1,496 research outputs found
Guidance for using pilot studies to inform the design of intervention trials with continuous outcomes
Background: A pilot study can be an important step in the assessment of an intervention by providing information to design the future definitive trial. Pilot studies can be used to estimate the recruitment and retention rates and population variance and to provide preliminary evidence of efficacy potential. However, estimation is poor because pilot studies are small, so sensitivity analyses for the main trial's sample size calculations should be undertaken. Methods: We demonstrate how to carry out easy-to-perform sensitivity analysis for designing trials based on pilot data using an example. Furthermore, we introduce rules of thumb for the size of the pilot study so that the overall sample size, for both pilot and main trials, is minimized. Results: The example illustrates how sample size estimates for the main trial can alter dramatically by plausibly varying assumptions. Required sample size for 90% power varied from 392 to 692 depending on assumptions. Some scenarios were not feasible based on the pilot study recruitment and retention rates. Conclusion: Pilot studies can be used to help design the main trial, but caution should be exercised. We recommend the use of sensitivity analyses to assess the robustness of the design assumptions for a main trial
The Baryon-Dark Matter Ratio Via Moduli Decay After Affleck-Dine Baryogenesis
Low-scale supersymmetry breaking in string motivated theories implies the
presence of O(100) TeV scale moduli, which generically lead to a significant
modification of the history of the universe prior to Big Bang Nucleosynthesis.
Such an approach implies a non-thermal origin for dark matter resulting from
scalar decay, where the lightest supersymmetric particle can account for the
observed dark matter relic density. We study the further effect of the decay on
the baryon asymmetry of the universe, and find that this can satisfactorily
address the problem of the over-production of the baryon asymmetry by the
Affleck-Dine mechanism in the MSSM. Remarkably, there is a natural connection
between the baryon and dark matter abundances today, which leads to a solution
of the `Cosmic Coincidence Problem'.Comment: 12 pages, no figure. v2: references adde
Quartic Gauge Couplings and the Radiation Zero in pp to l nu gamma gamma events at the LHC
We report a study of the process pp to l nu gamma gamma at CERN's Large
Hadron Collider, using a leading order partonic-level event generator
interfaced to the Pythia program for showering and hadronisation and a with a
generic detector simulation. The process is sensitive to possible anomalous
quartic gauge boson couplings of the form W W gamma gamma. It is shown how
unitarity-safe limits may be placed on these anomalous couplings by applying a
binned maximum likelihood fit to the distribution of the two-photon invariant
mass, M(gamma gamma), below a cutoff of 1TeV. Assuming 30fb-1 of integrated
luminosity, the expected limits are two orders of magnitude tighter than those
available from LEP. It is also demonstrated how the Standard Model radiation
zero feature of the qq to W gamma gamma process may be observed in the
difference between the two-photon and charged lepton pseudo-rapidities.Comment: 9 pages, 7 figure
Relations between entanglement, Bell-inequality violation and teleportation fidelity for the two-qubit X states
Based on the assumption that the receiver Bob can apply any unitary
transformation, Horodecki {\it et al.} [Phys. Lett. A {\bf 222}, 21 (1996)]
proved that any mixed two spin-1/2 state which violates the Bell-CHSH
inequality is useful for teleportation. Here, we further show that any X state
which violates the Bell-CHSH inequality can also be used for nonclassical
teleportation even if Bob can only perform the identity or the Pauli rotation
operations. Moreover, we showed that the maximal difference between the two
average fidelities achievable via Bob's arbitrary transformations and via the
sole identity or the Pauli rotation is 1/9.Comment: 5 pages, to be published in "Quantum Information Processing
The Point of Origin of the Radio Radiation from the Unresolved Cores of Radio-Loud Quasars
Locating the exact point of origin of the core radiation in active galactic
nuclei (AGN) would represent important progress in our understanding of
physical processes in the central engine of these objects. However, due to our
inability to resolve the region containing both the central compact object and
the jet base, this has so far been difficult. Here, using an analysis in which
the lack of resolution does not play a significant role, we demonstrate that it
may be impossible even in most radio loud sources for more than a small
percentage of the core radiation at radio wavelengths to come from the jet
base. We find for 3C279 that percent of the core flux at 15 GHz must
come from a separate, reasonably stable, region that is not part of the jet
base, and that then likely radiates at least quasi-isotropically and is
centered on the black hole. The long-term stability of this component also
suggests that it may originate in a region that extends over many Schwarzschild
radii.Comment: 7 pages with 3 figures, accepted for publication in Astrophysics and
Space Scienc
Measurement of liver iron by magnetic resonance imaging in the UK Biobank population
The burden of liver disease continues to increase in the UK, with liver cirrhosis reported to be the third most common cause of premature death. Iron overload, a condition that impacts liver health, was traditionally associated with genetic disorders such as hereditary haemochromatosis, however, it is now increasingly associated with obesity, type-2 diabetes and non-alcoholic fatty liver disease. The aim of this study was to assess the prevalence of elevated levels of liver iron within the UK Biobank imaging study in a cohort of 9108 individuals. Magnetic resonance imaging (MRI) was undertaken at the UK Biobank imaging centre, acquiring a multi-echo spoiled gradient-echo single-breath-hold MRI sequence from the liver. All images were analysed for liver iron and fat (expressed as proton density fat fraction or PDFF) content using LiverMultiScan™. Liver iron was measured in 97.3% of the cohort. The mean liver iron content was 1.32 ± 0.32 mg/g while the median was 1.25 mg/g (min: 0.85 max: 6.44 mg/g). Overall 4.82% of the population were defined as having elevated liver iron, above commonly accepted 1.8 mg/g threshold based on biochemical iron measurements in liver specimens obtained by biopsy. Further analysis using univariate models showed elevated liver iron to be related to male sex (p5.6% PDFF) was associated with a slight increase in prevalence of elevated liver iron (4.4% vs 6.3%, p=0.0007). This study shows that population studies including measurement of liver iron concentration are feasible, which may in future be used to better inform patient stratification and treatment
Partial Description of Quantum States
One of the most central and controversial element of quantum mechanics is the
use of non zero vectors of a Hilbert space (or, more generally, of one
dimension subspaces) for representing the state of a quantum system. In
particular, the question whether such a representation is complete has been
debated since almost the early days of quantum mechanics. In this article, we
develop an alternate way to formalize knowledge about the state of quantum
systems, based solely on experimentally accessible elements, namely on outcomes
of finite measurements. We introduce what we call partial description which,
given a feasible measurement, indicates some outcomes which are known to be
impossible (i.e. known to have a probability equal to 0 to occur) and hence
have to be discarded. Then, we introduce partial states (which are partial
descriptions providing as much information as possible) and compare this way to
describe quantum states to the orthodox one, using vector rays. Finally, we
show that partial states allow to describe quantum states in a strictly more
expressive way that the orthodox description does
Molecular dynamics simulation of the order-disorder phase transition in solid NaNO
We present molecular dynamics simulations of solid NaNO using pair
potentials with the rigid-ion model. The crystal potential surface is
calculated by using an \emph{a priori} method which integrates the \emph{ab
initio} calculations with the Gordon-Kim electron gas theory. This approach is
carefully examined by using different population analysis methods and comparing
the intermolecular interactions resulting from this approach with those from
the \emph{ab initio} Hartree-Fock calculations. Our numerics shows that the
ferroelectric-paraelectric phase transition in solid NaNO is triggered by
rotation of the nitrite ions around the crystallographical c axis, in agreement
with recent X-ray experiments [Gohda \textit{et al.}, Phys. Rev. B \textbf{63},
14101 (2000)]. The crystal-field effects on the nitrite ion are also addressed.
Remarkable internal charge-transfer effect is found.Comment: RevTeX 4.0, 11 figure
Anharmonic force fields of perchloric acid, HClO, and perchloric anhydride, ClO. An extreme case of inner polarization
DFT (density functional theory) anharmonic force fields with basis sets near
the Kohn-Sham limit have been obtained for perchloric acid, HClO, and
perchloric anhydride, ClO. Calculated fundamental frequencies are in
very good agreement with available experimental data. Some reassignments in the
vibrational spectra of ClO are proposed based on our calculations.
HClO and ClO are particularly severe examples of the `inner
polarization' phenomenon. The polarization consistent basis sets pc-1 and pc-2
(as well as their augmented counterparts) should be supplemented with two
(preferably three) and one (preferably two) high-exponent functions,
respectively, on second-row atoms. Complete anharmonic force fields are
available as electronic supporting information.Comment: J. Mol. Struct., in press (special issue); Electronic Supporting
Information at http://theochem.weizmann.ac.il/web/papers/Cl2O7.htm
Integrin activation - the importance of a positive feedback
Integrins mediate cell adhesion and are essential receptors for the
development and functioning of multicellular organisms. Integrin activation is
known to require both ligand and talin binding and to correlate with cluster
formation but the activation mechanism and precise roles of these processes are
not yet resolved. Here mathematical modeling, with known experimental
parameters, is used to show that the binding of a stabilizing factor, such as
talin, is alone insufficient to enable ligand-dependent integrin activation for
all observed conditions; an additional positive feedback is required.Comment: in press in Bulletin of Mathematical Biolog
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