5,258 research outputs found
Strange Quark Contribution to the Nucleon Spin from Electroweak Elastic Scattering Data
The total contribution of strange quarks to the intrinsic spin of the nucleon
can be determined from a measurement of the strange-quark contribution to the
nucleon's elastic axial form factor. We have studied the strangeness
contribution to the elastic vector and axial form factors of the nucleon, using
elastic electroweak scattering data. Specifically, we combine elastic
and scattering cross section data from the Brookhaven E734
experiment with elastic and quasi-elastic and -He scattering
parity-violating asymmetry data from the SAMPLE, HAPPEx, G0 and PVA4
experiments. We have not only determined these form factors at individual
values of momentum-transfer (), but also have fit the -dependence of
these form factors using simple functional forms. We present the results of
these fits, along with some expectations of how our knowledge of these form
factors can be improved with data from Fermilab experiments.Comment: 3 pages, 1 figure, CIPANP 201
A Photometric System for Detection of Water and Methane Ices on Kuiper Belt Objects
We present a new near-infrared photometric system for detection of water ice
and methane ice in the solar system. The system consists of two medium-band
filters in the K-band region of the near-infrared, which are sensitive to water
ice and methane ice, plus continuum observations in the J-band and Y-band. The
primary purpose of this system is to distinguish between three basic types of
Kuiper Belt Objects (KBOs) --- those rich in water ice, those rich in methane
ice, and those with little absorbance. In this work, we present
proof-of-concept observations of 51 KBOs using our filter system, 21 of which
have never been observed in the near-IR spectroscopically. We show that our
custom photometric system is consistent with previous spectroscopic
observations while reducing telescope observing time by a factor of 3. We use
our filters to identify Haumea collisional family members, which are thought to
be collisional remnants of a much larger body and are characterized by large
fractions of water ice on their surfaces. We add 2009 YE7 to the Haumea
collisional family based on our water ice band observations(J-H2O = -1.03 +/-
0.27) which indicate a high amount of water ice absorption, our calculated
proper orbital elements, and the neutral optical colors we measured, V-R = 0.38
+/- 0.04, which are all consistent with the rest of the Haumea family. We
identify several objects dynamically similar to Haumea as being distinct from
the Haumea family as they do not have water ice on their surfaces. In addition,
we find that only the largest KBOs have methane ice, and we find that Haumea
itself has significantly less water ice absorption than the smaller Haumea
family members. We find no evidence for other families in the Kuiper Belt.Comment: 38 pages, 7 figure
Scattering polarization in the CaII Infrared Triplet with Velocity Gradients
Magnetic field topology, thermal structure and plasma motions are the three
main factors affecting the polarization signals used to understand our star. In
this theoretical investigation, we focus on the effect that gradients in the
macroscopic vertical velocity field have on the non-magnetic scattering
polarization signals, establishing the basis for general cases. We demonstrate
that the solar plasma velocity gradients have a significant effect on the
linear polarization produced by scattering in chromospheric spectral lines. In
particular, we show the impact of velocity gradients on the anisotropy of the
radiation field and on the ensuing fractional alignment of the CaII levels, and
how they can lead to an enhancement of the zero-field linear polarization
signals. This investigation remarks the importance of knowing the dynamical
state of the solar atmosphere in order to correctly interpret
spectropolarimetric measurements, which is important, among other things, for
establishing a suitable zero field reference case to infer magnetic fields via
the Hanle effect.Comment: 14 pages, 10 figures, 3 appendixes, accepted for publication in Ap
Hints against the cold and collisionless nature of dark matter from the galaxy velocity function
The observed number of dwarf galaxies as a function of rotation velocity is
significantly smaller than predicted by the standard model of cosmology. This
discrepancy cannot be simply solved by assuming strong baryonic feedback
processes, since they would violate the observed relation between maximum
circular velocity () and baryon mass of galaxies. A speculative
but tantalising possibility is that the mismatch between observation and theory
points towards the existence of non-cold or non-collisionless dark matter (DM).
In this paper, we investigate the effects of warm, mixed (i.e warm plus cold),
and self-interacting DM scenarios on the abundance of dwarf galaxies and the
relation between observed HI line-width and maximum circular velocity. Both
effects have the potential to alleviate the apparent mismatch between the
observed and theoretical abundance of galaxies as a function of .
For the case of warm and mixed DM, we show that the discrepancy disappears,
even for luke-warm models that evade stringent bounds from the Lyman-
forest. Self-interacting DM scenarios can also provide a solution as long as
they lead to extended ( kpc) dark matter cores in the density
profiles of dwarf galaxies. Only models with velocity-dependent cross sections
can yield such cores without violating other observational constraints at
larger scales.Comment: Matches published versio
Stability and optimality in parametric convex programming models
Equivalent conditions for structural stability are given for convex programming models in terms of three point-to-set mappings. These mappings are then used to characterize locally optimal parameters. For Lagrange models and, in particular, LFS models the
characterizations are given relative to general (possibly unstable) perturbations
The Height of Chromospheric Loops in an Emerging Flux Region
Context. The chromospheric layer observable with the He I 10830 {\AA} triplet
is strongly warped. The analysis of the magnetic morphology of this layer
therefore requires a reliable technique to determine the height at which the He
I absorption takes place.
Aims. The He I absorption signature connecting two pores of opposite polarity
in an emerging flux region is investigated. This signature is suggestive of a
loop system connecting the two pores. We aim to show that limits can be set on
the height of this chromospheric loop system.
Methods. The increasing anisotropy in the illumination of a thin, magnetic
structure intensifies the linear polarization signal observed in the He I
triplet with height. This signal is altered by the Hanle effect. We apply an
inversion technique incorporating the joint action of the Hanle and Zeeman
effects, with the absorption layer height being one of the free parameters.
Results. The observed linear polarization signal can be explained only if the
loop apex is higher than \approx5 Mm. Best agreement with the observations is
achieved for a height of 6.3 Mm.
Conclusions. The strength of the linear polarization signal in the loop apex
is inconsistent with the assumption of a He I absorption layer at a constant
height level. The determined height supports the earlier conclusion that dark
He 10830 {\AA} filaments in emerging flux regions trace emerging loops.Comment: 7 pages, 4 figure
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