349 research outputs found
Pion-Muon Asymmetry Revisited
Long ago an unexpected and unexplainable phenomena was observed. The
distribution of muons from positive pion decay at rest was anisotropic with an
excess in the backward direction relative to the direction of the proton beam
from which the pions were created. Although this effect was observed by several
different groups with pions produced by different means, the result was not
accepted by the physics community, because it is in direct conflict with a
large set of other experiments indicating that the pion is a pseudoscalar
particle. It is possible to satisfy both sets of experiments if helicity-zero
vector particles exist and the pion is such a particle. Helicity-zero vector
particles have direction but no net spin. For the neutral pion to be a vector
particle requires an additional modification to conventional theory as
discussed herein. An experiment is proposed which can prove that the asymmetry
in the distribution of muons from pion decay is a genuine physical effect
because the asymmetry can be modified in a controllable manner. A positive
result will also prove that the pion is NOT a pseudoscalar particle.Comment: 9 pages, 3 figure
Robust pricing and hedging of double no-touch options
Double no-touch options, contracts which pay out a fixed amount provided an
underlying asset remains within a given interval, are commonly traded,
particularly in FX markets. In this work, we establish model-free bounds on the
price of these options based on the prices of more liquidly traded options
(call and digital call options). Key steps are the construction of super- and
sub-hedging strategies to establish the bounds, and the use of Skorokhod
embedding techniques to show the bounds are the best possible.
In addition to establishing rigorous bounds, we consider carefully what is
meant by arbitrage in settings where there is no {\it a priori} known
probability measure. We discuss two natural extensions of the notion of
arbitrage, weak arbitrage and weak free lunch with vanishing risk, which are
needed to establish equivalence between the lack of arbitrage and the existence
of a market model.Comment: 32 pages, 5 figure
The Isgur-Wise function in a relativistic model for system
We use the Dirac equation with a ``(asymptotically free) Coulomb + (Lorentz
scalar) linear '' potential to estimate the light quark wavefunction for mesons in the limit . We use these wavefunctions to
calculate the Isgur-Wise function for orbital and radial
ground states in the phenomenologically interesting range . We find a simple expression for the zero-recoil slope, , where is the energy eigenvalue
of the light quark, which can be identified with the parameter
of the Heavy Quark Effective Theory. This result implies an upper bound of
for the slope . Also, because for a very light quark the size of the meson is determined mainly by the
``confining'' term in the potential , the shape of
is seen to be mostly sensitive to the dimensionless
ratio . We present results for the ranges of
parameters , and
light quark masses and compare to existing
experimental data and other theoretical estimates. Fits to the data give:
,
and [ARGUS
'93]; , and
[CLEO '93]; ${\bar\Lambda_{u,d}}^2/Comment: 22 pages, Latex, 4 figures (not included) available by fax or via
email upon reques
Enhancement of the upper critical field by nonmagnetic impurities in dirty two-gap superconductors
Quasiclassic Uzadel equations for two-band superconductors in the dirty limit
with the account of both intraband and interband scattering by nonmagnetic
impurities are derived for any anisotropic Fermi surface. From these equations
the Ginzburg-Landau equations, and the critical temperature are obtained.
An equation for the upper critical field, which determines both the temperature
dependence of and the orientational dependence of
as a function of the angle between and the c-axis is
obtained. It is shown that the shape of the curve essentially
depends on the ratio of the intraband electron diffusivities and ,
and can be very different from the standard one-gap dirty limit theory. In
particular, the value can considerably exceed ,
which can have important consequences for applications of . A scaling
relation is proposed which enables one to obtain the angular dependence of
from the equation for at . It is shown
that, depending on the relation between and , the ratio of the upper
critical field for and can both increase and decrease as the temperature decreases. Implications
of the obtained results for are discussed
Spin Excitations in La2CuO4: Consistent Description by Inclusion of Ring-Exchange
We consider the square lattice Heisenberg antiferromagnet with plaquette ring
exchange and a finite interlayer coupling leading to a consistent description
of the spin-wave excitation spectrum in La2CuO4. The values of the in-plane
exchange parameters, including ring-exchange J_{\Box}, are obtained
consistently by an accurate fit to the experimentally observed in-plane
spin-wave dispersion, while the out-of-plane exchange interaction is found from
the temperature dependence of the sublattice magnetization at low temperatures.
The fitted exchange interactions J=151.9 meV and J_{\Box}=0.24 J give values
for the spin stiffness and the Neel temperature in excellent agreement with the
experimental data.Comment: 4 pages, 1 figure, RevTe
Maternal selenium, copper and zinc concentrations in early pregnancy, and the association with fertility
Trace elements such as zinc, copper, and selenium are essential for reproductive health, but there is limited work examining how circulating trace elements may associate with fertility in humans. The aim of this study was to determine the association between maternal plasma concentrations of zinc, copper, and selenium, and time to pregnancy and subfertility. Australian women (n = 1060) who participated in the multi-centre prospective Screening for Pregnancy Endpoints study were included. Maternal plasma concentrations of copper, zinc and selenium were assessed at 15 ± 1 weeks' gestation. Estimates of retrospectively reported time to pregnancy were documented as number of months to conceive; subfertility was defined as taking more than 12 months to conceive. A range of maternal and paternal adjustments were included. Women who had lower zinc (time ratio, 1.20 (0.99-1.44)) or who had lower selenium concentrations (1.19 (1.01-1.40)) had a longer time to pregnancy, equivalent to a median difference in time to pregnancy of around 0.6 months. Women with low selenium concentrations were also at a 1.46 (1.06-2.03) greater relative risk for subfertility compared to women with higher selenium concentrations. There were no associations between copper and time to pregnancy or subfertility. Lower selenium and zinc trace element concentrations, which likely reflect lower dietary intakes, associate with a longer time to pregnancy. Further research supporting our work is required, which may inform recommendations to increase maternal trace element intake in women planning a pregnancy.Jessica A. Grieger, Luke E. Grzeskowiak, Rebecca L. Wilson, Tina Bianco-Miotto, Shalem Y. Leemaqz, Tanja Jankovic-Karasoulos, Anthony V. Perkins, Robert J. Norman, Gus A. Dekker and Claire T. Robert
Scale-free memory model for multiagent reinforcement learning. Mean field approximation and rock-paper-scissors dynamics
A continuous time model for multiagent systems governed by reinforcement
learning with scale-free memory is developed. The agents are assumed to act
independently of one another in optimizing their choice of possible actions via
trial-and-error search. To gain awareness about the action value the agents
accumulate in their memory the rewards obtained from taking a specific action
at each moment of time. The contribution of the rewards in the past to the
agent current perception of action value is described by an integral operator
with a power-law kernel. Finally a fractional differential equation governing
the system dynamics is obtained. The agents are considered to interact with one
another implicitly via the reward of one agent depending on the choice of the
other agents. The pairwise interaction model is adopted to describe this
effect. As a specific example of systems with non-transitive interactions, a
two agent and three agent systems of the rock-paper-scissors type are analyzed
in detail, including the stability analysis and numerical simulation.
Scale-free memory is demonstrated to cause complex dynamics of the systems at
hand. In particular, it is shown that there can be simultaneously two modes of
the system instability undergoing subcritical and supercritical bifurcation,
with the latter one exhibiting anomalous oscillations with the amplitude and
period growing with time. Besides, the instability onset via this supercritical
mode may be regarded as "altruism self-organization". For the three agent
system the instability dynamics is found to be rather irregular and can be
composed of alternate fragments of oscillations different in their properties.Comment: 17 pages, 7 figur
Evidence of Color Coherence Effects in W+jets Events from ppbar Collisions at sqrt(s) = 1.8 TeV
We report the results of a study of color coherence effects in ppbar
collisions based on data collected by the D0 detector during the 1994-1995 run
of the Fermilab Tevatron Collider, at a center of mass energy sqrt(s) = 1.8
TeV. Initial-to-final state color interference effects are studied by examining
particle distribution patterns in events with a W boson and at least one jet.
The data are compared to Monte Carlo simulations with different color coherence
implementations and to an analytic modified-leading-logarithm perturbative
calculation based on the local parton-hadron duality hypothesis.Comment: 13 pages, 6 figures. Submitted to Physics Letters
Advances in ab-initio theory of Multiferroics. Materials and mechanisms: modelling and understanding
Within the broad class of multiferroics (compounds showing a coexistence of
magnetism and ferroelectricity), we focus on the subclass of "improper
electronic ferroelectrics", i.e. correlated materials where electronic degrees
of freedom (such as spin, charge or orbital) drive ferroelectricity. In
particular, in spin-induced ferroelectrics, there is not only a {\em
coexistence} of the two intriguing magnetic and dipolar orders; rather, there
is such an intimate link that one drives the other, suggesting a giant
magnetoelectric coupling. Via first-principles approaches based on density
functional theory, we review the microscopic mechanisms at the basis of
multiferroicity in several compounds, ranging from transition metal oxides to
organic multiferroics (MFs) to organic-inorganic hybrids (i.e. metal-organic
frameworks, MOFs)Comment: 22 pages, 9 figure
Single Spin Asymmetry in Polarized Proton-Proton Elastic Scattering at GeV
We report a high precision measurement of the transverse single spin
asymmetry at the center of mass energy GeV in elastic
proton-proton scattering by the STAR experiment at RHIC. The was measured
in the four-momentum transfer squared range \GeVcSq, the region of a significant interference between the
electromagnetic and hadronic scattering amplitudes. The measured values of
and its -dependence are consistent with a vanishing hadronic spin-flip
amplitude, thus providing strong constraints on the ratio of the single
spin-flip to the non-flip amplitudes. Since the hadronic amplitude is dominated
by the Pomeron amplitude at this , we conclude that this measurement
addresses the question about the presence of a hadronic spin flip due to the
Pomeron exchange in polarized proton-proton elastic scattering.Comment: 12 pages, 6 figure
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