235 research outputs found
Direct evidence for charge stripes in a layered cobalt oxide
Recent experiments indicate that static stripe-like charge order is generic to the hole-doped copper oxide superconductors and competes with superconductivity. Here we show that a similar type of charge order is present in La5/3 Sr1/3 CoO4 , an insulating analogue of the copper oxide superconductors containing cobalt in place of copper. The stripe phase we have detected is accompanied by short-range, quasi-one-dimensional, antiferromagnetic order, and provides a natural explanation for the distinctive hour- glass shape of the magnetic spectrum previously observed in neutron scattering mea- surements of La2âxSrx CoO4 and many hole-doped copper oxide superconductors. The results establish a solid empirical basis for theories of the hourglass spectrum built on short-range, quasi-static, stripe correlations
On measurement of top polarization as a probe of production mechanisms at the LHC
In this note we demonstrate the use of top polarization in the study of resonances at the LHC, in the possible case where the dynamics implies
a non-zero top polarization. As a probe of top polarization we construct an
asymmetry in the decay-lepton azimuthal angle distribution (corresponding to
the sign of ) in the laboratory. The asymmetry is non-vanishing
even for a symmetric collider like the LHC, where a positive axis is not
uniquely defined. The angular distribution of the leptons has the advantage of
being a faithful top-spin analyzer, unaffected by possible anomalous
couplings, to linear order. We study, for purposes of demonstration, the case
of a as might exist in the little Higgs models. We identify kinematic cuts
which ensure that our asymmetry reflects the polarization in sign and
magnitude. We investigate possibilities at the LHC with two energy options:
TeV and TeV, as well as at the Tevatron. At the
LHC the model predicts net top quark polarization of the order of a few per
cent for GeV, being as high as for a smaller mass
of the of GeV and for the largest allowed coupling in the model, the
values being higher for the TeV option. These polarizations translate to a
deviation from the standard-model value of azimuthal asymmetry of up to about
() for () TeV LHC, whereas for the Tevatron, values as high as
are attained. For the TeV LHC with an integrated luminosity of 10
fb, these numbers translate into a sensitivity over a large
part of the range GeV.Comment: 28 page, LaTeX, requires JHEP style file, 12 figures. Typos corrected
and references adde
Rare Z-decay into light CP-odd Higgs bosons: a comparative study in different new physics models
Various new physics models predict a light CP-odd Higgs boson (labeled as
) and open up new decay modes for Z-boson, such as ,
and , which could be explored at the GigaZ option of
the ILC. In this work we investigate these rare decays in several new physics
models, namely the type-II two Higgs doublet model (type-II 2HDM), the
lepton-specific two Higgs doublet model (L2HDM), the nearly minimal
supersymetric standard model (nMSSM) and the next-to-minimal supersymmetric
standard model (NMSSM). We find that in the parameter space allowed by current
experiments, the branching ratios can reach for
(), for and for , which
implies that the decays and may be accessible
at the GigaZ option. Moreover, since different models predict different
patterns of the branching ratios, the measurement of these rare decays at the
GigaZ may be utilized to distinguish the models.Comment: Version in JHEP (discussions added, errors corrected
Multiple Parton Interactions in Z+jets production at the LHC. A comparison of factorized and non--factorized double parton distribution functions
We examine the contribution of Multiple Parton Interactions to Z+n-jets
production at the LHC, n=2,3,4, where the Z boson is assumed to decay
leptonically. We compare the results obtained with the correlated GS09 double
parton distribution function with those obtained with two instances of fully
factorized single parton distribution functions: MSTW2008LO and CTEQ6LO. It
appears quite feasible to measure the MPI contribution to Z+2/3/4 jets already
in the first phase of the LHC with a total luminosity of one inverse femtobarn
at 7 TeV. If as expected the trigger threshold for single photons is around 80
GeV, Z+2-jets production may well turn out to be more easily observable than
the gamma+3-jets channel. The MPI cross section is dominated by relatively soft
events with two jets balancing in transverse momentum.Comment: 15 pages, 3 plot
Dark matter scenarios in the minimal SUSY B-L model
We perform a study of the dark matter candidates of a constrained version of
the minimal R-parity-conserving supersymmetric model with a gauged
. It turns out that there are four additional candidates for dark
matter in comparison to the MSSM: two kinds of neutralino, which either
correspond to the gaugino of the or to a fermionic bilepton, as
well as "right-handed" CP-even and -odd sneutrinos. The correct dark matter
relic density of the neutralinos can be obtained due to different mechanisms
including new co-annihilation regions and resonances. The large additional
Yukawa couplings required to break the radiatively often lead to
large annihilation cross sections for the sneutrinos. The correct treatment of
gauge kinetic mixing is crucial to the success of some scenarios. All
candidates are consistent with the exclusion limits of Xenon100.Comment: 45 pages, 22 figures; v2: extended discussion of direct detection
cross section, matches published versio
The Universal One-Loop Effective Action
We present the universal one-loop effective action for all operators of
dimension up to six obtained by integrating out massive, non-degenerate
multiplets. Our general expression may be applied to loops of heavy fermions or
bosons, and has been checked against partial results available in the
literature. The broad applicability of this approach simplifies one-loop
matching from an ultraviolet model to a lower-energy effective field theory
(EFT), a procedure which is now reduced to the evaluation of a combination of
matrices in our universal expression, without any loop integrals to evaluate.
We illustrate the relationship of our results to the Standard Model (SM) EFT,
using as an example the supersymmetric stop and sbottom squark Lagrangian and
extracting from our universal expression the Wilson coefficients of
dimension-six operators composed of SM fields.Comment: 30 pages, v2 contains additional comments and corrects typos, version
accepted for publication in JHE
Radiative contribution to neutrino masses and mixing in SSM
In an extension of the minimal supersymmetric standard model (popularly known
as the SSM), three right handed neutrino superfields are introduced to
solve the -problem and to accommodate the non-vanishing neutrino masses
and mixing. Neutrino masses at the tree level are generated through parity
violation and seesaw mechanism. We have analyzed the full effect of one-loop
contributions to the neutrino mass matrix. We show that the current three
flavour global neutrino data can be accommodated in the SSM, for both
the tree level and one-loop corrected analyses. We find that it is relatively
easier to accommodate the normal hierarchical mass pattern compared to the
inverted hierarchical or quasi-degenerate case, when one-loop corrections are
included.Comment: 51 pages, 14 figures (58 .eps files), expanded introduction, other
minor changes, references adde
Strong interface-induced spin-orbit coupling in graphene on WS2
Interfacial interactions allow the electronic properties of graphene to be
modified, as recently demonstrated by the appearance of satellite Dirac cones
in the band structure of graphene on hexagonal boron nitride (hBN) substrates.
Ongoing research strives to explore interfacial interactions in a broader class
of materials in order to engineer targeted electronic properties. Here we show
that at an interface with a tungsten disulfide (WS2) substrate, the strength of
the spin-orbit interaction (SOI) in graphene is very strongly enhanced. The
induced SOI leads to a pronounced low-temperature weak anti-localization (WAL)
effect, from which we determine the spin-relaxation time. We find that
spin-relaxation time in graphene is two-to-three orders of magnitude smaller on
WS2 than on SiO2 or hBN, and that it is comparable to the intervalley
scattering time. To interpret our findings we have performed first-principle
electronic structure calculations, which both confirm that carriers in
graphene-on-WS2 experience a strong SOI and allow us to extract a
spin-dependent low-energy effective Hamiltonian. Our analysis further shows
that the use of WS2 substrates opens a possible new route to access topological
states of matter in graphene-based systems.Comment: Originally submitted version in compliance with editorial guidelines.
Final version with expanded discussion of the relation between theory and
experiments to be published in Nature Communication
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Terrestrial implications for the maritime geoarchaeological resource: A view from the Lower Palaeolithic
Stone tools and faunal remains have been recovered from the English Channel and the North Sea through trawling, dredging for aggregates, channel clearance, and coring. These finds highlight the potential for a maritime Lower Palaeolithic archaeological resource. It is proposed here that any Lower Palaeolithic artefacts, faunal remains, and sediments deposited in the maritime zone during dry, low-stand phases were once (and may still be) contextually similar to their counterparts in the terrestrial Lower Palaeolithic records of north-western Europe. Given these similarities, can interpretive models and analytical frameworks developed for terrestrial archaeology be profitably applied to an assessment of the potential value of any maritime resource? The terrestrial geoarchaeological resource for the Lower Palaeolithic is dominated by artefacts and ecofacts that have been fluvially reworked. The spatio-temporal resolution of these data varies from entire river valleys and marine isotope stages to river channel gravel bar surfaces and decadal timescales, thus supporting a variety of questions and approaches. However, the structure of the terrestrial resource also highlights two fundamental limitations in current maritime knowledge that can restrict the application of terrestrial approaches to any potential maritime resource: (i) how have the repetitive transgressions and regressions of the Middle and Late Pleistocene modified the terrace landforms and sediments associated with the river systems of the English Channel and southern North Sea basins?; and (ii) do the surviving submerged terrace landforms and fluvial sedimentary deposits support robust geochronological models, as is the case with the classical terrestrial terrace sequences? This paper highlights potential approaches to these questions, and concludes that the fluvial palaeogeography, Pleistocene fossils, and potential Lower Palaeolithic artefacts of the maritime geoarchaeological resource can be profitably investigated in future as derived, low-resolution data sets, facilitating questions of colonisation, occupation, demography, and material culture
A novel role for RecA under non-stress: promotion of swarming motility in Escherichia coli K-12
BACKGROUND: Bacterial motility is a crucial factor in the colonization of natural environments. Escherichia coli has two flagella-driven motility types: swimming and swarming. Swimming motility consists of individual cell movement in liquid medium or soft semisolid agar, whereas swarming is a coordinated cellular behaviour leading to a collective movement on semisolid surfaces. It is known that swimming motility can be influenced by several types of environmental stress. In nature, environmentally induced DNA damage (e.g. UV irradiation) is one of the most common types of stress. One of the key proteins involved in the response to DNA damage is RecA, a multifunctional protein required for maintaining genome integrity and the generation of genetic variation. RESULTS: The ability of E. coli cells to develop swarming migration on semisolid surfaces was suppressed in the absence of RecA. However, swimming motility was not affected. The swarming defect of a ÎrecA strain was fully complemented by a plasmid-borne recA gene. Although the ÎrecA cells grown on semisolidsurfaces exhibited flagellar production, they also presented impaired individual movement as well as a fully inactive collective swarming migration. Both the comparative analysis of gene expression profiles in wild-type and ÎrecA cells grown on a semisolid surface and the motility of lexA1 [Ind-] mutant cells demonstrated that the RecA effect on swarming does not require induction of the SOS response. By using a RecA-GFP fusion protein we were able to segregate the effect of RecA on swarming from its other functions. This protein fusion failed to regulate the induction of the SOS response, the recombinational DNA repair of UV-treated cells and the genetic recombination, however, it was efficient in rescuing the swarming motility defect of the ÎrecA mutant. The RecA-GFP protein retains a residual ssDNA-dependent ATPase activity but does not perform DNA strand exchange. CONCLUSION: The experimental evidence presented in this work supports a novel role for RecA: the promotion of swarming motility. The defective swarming migration of ÎrecA cells does not appear to be associated with defective flagellar production; rather, it seems to be associated with an abnormal flagellar propulsion function. Our results strongly suggest that the RecA effect on swarming motility does not require an extensive canonical RecA nucleofilament formation. RecA is the first reported cellular factor specifically affecting swarming but not swimming motility in E. coli. The integration of two apparently disconnected biologically important processes, such as the maintenance of genome integrity and motility in a unique protein, may have important evolutive consequences
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