3,068 research outputs found
Escape rates in periodically driven Markov processes
We present an approximate analytical expression for the escape rate of
time-dependent driven stochastic processes with an absorbing boundary such as
the driven leaky integrate-and-fire model for neural spiking. The novel
approximation is based on a discrete state Markovian modeling of the full
long-time dynamics with time-dependent rates. It is valid in a wide parameter
regime beyond the restraining limits of weak driving (linear response) and/or
weak noise. The scheme is carefully tested and yields excellent agreement with
three different numerical methods based on the Langevin equation, the
Fokker-Planck equation and an integral equation.Comment: 10 pages, 5 figure
Observation of Dirac plasmons in a topological insulator
Plasmons are the quantized collective oscillations of electrons in metals and
doped semiconductors. The plasmons of ordinary, massive electrons are since a
long time basic ingredients of research in plasmonics and in optical
metamaterials. Plasmons of massless Dirac electrons were instead recently
observed in a purely two-dimensional electron system (2DEG)like graphene, and
their properties are promising for new tunable plasmonic metamaterials in the
terahertz and the mid-infrared frequency range. Dirac quasi-particles are known
to exist also in the two-dimensional electron gas which forms at the surface of
topological insulators due to a strong spin-orbit interaction. Therefore,one
may look for their collective excitations by using infrared spectroscopy. Here
we first report evidence of plasmonic excitations in a topological insulator
(Bi2Se3), that was engineered in thin micro-ribbon arrays of different width W
and period 2W to select suitable values of the plasmon wavevector k. Their
lineshape was found to be extremely robust vs. temperature between 6 and 300 K,
as one may expect for the excitations of topological carriers. Moreover, by
changing W and measuring in the terahertz range the plasmonic frequency vP vs.
k we could show, without using any fitting parameter, that the dispersion curve
is in quantitative agreement with that predicted for Dirac plasmons.Comment: 11 pages, 3 figures, published in Nature Nanotechnology (2013
Experimental evidence of cut-wire-induced enhanced transmission of transverse-electric fields through sub-wavelength slits in a thin metallic screen
Recent numerical studies have demonstrated the possibility of achieving
substantial enhancements in the transmission of transverse-electric-polarized
electromagnetic fields through subwavelength slits in a thin metallic screen by
placing single or paired metallic cut-wire arrays at a close distance from the
screen. In this Letter, we report on the first experimental evidence of such
extraordinary transmission phenomena, via microwave (X/Ku-band) measurements on
printed-circuit-board prototypes. Experimental results agree very well with
full-wave numerical predictions, and indicate an intrinsic robustness of the
enhanced transmission phenomena with respect to fabrication tolerances and
experimental imperfections.Comment: 6 pages, 4 figures (slight revision
Quantifying the contribution of established risk factors to cardiovascular mortality differences between Russia and Norway.
Surprisingly few attempts have been made to quantify the simultaneous contribution of well-established risk factors to CVD mortality differences between countries. We aimed to develop and critically appraise an approach to doing so, applying it to the substantial CVD mortality gap between Russia and Norway using survey data in three cities and mortality risks from the Emerging Risk Factor Collaboration. We estimated the absolute and relative differences in CVD mortality at ages 40-69 years between countries attributable to the risk factors, under the counterfactual that the age- and sex-specific risk factor profile in Russia was as in Norway, and vice-versa. Under the counterfactual that Russia had the Norwegian risk factor profile, the absolute age-standardized CVD mortality gap would decline by 33.3% (95% CI 25.1-40.1) among men and 22.1% (10.4-31.3) among women. In relative terms, the mortality rate ratio (Russia/Norway) would decline from 9-10 to 7-8. Under the counterfactual that Norway had the Russian risk factor profile, the mortality gap reduced less. Well-established CVD risk factors account for a third of the male and around a quarter of the female CVD mortality gap between Russia and Norway. However, these estimates are based on widely held epidemiological assumptions that deserve further scrutiny
Landau-Zener-Stuckelberg interferometry
A transition between energy levels at an avoided crossing is known as a
Landau-Zener transition. When a two-level system (TLS) is subject to periodic
driving with sufficiently large amplitude, a sequence of transitions occurs.
The phase accumulated between transitions (commonly known as the Stuckelberg
phase) may result in constructive or destructive interference. Accordingly, the
physical observables of the system exhibit periodic dependence on the various
system parameters. This phenomenon is often referred to as
Landau-Zener-Stuckelberg (LZS) interferometry. Phenomena related to LZS
interferometry occur in a variety of physical systems. In particular, recent
experiments on LZS interferometry in superconducting TLSs (qubits) have
demonstrated the potential for using this kind of interferometry as an
effective tool for obtaining the parameters characterizing the TLS as well as
its interaction with the control fields and with the environment. Furthermore,
strong driving could allow for fast and reliable control of the quantum system.
Here we review recent experimental results on LZS interferometry, and we
present related theory.Comment: 34 single-column pages, 11 figure
Local scale-invariance and ageing in noisy systems
The influence of the noise on the long-time ageing dynamics of a quenched
ferromagnetic spin system with a non-conserved order parameter and described
through a Langevin equation with a thermal noise term and a disordered initial
state is studied. If the noiseless part of the system is Galilei-invariant and
scale-invariant with dynamical exponent z=2, the two-time linear response
function is independent of the noise and therefore has exactly the form
predicted from the local scale-invariance of the noiseless part. The two-time
correlation function is exactly given in terms of certain noiseless three- and
four-point response functions. An explicit scaling form of the two-time
autocorrelation function follows. For disordered initial states, local
scale-invariance is sufficient for the equality of the autocorrelation and
autoresponse exponents in phase-ordering kinetics. The results for the scaling
functions are confirmed through tests in the kinetic spherical model, the
spin-wave approximation of the XY model, the critical voter model and the free
random walk.Comment: Latex2e, 45 pages, no figures, final for
Dane developments for the KLOE-2 experimental run
Recently the peak luminosity achieved on the DA{\Phi}NE collider has been
improved by almost a factor three by implementing a novel collision scheme
based on large Piwinski angle and Crab-Waist. This encouraging result opened
new perspectives for physics research and a new run with the KLOE-2 detector
has been scheduled to start by spring 2010. The KLOE-2 installation is a
complex operation requiring a careful design effort and a several months long
shutdown. The high luminosity interaction region has been deeply revised in
order to take into account the effect on the beam caused by the solenoidal
field of the experimental detector and to ensure background rejection. The
shutdown has been also used to implement several other modifications aimed at
improving beam dynamics: the wiggler poles have been displaced from the magnet
axis in order to cancel high order terms in the field, the feedback systems
have been equipped with stronger power supplies and more efficient kickers and
electrodes have been inserted inside the wiggler and the dipole vacuum
chambers, in the positron ring, to avoid the e-cloud formation. A low level RF
feedback has been added to the cavity control in both rings.Comment: 3 pages, 3 figures, presented to the IPAC10 conferenc
Differential branching fraction and angular analysis of the decay B0→K∗0μ+μ−
The angular distribution and differential branching fraction of the decay B 0→ K ∗0 μ + μ − are studied using a data sample, collected by the LHCb experiment in pp collisions at s√=7 TeV, corresponding to an integrated luminosity of 1.0 fb−1. Several angular observables are measured in bins of the dimuon invariant mass squared, q 2. A first measurement of the zero-crossing point of the forward-backward asymmetry of the dimuon system is also presented. The zero-crossing point is measured to be q20=4.9±0.9GeV2/c4 , where the uncertainty is the sum of statistical and systematic uncertainties. The results are consistent with the Standard Model predictions
Observation of two new baryon resonances
Two structures are observed close to the kinematic threshold in the mass spectrum in a sample of proton-proton collision data, corresponding
to an integrated luminosity of 3.0 fb recorded by the LHCb experiment.
In the quark model, two baryonic resonances with quark content are
expected in this mass region: the spin-parity and
states, denoted and .
Interpreting the structures as these resonances, we measure the mass
differences and the width of the heavier state to be
MeV,
MeV,
MeV, where the first and second
uncertainties are statistical and systematic, respectively. The width of the
lighter state is consistent with zero, and we place an upper limit of
MeV at 95% confidence level. Relative
production rates of these states are also reported.Comment: 17 pages, 2 figure
Study of and decays and determination of the CKM angle
We report a study of the suppressed and favored
decays, where the neutral meson is detected
through its decays to the and CP-even and
final states. The measurement is carried out using a proton-proton
collision data sample collected by the LHCb experiment, corresponding to an
integrated luminosity of 3.0~fb. We observe the first significant
signals in the CP-even final states of the meson for both the suppressed
and favored modes, as well as
in the doubly Cabibbo-suppressed final state of the decay. Evidence for the ADS suppressed decay , with , is also presented. From the observed
yields in the , and their
charge conjugate decay modes, we measure the value of the weak phase to be
. This is one of the most precise
single-measurement determinations of to date.Comment: 22 pages, 9 figures; All figures and tables, along with any
supplementary material and additional information, are available at
https://lhcbproject.web.cern.ch/lhcbproject/Publications/LHCbProjectPublic/LHCb-PAPER-2015-020.htm
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