8,740 research outputs found
B(E2) Evaluation for 0+ to 2+ Transitions in Even-Even Nuclei
A collaborative study by Brookhaven-McMaster-Central Michigan is underway to
evaluate B(E2) for 0 2 transitions.
This work is a continuation of a previous USNDP evaluation and has been
motivated by a large number of recent measurements and nuclear theory
developments. It includes an extended compilation, data evaluation procedures
and shell model calculations. The subset of B(E2) recommended values
for nuclei of relevance to the double-beta decay problem is presented, and
evaluation policies of experimental data and systematics are discussed. Future
plans for completion of the B(E2;0 2)
evaluation project are also described.Comment: 3 pages, 2 tables, 2 figure
K-Rational D-Brane Crystals
In this paper the problem of constructing spacetime from string theory is
addressed in the context of D-brane physics. It is suggested that the knowledge
of discrete configurations of D-branes is sufficient to reconstruct the motivic
building blocks of certain Calabi-Yau varieties. The collections of D-branes
involved have algebraic base points, leading to the notion of K-arithmetic
D-crystals for algebraic number fields K. This idea can be tested for D0-branes
in the framework of toroidal compactifications via the conjectures of Birch and
Swinnerton-Dyer. For the special class of D0-crystals of Heegner type these
conjectures can be interpreted as formulae that relate the canonical Neron-Tate
height of the base points of the D-crystals to special values of the motivic
L-function at the central point. In simple cases the knowledge of the
D-crystals of Heegner type suffices to uniquely determine the geometry.Comment: 36 page
Configurational order-disorder induced metal-nonmetal transition in BC studied with first-principles superatom-special quasirandom structure method
Due to a large discrepancy between theory and experiment, the electronic
character of crystalline boron carbide BC has been a controversial
topic in the field of icosahedral boron-rich solids. We demonstrate that this
discrepancy is removed when configurational disorder is accurately considered
in the theoretical calculations. We find that while ordered ground state
BC is metallic, configurationally disordered BC,
modeled with a superatom-special quasirandom structure method, goes through a
metal to non-metal transition as the degree of disorder is increased with
increasing temperature. Specifically, one of the chain-end carbon atoms in the
CBC chains substitutes a neighboring equatorial boron atom in a B
icosahedron bonded to it, giving rise to a BC(BBC) unit. The
atomic configuration of the substitutionally disordered BC thus
tends to be dominated by a mixture between B(CBC) and
BC(BBC). Due to splitting of valence states in
BC(BBC), the electron deficiency in B(CBC) is gradually
compensated
Helioseismology of Pre-Emerging Active Regions II: Average Emergence Properties
We report on average subsurface properties of pre-emerging active regions as
compared to areas where no active region emergence was detected. Helioseismic
holography is applied to samples of the two populations (pre-emergence and
without emergence), each sample having over 100 members, which were selected to
minimize systematic bias, as described in Leka et al. We find that there are
statistically significant signatures (i.e., difference in the means of more
than a few standard errors) in the average subsurface flows and the apparent
wave speed that precede the formation of an active region. The measurements
here rule out spatially extended flows of more than about 15 m/s in the top 20
Mm below the photosphere over the course of the day preceding the start of
visible emergence. These measurements place strong constraints on models of
active region formation.Comment: 15 pages, 10 figures, ApJ (published
Validating Forward Modeling and Inversions of Helioseismic Holography Measurements
Here we use synthetic data to explore the performance of forward models and
inverse methods for helioseismic holography. Specifically, this work presents
the first comprehensive test of inverse modeling for flows using
lateral-vantage (deep-focus) holography. We derive sensitivity functions in the
Born approximation. We then use these sensitivity functions in a series of
forward models and inversions of flows from a publicly available
magnetohydrodynamic quiet-Sun simulation. The forward travel times computed
using the kernels generally compare favorably with measurements obtained by
applying holography, in a lateral-vantage configuration, on a 15-hour time
series of artificial Dopplergrams extracted from the simulation. Inversions for
the horizontal flow components are able to reproduce the flows in the upper 3Mm
of the domain, but are compromised by noise at greater depths.Comment: accepted for publication by the Astrophysical
SDO/HMI survey of emerging active regions for helioseismology
Observations from the Solar Dynamics Observatory (SDO) have the potential for
allowing the helioseismic study of the formation of hundreds of active regions,
which would enable us to perform statistical analyses. Our goal is to collate a
uniform data set of emerging active regions observed by the SDO/HMI instrument
suitable for helioseismic analysis up to seven days before emergence. We
restricted the sample to active regions that were visible in the continuum and
emerged into quiet Sun largely avoiding pre-existing magnetic regions. As a
reference data set we paired a control region (CR), with the same latitude and
distance from central meridian, with each emerging active region (EAR). We call
this data set, which is currently comprised of 105 emerging active regions
observed between May 2010 and November 2012, the SDO Helioseismic Emerging
Active Region (SDO/HEAR) survey. To demonstrate the utility of a data set of a
large number of emerging active regions, we measure the relative east-west
velocity of the leading and trailing polarities from the line-of-sight
magnetogram maps during the first day after emergence. The latitudinally
averaged line-of-sight magnetic field of all the EARs shows that, on average,
the leading (trailing) polarity moves in a prograde (retrograde) direction with
a speed of 121 +/- 22 m/s (-70 +/- 13 m/s) relative to the Carrington rotation
rate in the first day. However, relative to the differential rotation of the
surface plasma, the east-west velocity is symmetric, with a mean of 95 +/- 13
m/s. The SDO/HEAR data set will not only be useful for helioseismic studies,
but will also be useful to study other features such as the surface magnetic
field evolution of a large sample of EARs.Comment: Accepted by Astronomy and Astrophysics, 11 figures, one longtable;
update corrects units in Figure
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