21,379 research outputs found
CsCl-type compounds in binary alloys of rare-earth metals with gold and silver
In binary alloys of silver with Sm, Tb, Ho, and Tm, and of gold with Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm, intermediate phases containing stoichiometric proportions of the two metals were found to crystallize into the CsCl (B2)-type structure. The lattice parameters of these phases are reported and a correlation has been found between these lattice parameters and the trivalent ionic radii of the rare-earth metals
Unbounded Human Learning: Optimal Scheduling for Spaced Repetition
In the study of human learning, there is broad evidence that our ability to
retain information improves with repeated exposure and decays with delay since
last exposure. This plays a crucial role in the design of educational software,
leading to a trade-off between teaching new material and reviewing what has
already been taught. A common way to balance this trade-off is spaced
repetition, which uses periodic review of content to improve long-term
retention. Though spaced repetition is widely used in practice, e.g., in
electronic flashcard software, there is little formal understanding of the
design of these systems. Our paper addresses this gap in three ways. First, we
mine log data from spaced repetition software to establish the functional
dependence of retention on reinforcement and delay. Second, we use this memory
model to develop a stochastic model for spaced repetition systems. We propose a
queueing network model of the Leitner system for reviewing flashcards, along
with a heuristic approximation that admits a tractable optimization problem for
review scheduling. Finally, we empirically evaluate our queueing model through
a Mechanical Turk experiment, verifying a key qualitative prediction of our
model: the existence of a sharp phase transition in learning outcomes upon
increasing the rate of new item introductions.Comment: Accepted to the ACM SIGKDD Conference on Knowledge Discovery and Data
Mining 201
Non-Newtonian gravity or gravity anomalies?
Geophysical measurements of G differ from laboratory values, indicating that gravity may be non-Newtonian. A spherical harmonic formulation is presented for the variation of (Newtonian) gravity inside the Earth. Using the GEM-10B Earth Gravitational Field Model, it is shown that long-wavelength gravity anomalies, if not corrected, may masquerade as non-Newtonian gravity by providing significant influences on experimental observation of delta g/delta r and G. An apparent contradiction in other studies is also resolved: i.e., local densities appear in equations when average densities of layers seem to be called for
A tracking algorithm for the stable spin polarization field in storage rings using stroboscopic averaging
Polarized protons have never been accelerated to more than about GeV. To
achieve polarized proton beams in RHIC (250GeV), HERA (820GeV), and the
TEVATRON (900GeV), ideas and techniques new to accelerator physics are needed.
In this publication we will stress an important aspect of very high energy
polarized proton beams, namely the fact that the equilibrium polarization
direction can vary substantially across the beam in the interaction region of a
high energy experiment when no countermeasure is taken. Such a divergence of
the polarization direction would not only diminish the average polarization
available to the particle physics experiment, but it would also make the
polarization involved in each collision analyzed in a detector strongly
dependent on the phase space position of the interacting particle. In order to
analyze and compensate this effect, methods for computing the equilibrium
polarization direction are needed. In this paper we introduce the method of
stroboscopic averaging, which computes this direction in a very efficient way.
Since only tracking data is needed, our method can be implemented easily in
existing spin tracking programs. Several examples demonstrate the importance of
the spin divergence and the applicability of stroboscopic averaging.Comment: 39 page
Cancellation of Infrared Divergences in Hadronic Annihilation Decays of Heavy Quarkonia
In the framework of a newly developed factorization formalism which is based
on NRQCD, explicit cancellations are shown for the infrared divergences that
appeared in the previously calculated hadronic annihilation decay rates of
P-wave and D-wave heavy quarkonia. We extend them to a more general case that
to leading order in and next-to-leading order in , the infrared
divergences in the annihilation amplitudes of color-singlet
pair can be removed by including the contributions of
color-octet operators ,
, ... in NRQCD. We also give the decay widths of
at leading order in .Comment: 8 pages, LaTex(3 figures included), to be publishe
The meson annihilation to leptons and inclusive light hadrons
The annihilation of the meson to leptons and inclusive light hadrons is
analyzed in the framework of nonrelativistic QCD (NRQCD) factorization. We find
that the decay mode, which escapes from the helicity suppression, contributes a
sizable fraction width. According to the analysis, the branching ratio due to
the contribution from the color-singlet component of the meson can be of
order (10^{-2}). We also estimate the contributions from the color-octet
components. With the velocity scaling rule of NRQCD, we find that the
color-octet contributions are sizable too, especially, in certain phase space
of the annihilation they are greater than (or comparative to) the color-singlet
component. A few observables relevant to the spectrum of charged lepton are
suggested, that may be used as measurements on the color-octet and
color-singlet components in the future experiments. A typical long
distance contribution in the annihilation is estimated too.Comment: 26 pages, 5 figures (6 eps-files), submitted to Phys. Rev.
transition form factor within Light Front Quark Model
We study the transition form factor of as a
function of the momentum transfer within the light-front quark model
(LFQM). We compare our result with the experimental data by BaBar as well as
other calculations based on the LFQM in the literature. We show that our
predicted form factor fits well with the experimental data, particularly those
at the large region.Comment: 11 pages, 4 figures, accepted for publication in PR
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