3,537 research outputs found
A note on compactly generated co-t-structures
The idea of a co-t-structure is almost "dual" to that of a t-structure, but
with some important differences. This note establishes co-t-structure analogues
of Beligiannis and Reiten's corresponding results on compactly generated
t-structures.Comment: 10 pages; details added to proofs, small correction in the main
resul
The CIPM list "Recommended values of standard frequencies": 2021 update
This paper gives a detailed account of the analysis underpinning the 2021
update to the list of standard reference frequency values recommended by the
International Committee for Weights and Measures (CIPM). This update focused on
a subset of atomic transitions that are secondary representations of the second
(SRS) or considered as potential SRS. As in previous updates in 2015 and 2017,
methods for analysing over-determined data sets were applied to make optimum
use of the worldwide body of published clock comparison data. To ensure that
these methods were robust, three independent calculations were performed using
two different algorithms. The 2021 update differed from previous updates in
taking detailed account of correlations among the input data, a step shown to
be important in deriving unbiased frequency values and avoiding underestimation
of their uncertainties. It also differed in the procedures used to assess input
data and to assign uncertainties to the recommended frequency values, with
previous practice being adapted to produce a fully consistent output data set
consisting of frequency ratio values as well as absolute frequencies. These
changes are significant in the context of an anticipated redefinition of the
second in terms of an optical transition or transitions, since optical
frequency ratio measurements will be critical for verifying the international
consistency of optical clocks prior to the redefinition. In the meantime, the
reduced uncertainties for optical SRS resulting from this analysis
significantly increases the weight that secondary frequency standards based on
these transitions can have in the steering of International Atomic Time (TAI).Comment: 22 pages, 5 figure
Effects of a mandatory safety belt law on hospital admissions
Although the effectiveness of automobile safety belts in reducing risk of serious injury in traffic crashes is well documented, safety belt use in many U.S. jurisdictions remains low. Michigan's mandatory safety belt law for front-seat occupants, implemented in July 1985, is one of 34 similar laws in the United States intended to increase belt use and reduce crash-related injuries. Using time-series intervention analyses of data from 14 hospitals throughout the state, we found a 19% reduction in the rate of admitted patients for all automobile occupant injuries and a 20% reduction in the rate of admitted patients with extremity injuries following implementation of the safety belt law. The utility of hospital data for the evaluation of interventions like the safety belt law reinforce the importance of consistently recording E-codes for all injury patients.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28539/1/0000337.pd
Implications of Source Abundances of Ultraheavy Cosmic Rays
The ratio of cosmic ray source abundance to solar-system abundance was examined for individual elements. Correlations of these ratios with first-ionization potential (FIP) and the expected mass-to-charge ratio (A/Q) of the elements in a million-degree plasma are analyzed. The FIP correlation was examined and it is shown that the correlation is affected by the choice of C2 or C1 chondritic meteorites as the solar-system standard for comparison. An A/Q correlation is suggested as a consequence of the shock acceleration model in the hot interstellar medium. The correlations are presented
Photoionization Broadening of the 1S-2S Transition in a Beam of Atomic Hydrogen
We consider the excitation dynamics of the two-photon \sts transition in a
beam of atomic hydrogen by 243 nm laser radiation. Specifically, we study the
impact of ionization damping on the transition line shape, caused by the
possibility of ionization of the 2S level by the same laser field. Using a
Monte-Carlo simulation, we calculate the line shape of the \sts transition for
the experimental geometry used in the two latest absolute frequency
measurements (M. Niering {\it et al.}, PRL 84, 5496 (2000) and M. Fischer {\it
et al.}, PRL 92, 230802 (2004)). The calculated line shift and line width are
in excellent agreement with the experimentally observed values. From this
comparison we can verify the values of the dynamic Stark shift coefficient for
the \sts transition for the first time on a level of 15%. We show that the
ionization modifies the velocity distribution of the metastable atoms, the line
shape of the \sts transition, and has an influence on the derivation of its
absolute frequency.Comment: 10 pages, 5 figure
Contemplative Science: An Insider's Prospectus
This chapter describes the potential farâreaching consequences of contemplative higher education for the fields of science and medicine
Optical Clocks in Space
The performance of optical clocks has strongly progressed in recent years,
and accuracies and instabilities of 1 part in 10^18 are expected in the near
future. The operation of optical clocks in space provides new scientific and
technological opportunities. In particular, an earth-orbiting satellite
containing an ensemble of optical clocks would allow a precision measurement of
the gravitational redshift, navigation with improved precision, mapping of the
earth's gravitational potential by relativistic geodesy, and comparisons
between ground clocks.Comment: Proc. III International Conference on Particle and Fundamental
Physics in Space (SpacePart06), Beijing 19 - 21 April 2006, to appear in
Nucl. Phys.
Proposal for the determination of nuclear masses by high-precision spectroscopy of Rydberg states
The theoretical treatment of Rydberg states in one-electron ions is
facilitated by the virtual absence of the nuclear-size correction, and
fundamental constants like the Rydberg constant may be in the reach of planned
high-precision spectroscopic experiments. The dominant nuclear effect that
shifts transition energies among Rydberg states therefore is due to the nuclear
mass. As a consequence, spectroscopic measurements of Rydberg transitions can
be used in order to precisely deduce nuclear masses. A possible application of
this approach to the hydrogen and deuterium, and hydrogen-like lithium and
carbon is explored in detail. In order to complete the analysis, numerical and
analytic calculations of the quantum electrodynamic (QED) self-energy remainder
function for states with principal quantum number n=5,...,8 and with angular
momentum L=n-1 and L=n-2 are described (j = L +/- 1/2).Comment: 21 pages; LaTe
Abundances of 'secondary' elements among the ultraheavy cosmic rays: Results from HEAO-3
The HEAO-3 Heavy Nuclei Experiment has measured elemental
abundances of ultraheavy cosmic rays near earth. The
elements with atomic number (Z) in the intervals 44â€Zâ€48 and
62â€Zâ€74 arriving at earth are expected to have significant
secondary components. However, their source abundances are
unlikely to be low enough to warrant treating them as pure
secondaries. Our results are consistent with solar system
abundances modified for first ionization potential with possibly
some enhancement of tho r to s ratio
- âŠ