3,432 research outputs found
Search for New Physics via Single Top Production at the LHC
We consider single-top production as a probe for new physics effects at the
Large Hadron Collider (LHC). We argue that for natural theories a small
deviation from the Standard Model tree-level couplings in this reaction can be
parameterized by 3 higher dimension operators. Precision measurement of these
effective couplings in the single-top events, via studying their interference
effects with the SM contributions, can discriminate several new physics models.
In particular, combining the production rate of three single-top production
modes will provide a severe test of the Little Higgs model with T-parity. We
find that at the LHC, a 5% accuracy in the measurement of the single-top cross
sections would probe the new physics scale up to about .Comment: version to appear in Physics Letters
The quark - antiquark asymmetry of the strange sea of the nucleon
The strange sea of the proton is generally assumed to have quark - antiquark
symmetry. However it has been known for some time that non-perturbative
processes involving the meson cloud of the proton may break this symmetry.
Recently this has been of interest as it affects the analysis of the so-called
`NuTeV anomaly', and could explain the large discrepancy between the NuTeV
measurement of and the currently accepted value. In this
paper we re-examine strange - anti-strange asymmetry using the meson cloud
model. We calculate contributions to the strange sea arising from fluctuations
in the proton wavefunction to states containing either Lambda or Sigma hyperons
together with either Kaons or pseudovector mesons. We find that we
should not ignore fluctuations involving mesons in this picture. The
strange sea asymmetry is found to be small, and is unlikely to affect the
analysis of the Llewellyn-Smith cross section ratios or the Paschos-Wolfenstein
relationship.Comment: 12 pages, 3 figure
Vesicles and actin are targeted to the cleavage furrow via furrow microtubules and the central spindle
During cytokinesis, cleavage furrow invagination requires an actomyosin-based contractile ring and addition of new membrane. Little is known about how this actin and membrane traffic to the cleavage furrow. We address this through live analysis of fluorescently tagged vesicles in postcellularized Drosophila melanogaster embryos. We find that during cytokinesis, F-actin and membrane are targeted as a unit to invaginating furrows through formation of F-actin–associated vesicles. F-actin puncta strongly colocalize with endosomal, but not Golgi-derived, vesicles. These vesicles are recruited to the cleavage furrow along the central spindle and a distinct population of microtubules (MTs) in contact with the leading furrow edge (furrow MTs). We find that Rho-specific guanine nucleotide exchange factor mutants, pebble (pbl), severely disrupt this F-actin–associated vesicle transport. These transport defects are a consequence of the pbl mutants' inability to properly form furrow MTs and the central spindle. Transport of F-actin–associated vesicles on furrow MTs and the central spindle is thus an important mechanism by which actin and membrane are delivered to the cleavage furrow
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Nuf, a Rab11 Effector, Maintains Cytokinetic Furrow Integrity by Promoting Local Actin Polymerization
Plasma membrane ingression during cytokinesis involves both actin remodeling and vesicle-mediated membrane addition. Vesicle-based membrane delivery from the recycling endosome (RE) has an essential but ill-defined involvement in cytokinesis. In the Drosophila melanogaster early embryo, Nuf (Nuclear fallout), a Rab11 effector which is essential for RE function, is required for F-actin and membrane integrity during furrow ingression. We find that in nuf mutant embryos, an initial loss of F-actin at the furrow is followed by loss of the associated furrow membrane. Wild-type embryos treated with Latrunculin A or Rho inhibitor display similar defects. Drug- or Rho-GTP–induced increase of actin polymerization or genetically mediated decrease of actin depolymerization suppresses the nuf mutant F-actin and membrane defects. We also find that RhoGEF2 does not properly localize at the furrow in nuf mutant embryos and that RhoGEF2–Rho1 pathway components show strong specific genetic interactions with Nuf. We propose a model in which RE-derived vesicles promote furrow integrity by regulating the rate of actin polymerization through the RhoGEF2–Rho1 pathway
General Analysis of Single Top Production and W Helicity in Top Decay
We provide a framework for the analysis of the W boson helicity in the decay
of the top quark that is based on a general effective tbW coupling. Four
independent coupling coefficients can be uniquely determined by the fractions
of longitudinal and transverse W boson polarizations as well as the single top
production rates for the t-channel and the s-channel processes. The knowledge
of these coefficients can be used to discriminate models of electroweak
symmetry breaking.Comment: Typo on Eqs. 5 and 6 correcte
Nuf, a Rab11 effector, maintains cytokinetic furrow integrity by promoting local actin polymerization
Plasma membrane ingression during cytokinesis involves both actin remodeling and vesicle-mediated membrane addition. Vesicle-based membrane delivery from the recycling endosome (RE) has an essential but ill-defined involvement in cytokinesis. In the Drosophila melanogaster early embryo, Nuf (Nuclear fallout), a Rab11 effector which is essential for RE function, is required for F-actin and membrane integrity during furrow ingression. We find that in nuf mutant embryos, an initial loss of F-actin at the furrow is followed by loss of the associated furrow membrane. Wild-type embryos treated with Latrunculin A or Rho inhibitor display similar defects. Drug- or Rho-GTP–induced increase of actin polymerization or genetically mediated decrease of actin depolymerization suppresses the nuf mutant F-actin and membrane defects. We also find that RhoGEF2 does not properly localize at the furrow in nuf mutant embryos and that RhoGEF2–Rho1 pathway components show strong specific genetic interactions with Nuf. We propose a model in which RE-derived vesicles promote furrow integrity by regulating the rate of actin polymerization through the RhoGEF2–Rho1 pathway
Wolbachia Utilizes Host Microtubules and Dynein for Anterior Localization in the Drosophila Oocyte
To investigate the role of the host cytoskeleton in the maternal transmission of the endoparasitic bacteria Wolbachia, we have characterized their distribution in the female germ line of Drosophila melanogaster. In the germarium, Wolbachia are distributed to all germ cells of the cyst, establishing an early infection in the cell destined to become the oocyte. During mid-oogenesis, Wolbachia exhibit a distinct concentration between the anterior cortex and the nucleus in the oocyte, where many bacteria appear to contact the nuclear envelope. Following programmed rearrangement of the microtubule network, Wolbachia dissociate from this anterior position and become dispersed throughout the oocyte. This localization pattern is distinct from mitochondria and all known axis determinants. Manipulation of microtubules and cytoplasmic Dynein and Dynactin, but not Kinesin-1, disrupts anterior bacterial localization in the oocyte. In live egg chambers, Wolbachia exhibit movement in nurse cells but not in the oocyte, suggesting that the bacteria are anchored by host factors. In addition, we identify mid-oogenesis as a period in the life cycle of Wolbachia in which bacterial replication occurs. Total bacterial counts show that Wolbachia increase at a significantly higher rate in the oocyte than in the average nurse cell, and that normal Wolbachia levels in the oocyte depend on microtubules. These findings demonstrate that Wolbachia utilize the host microtubule network and associated proteins for their subcellular localization in the Drosophila oocyte. These interactions may also play a role in bacterial motility and replication, ultimately leading to the bacteria's efficient maternal transmission
Search for precursor eruptions among Type IIb supernovae
The progenitor stars of several Type IIb supernovae (SNe) show indications
for extended hydrogen envelopes. These envelopes might be the outcome of
luminous energetic pre-explosion events, so-called precursor eruptions. We use
the Palomar Transient Factory (PTF) pre-explosion observations of a sample of
27 nearby Type IIb SNe to look for such precursors during the final years prior
to the SN explosion. No precursors are found when combining the observations in
15-day bins, and we calculate the absolute-magnitude-dependent upper limit on
the precursor rate. At the 90% confidence level, Type IIb SNe have on average
precursors as bright as absolute -band magnitude in the final
3.5 years before the explosion and events over the final year. In
contrast, precursors among SNe IIn have a times higher rate. The
kinetic energy required to unbind a low-mass stellar envelope is comparable to
the radiated energy of a few-weeks-long precursor which would be detectable for
the closest SNe in our sample. Therefore, mass ejections, if they are common in
such SNe, are radiatively inefficient or have durations longer than months.
Indeed, when using 60-day bins a faint precursor candidate is detected prior to
SN 2012cs (% false-alarm probability). We also report the detection of
the progenitor of SN 2011dh which does not show detectable variability over the
final two years before the explosion. The suggested progenitor of SN 2012P is
still present, and hence is likely a compact star cluster, or an unrelated
object.Comment: 16 pages, 10 figure
FCNC-induced heavy-quark events at the LHC from Supersymmetry
We analyze the production and subsequent decay of the neutral Higgs bosons of
the MSSM into electrically neutral quark pairs qq'=bs,tc of different flavors
at the LHC and compare with the direct FCNC production mechanisms. The
cross-sections are computed in the unconstrained MSSM with minimal
flavor-mixing sources and taking into account the stringent bounds from
radiative B-meson decays. We extend the results previously found for these FCNC
processes, which are singularly uncommon in the SM. Specifically, we report
here on the SUSY-EW contribution of the Higgs-mediated FCNC cross-section into
bs and tc final states and the SUSY-QCD and SUSY-EW contributions to
bs-production. In this way, the complete map of MSSM predictions for the
qq'-pairs produced at the LHC becomes available. The upshot is that the most
favorable channels are: 1) the Higgs boson FCNC decays into bs, and 2) the
direct production of tc pairs, both of them at the 1 pb level and mediated by
SUSY-QCD effects. If, however, the latter are suppressed, we find a small
SUSY-EW yield for the tc-production through Higgs decays but, at the same time,
a cross-section of 0.1-1 pb for bs-production, which implies a significant
number (10^4-10^5) of bs-pairs per 100 inverse femtobarn of integrated
luminosity.Comment: LaTeX, 17 pages, 4 figures, 4 tables. Extended discussion. Accepted
in Phys. Lett.
iPTF13beo: The Double-Peaked Light Curve of a Type Ibn Supernova Discovered Shortly after Explosion
We present optical photometric and spectroscopic observations of the Type Ibn
(SN 2006jc-like) supernova iPTF13beo. Detected by the intermediate Palomar
Transient Factory ~3 hours after the estimated first light, iPTF13beo is the
youngest and the most distant (~430 Mpc) Type Ibn event ever observed. The
iPTF13beo light curve is consistent with light curves of other Type Ibn SNe and
with light curves of fast Type Ic events, but with a slightly faster rise-time
of two days. In addition, the iPTF13beo R-band light curve exhibits a
double-peak structure separated by ~9 days, not observed before in any Type Ibn
SN. A low-resolution spectrum taken during the iPTF13beo rising stage is
featureless, while a late-time spectrum obtained during the declining stage
exhibits narrow and intermediate-width He I and Si II features with FWHM ~
2000-5000 km/s and is remarkably similar to the prototypical SN Ibn 2006jc
spectrum. We suggest that our observations support a model of a massive star
exploding in a dense He-rich circumstellar medium (CSM). A shock breakout in a
CSM model requires an eruption releasing a total mass of ~0.1 Msun over a time
scale of couple of weeks prior to the SN explosion.Comment: 8 pages, 5 figures, submitted to MNRA
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