6,589 research outputs found
Lifetime Measurement of the 8s Level in Francium
We measure the lifetime of the 8s level on a magneto-optically trapped sample
of ^{210}Fr atoms with time-correlated single-photon counting. The 7P_{1/2}
state serves as the resonant intermediate level for two-photon excitation of
the 8s level completed with a 1300 nm laser. Analysis of the fluorescence decay
through the the 7P_{3/2} level gives 53.30 +- 0.44 ns for the 8s level
lifetime.Comment: 4 pages, 4 figure
Dimensional Reduction, Hard Thermal Loops and the Renormalization Group
We study the realization of dimensional reduction and the validity of the
hard thermal loop expansion for lambda phi^4 theory at finite temperature,
using an environmentally friendly finite-temperature renormalization group with
a fiducial temperature as flow parameter. The one-loop renormalization group
allows for a consistent description of the system at low and high temperatures,
and in particular of the phase transition. The main results are that
dimensional reduction applies, apart from a range of temperatures around the
phase transition, at high temperatures (compared to the zero temperature mass)
only for sufficiently small coupling constants, while the HTL expansion is
valid below (and rather far from) the phase transition, and, again, at high
temperatures only in the case of sufficiently small coupling constants. We
emphasize that close to the critical temperature, physics is completely
dominated by thermal fluctuations that are not resummed in the hard thermal
loop approach and where universal quantities are independent of the parameters
of the fundamental four-dimensional theory.Comment: 20 pages, 13 eps figures, uses epsfig and pstrick
Lifetime Measurement of the 6s Level of Rubidium
We present a lifetime measurements of the 6s level of rubidium. We use a
time-correlated single-photon counting technique on two different samples of
rubidium atoms. A vapor cell with variable rubidium density and a sample of
atoms confined and cooled in a magneto-optical trap. The 5P_{1/2} level serves
as the resonant intermediate step for the two step excitation to the 6s level.
We detect the decay of the 6s level through the cascade fluorescence of the
5P_{3/2} level at 780 nm. The two samples have different systematic effects,
but we obtain consistent results that averaged give a lifetime of 45.57 +- 0.17
ns.Comment: 10 pages, 9 figure
On the "renormalization" transformations induced by cycles of expansion and contraction in causal set cosmology
We study the ``renormalization group action'' induced by cycles of cosmic
expansion and contraction, within the context of a family of stochastic
dynamical laws for causal sets derived earlier. We find a line of fixed points
corresponding to the dynamics of transitive percolation, and we prove that
there exist no other fixed points and no cycles of length two or more. We also
identify an extensive ``basin of attraction'' of the fixed points but find that
it does not exhaust the full parameter space. Nevertheless, we conjecture that
every trajectory is drawn toward the fixed point set in a suitably weakened
sense.Comment: 22 pages, 1 firgure, submitted to Phys. Rev.
Modelling radiobiology
Radiotherapy has played an essential role in cancer treatment for over a century, and remains one of the best-studied methods of cancer treatment. Because of its close links with the physical sciences, it has been the subject of extensive quantitative mathematical modelling, but a complete understanding of the mechanisms of radiotherapy has remained elusive. In part this is because of the complexity and range of scales involved in radiotherapy—from physical radiation interactions occurring over nanometres to evolution of patient responses over months and years. This review presents the current status and ongoing research in modelling radiotherapy responses across these scales, including basic physical mechanisms of DNA damage, the immediate biological responses this triggers, and genetic- and patient-level determinants of response. Finally, some of the major challenges in this field and potential avenues for future improvements are also discussed
Correlated Gravitational Wave and Neutrino Signals from General-Relativistic Rapidly Rotating Iron Core Collapse
We present results from a new set of 3D general-relativistic hydrodynamic
simulations of rotating iron core collapse. We assume octant symmetry and focus
on axisymmetric collapse, bounce, the early postbounce evolution, and the
associated gravitational wave (GW) and neutrino signals. We employ a
finite-temperature nuclear equation of state, parameterized electron capture in
the collapse phase, and a multi-species neutrino leakage scheme after bounce.
The latter captures the important effects of deleptonization, neutrino cooling
and heating and enables approximate predictions for the neutrino luminosities
in the early evolution after core bounce. We consider 12-solar-mass and
40-solar-mass presupernova models and systematically study the effects of (i)
rotation, (ii) progenitor structure, and (iii) postbounce neutrino leakage on
dynamics, GW, and, neutrino signals. We demonstrate, that the GW signal of
rapidly rotating core collapse is practically independent of progenitor mass
and precollapse structure. Moreover, we show that the effects of neutrino
leakage on the GW signal are strong only in nonrotating or slowly rotating
models in which GW emission is not dominated by inner core dynamics. In rapidly
rotating cores, core bounce of the centrifugally-deformed inner core excites
the fundamental quadrupole pulsation mode of the nascent protoneutron star. The
ensuing global oscillations (f~700-800 Hz) lead to pronounced oscillations in
the GW signal and correlated strong variations in the rising luminosities of
antineutrino and heavy-lepton neutrinos. We find these features in cores that
collapse to protoneutron stars with spin periods <~ 2.5 ms and rotational
energies sufficient to drive hyper-energetic core-collapse supernova
explosions. Hence, joint GW + neutrino observations of a core collapse event
could deliver strong evidence for or against rapid core rotation. [abridged]Comment: 29 pages, 14 figures. Replaced with version matching published
versio
Word matching using single closed contours for indexing handwritten historical documents
Effective indexing is crucial for providing convenient access to scanned versions of large collections of historically valuable handwritten manuscripts. Since traditional handwriting recognizers based on optical character recognition (OCR) do not perform well on historical documents, recently a holistic word recognition approach has gained in popularity as an attractive and more straightforward solution (Lavrenko et al. in proc. document Image Analysis for Libraries (DIAL’04), pp. 278–287, 2004). Such techniques attempt to recognize words based on scalar and profile-based features extracted from whole word images. In this paper, we propose a new approach to holistic word recognition for historical handwritten manuscripts based on matching word contours instead of whole images or word profiles. The new method consists of robust extraction of closed word contours and the application of an elastic contour matching technique proposed originally for general shapes (Adamek and O’Connor in IEEE Trans Circuits Syst Video Technol 5:2004). We demonstrate that multiscale contour-based descriptors can effectively capture intrinsic word features avoiding any segmentation of words into smaller subunits. Our experiments show a recognition accuracy of 83%, which considerably exceeds the performance of other systems reported in the literature
Localization of Eigenfunctions in the Stadium Billiard
We present a systematic survey of scarring and symmetry effects in the
stadium billiard. The localization of individual eigenfunctions in Husimi phase
space is studied first, and it is demonstrated that on average there is more
localization than can be accounted for on the basis of random-matrix theory,
even after removal of bouncing-ball states and visible scars. A major point of
the paper is that symmetry considerations, including parity and time-reversal
symmetries, enter to influence the total amount of localization. The properties
of the local density of states spectrum are also investigated, as a function of
phase space location. Aside from the bouncing-ball region of phase space,
excess localization of the spectrum is found on short periodic orbits and along
certain symmetry-related lines; the origin of all these sources of localization
is discussed quantitatively and comparison is made with analytical predictions.
Scarring is observed to be present in all the energy ranges considered. In
light of these results the excess localization in individual eigenstates is
interpreted as being primarily due to symmetry effects; another source of
excess localization, scarring by multiple unstable periodic orbits, is smaller
by a factor of .Comment: 31 pages, including 10 figure
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