful 1% of COUP flares), have sufficient statistics for the analysis.
A broad range of decay times (from 10 to 400 ks) are present in the sample.
Peak flare temperatures are often very high, with half of the flares in the
sample showing temperatures in excess of 100 MK. Significant sustained heating
is present in the majority of the flares. The magnetic structures which are
found, are in a number of cases very long, with semi-lengths up to 10^12 cm,
implying the presence of magnetic fields of hundreds of G extending to
comparable distance from the stellar photosphere. These very large sizes for
the flaring structures ($ >> R_*) are not found in more evolved stars, where,
almost invariably, the same type of analysis results in structures with L <=
R_*. As the majority of young stars in the ONC are surrounded by disks, we
speculate that the large magnetic structures which confine the flaring plasma
are actually the same type of structures which channel the plasma in the
magnetospheric accretion paradigm, connecting the star's photosphere with the
accretion disk.Comment: Accepted to ApJS, COUP special issu
Characteristic slepton signal in anomaly mediated SUSY breaking models via gauge boson fusion at the LHC
We point out that slepton pairs produced via gauge boson fusion in anomaly
mediated supersymmetry breaking (AMSB) model have very characteristic and
almost clean signal at the Large Hadron Collider. In this article, we discuss
how one lepton associated with missing energy and produced in between two
high- and high-mass forward jets can explore quite heavy sleptons in this
scenario.Comment: Version to appear in Physical Review
X-Ray flares in Orion Young Stars. II. Flares, Magnetospheres, and Protoplanetary Disks
We study the properties of powerful X-ray flares from 161 pre-main sequence
(PMS) stars observed with the Chandra X-ray Observatory in the Orion Nebula
region. Relationships between flare properties, protoplanetary disks and
accretion are examined in detail to test models of star-disk interactions at
the inner edge of the accretion disks. Previous studies had found no
differences in flaring between diskfree and accreting systems other than a
small overall diminution of X-ray luminosity in accreting systems. The most
important finding is that X-ray coronal extents in fast-rotating diskfree stars
can significantly exceed the Keplerian corotation radius, whereas X-ray loop
sizes in disky and accreting systems do not exceed the corotation radius. This
is consistent with models of star-disk magnetic interaction where the inner
disk truncates and confines the PMS stellar magnetosphere. We also find two
differences between flares in accreting and diskfree PMS stars. First, a
subclass of super-hot flares with peak plasma temperatures exceeding 100 MK are
preferentially present in accreting systems. Second, we tentatively find that
accreting stars produce flares with shorter durations. Both results may be
consequences of the distortion and destabilization of the stellar magnetosphere
by the interacting disk. Finally, we find no evidence that any flare types,
even slow-rise flat-top flares are produced in star-disk magnetic loops. All
are consistent with enhanced solar long-duration events with both footprints
anchored in the stellar surface.Comment: Accepted for publication in ApJ (07/17/08); 46 pages, 14 figures, 2
table
On the Absorption of X-rays in the Interstellar Medium
We present an improved model for the absorption of X-rays in the ISM intended
for use with data from future X-ray missions with larger effective areas and
increased energy resolution such as Chandra and XMM, in the energy range above
100eV. Compared to previous work, our formalism includes recent updates to the
photoionization cross section and revised abundances of the interstellar
medium, as well as a treatment of interstellar grains and the H2molecule. We
review the theoretical and observational motivations behind these updates and
provide a subroutine for the X-ray spectral analysis program XSPEC that
incorporates our model.Comment: ApJ, in press, for associated software see
http://astro.uni-tuebingen.de/nh
Direct probes of R-parity-violating supersymmetric couplings via single-top-squark production
We study the s-channel production of a single top squark in hadron collisions
through an R-parity-violating mechanism, examining in detail the case in which
the squark decays through an R-parity-conserving process into a bottom quark, a
lepton, and missing energy. We show that the top squark can be discovered if
its mass is less than 400 GeV, or that the current bound on the size of the
R-parity-violating couplings can be reduced by up to one order of magnitude
with existing data and by two orders of magnitude at the forthcoming run II of
the Fermilab Tevatron.Comment: To appear in Phys. Rev. D; 32 pgs., 17 ps figs., uses RevTeX; 1 new
fig., slight textual clarification
The subcellular organization of Madin-Darby canine kidney cells during the formation of a polarized epithelium.
Rotation and X-ray emission from protostars
The ASCA satellite has recently detected variable hard X-ray emission from
two Class I protostars in the rho Oph cloud, YLW15 (IRS43) and WL6, with a
characteristic time scale ~20h. In YLW15, the X-ray emission is in the form of
quasi-periodic energetic flares, which we explain in terms of strong magnetic
shearing and reconnection between the central star and the accretion disk. In
WL6, X-ray flaring is rotationally modulated, and appears to be more like the
solar-type magnetic activity ubiquitous on T Tauri stars. We find that YLW15 is
a fast rotator (near break-up), while WL6 rotates with a significantly longer
period. We derive a mass M_\star ~ 2 M_\odot and \simlt 0.4 M_\odot for the
central stars of YLW15 and WL6 respectively. On the long term, the interactions
between the star and the disk results in magnetic braking and angular momentum
loss of the star. On time scales t_{br} ~ a few 10^5 yrs, i.e., of the same
order as the estimated duration of the Class~I protostar stage. Close to the
birthline there must be a mass-rotation relation, t_{br} \simpropto M_\star,
such that stars with M_\star \simgt 1-2 M_\odot are fast rotators, while their
lower-mass counterparts have had the time to spin down. The rapid rotation and
strong star-disk magnetic interactions of YLW15 also naturally explain the
observation of X-ray ``superflares''. In the case of YLW15, and perhaps also of
other protostars, a hot coronal wind (T~10^6 K) may be responsible for the VLA
thermal radio emission. This paper thus proposes the first clues to the
rotation status and evolution of protostars.Comment: 13 pages with 6 figures. To be published in ApJ (April 10, 2000 Part
1 issue
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