4,655 research outputs found
Shape-induced magnetic anisotropy in dilute magnetic alloys
We extend the theory of the surface-induced magnetic anisotropy to mesoscopic
samples with arbitrary geometry. The shape-induced anisotropy of impurity spins
in small brick-shaped grains of dilute magnetic alloys is studied in detail.
The surface-induced blocking of a magnetic-impurity spin is shown to be very
sensitive to geometric parameters of a grain. This implies that the apparent
discrepancy between the experimental data of different groups on the size
dependence of the Kondo resistivity can result from different microstructure of
the used samples. In order to interpret recent experimental data on the
anomalous Hall effect in thin polycrystalline Fe doped Au films, we analyse the
magnetisation of impurity spins as a function of the impurity position and of
the grain shape.Comment: 10 pages, 6 figures, E-mail addresses: [email protected],
[email protected], [email protected]
Light Stop NLSPs at the Tevatron and LHC
How light can the stop be given current experimental constraints? Can it
still be lighter than the top? In this paper, we study this and related
questions in the context of gauge-mediated supersymmetry breaking, where a stop
NLSP decays into a W, b and gravitino. Focusing on the case of prompt decays,
we simulate several existing Tevatron and LHC analyses that would be sensitive
to this scenario, and find that they allow the stop to be as light as 150 GeV,
mostly due to the large top production background. With more data, the existing
LHC analyses will be able to push the limit up to at least 180 GeV. We hope
this work will motivate more dedicated experimental searches for this simple
scenario, in which, for most purposes, the only free parameters are the stop
mass and lifetime.Comment: 31 pages, 11 figures; v2: added minor clarifications and reference
On direct measurement of the W production charge asymmetry at the LHC
The prospects for making a direct measurement of the W production charge
asymmetry at the LHC are discussed. A modification to the method used at the
Tevatron is proposed for measurements at the LHC. The expected sensitivity for
such a measurement to parton distribution functions is compared to that for a
measurement of the lepton charge asymmetry. The direct measurement approach is
found to be less useful for placing constraints on parton distribution
functions at the LHC than a measurement of the lepton charge asymmetry.Comment: 18 pages, 10 figures, v2: references and keywords updated v3:
Additional paragraph discussing inclusion of W asymmetry in global fits adde
Prompt Decays of General Neutralino NLSPs at the Tevatron
Recent theoretical developments have shown that gauge mediation has a much
larger parameter space of possible spectra and mixings than previously
considered. Motivated by this, we explore the collider phenomenology of gauge
mediation models where a general neutralino is the lightest MSSM superpartner
(the NLSP), focusing on the potential reach from existing and future Tevatron
searches. Promptly decaying general neutralino NLSPs can give rise to final
states involving missing energy plus photons, Zs, Ws and/or Higgses. We survey
the final states and determine those where the Tevatron should have the most
sensitivity. We then estimate the reach of existing Tevatron searches in these
final states and discuss new searches (or optimizations of existing ones) that
should improve the reach. Finally we comment on the potential for discovery at
the LHC.Comment: 41 pages, minor changes, added refs and discussion of previous
literatur
Two Simple W' Models for the Early LHC
W' gauge bosons are good candidates for early LHC discovery. We define two
reference models, one containing a W'_R and one containing a W'_L, which may
serve as ``simplified models'' for presenting experimental results of W'
searches at the LHC. We present the Tevatron bounds on each model and compute
the constraints from precision electroweak observables. We find that indirect
low-energy constraints on the W'_L are quite strong. However, for a W'_R
coupling to right-handed fermions there exists a sizeable region in parameter
space beyond the bounds from the Tevatron and low-energy precision measurements
where even 50 inverse picobarns of integrated LHC luminosity are sufficient to
discover the W'_R. The most promising final states are two leptons and two
jets, or one lepton recoiling against a ``neutrino jet''. A neutrino jet is a
collimated object consisting of a hard lepton and two jets arising from the
decay of a highly boosted massive neutrino.Comment: 20 pages, 8 figures. v2: references adde
Color & Weak triplet scalars, the dimuon asymmetry in decay, the top forward-backward asymmetry, and the CDF dijet excess
The new physics required to explain the anomalies recently reported by the D0
and CDF collaborations, namely the top forward-backward asymmetry (FBA), the
like-sign dimuon charge asymmetry in semileptonic b decay, and the CDF dijet
excess, has to feature an amount of flavor symmetry in order to satisfy the
severe constrains arising from flavor violation. In this paper we show that,
once baryon number conservation is imposed, color & weak triplet scalars with
hypercharge can feature the required flavor structure as a consequence
of standard model gauge invariance. The color & weak triplet model can
simultaneously explain the top FBA and the dimuon charge asymmetry or the
dimuon charge asymmetry and the CDF dijet excess. However, the CDF dijet excess
appears to be incompatible with the top FBA in the minimal framework. Our model
for the dimuon asymmetry predicts the observed pattern in the
region of parameter space required to explain the top FBA, whereas our model
for the CDF dijet anomaly is characterized by the absence of beyond the SM
b-quark jets in the excess region. Compatibility of the color & weak triplet
with the electroweak constraints is also discussed. We show that a Higgs boson
mass exceeding the LEP bound is typically favored in this scenario, and that
both Higgs production and decay can be significantly altered by the triplet.
The most promising collider signature is found if the splitting among the
components of the triplet is of weak scale magnitude.Comment: references added, published versio
Effects of asymmetric contacts on single molecule conductances of HS(CH2)nCOOH in nano-electrical junctions
A scanning tunnelling microscope has been used to determine the conductance of single molecular wires with the configuration X-bridge-X, X-bridge-Y and Y-bridge-Y (X = thiol terminus and Y = COOH). We find that for molecular wires with mixed functional groups (X-bridge-Y) the single molecule conductance decreases with respect to the comparable symmetric molecules. These differences are confirmed by theoretical computations based on a combination of density functional theory and the non-equilibrium Green functions formalism. This study demonstrates that the apparent contact resistance, as well as being highly sensitive to the type of the anchoring group is also strongly influenced by contact-asymmetry of the single molecular junction which in this case decreases the transmission. This highlights that contact asymmetry is a significant factor to be considered when evaluating nano-electrical junctions incorporating single molecules
LHC Predictions from a Tevatron Anomaly in the Top Quark Forward-Backward Asymmetry
We examine the implications of the recent CDF measurement of the top-quark
forward-backward asymmetry, focusing on a scenario with a new color octet
vector boson at 1-3 TeV. We study several models, as well as a general
effective field theory, and determine the parameter space which provides the
best simultaneous fit to the CDF asymmetry, the Tevatron top pair production
cross section, and the exclusion regions from LHC dijet resonance and contact
interaction searches. Flavor constraints on these models are more subtle and
less severe than the literature indicates. We find a large region of allowed
parameter space at high axigluon mass and a smaller region at low mass; we
match the latter to an SU(3)xSU(3)/SU(3) coset model with a heavy vector-like
fermion. Our scenario produces discoverable effects at the LHC with only 1-2
inverse femtobarns of luminosity at 7-8 TeV. Lastly, we point out that a
Tevatron measurement of the b-quark forward-backward asymmetry would be very
helpful in characterizing the physics underlying the top-quark asymmetry.Comment: 35 pages, 10 figures, 4 table
Limit on the mass of a long-lived or stable gluino
We reinterpret the generic CDF charged massive particle limit to obtain a
limit on the mass of a stable or long-lived gluino. Various sources of
uncertainty are examined. The -hadron spectrum and scattering cross sections
are modeled based on known low-energy hadron physics and the resultant
uncertainties are quantified and found to be small compared to uncertainties
from the scale dependence of the NLO pQCD production cross sections. The
largest uncertainty in the limit comes from the unknown squark mass: when the
squark -- gluino mass splitting is small, we obtain a gluino mass limit of 407
GeV, while in the limit of heavy squarks the gluino mass limit is 397 GeV. For
arbitrary (degenerate) squark masses, we obtain a lower limit of 322 GeV on the
gluino mass. These limits apply for any gluino lifetime longer than
ns, and are the most stringent limits for such a long-lived or stable gluino.Comment: 15 pages, 5 figures, accepted for publication in JHE
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