1,493 research outputs found
Candidate molecular ions for an electron electric dipole moment experiment
This paper is a theoretical work in support of a newly proposed experiment
(R. Stutz and E. Cornell, Bull. Am. Soc. Phys. 89, 76 2004) that promises
greater sensitivity to measurements of the electron's electric dipole moment
(EDM) based on the trapping of molecular ions. Such an experiment requires the
choice of a suitable molecule that is both experimentally feasible and
possesses an expectation of a reasonable EDM signal. We find that the molecular
ions PtH+, HfH+, and HfF+ are suitable candidates in their low-lying triplet
Delta states. In particular, we anticipate that the effective electric fields
generated inside these molecules are approximately of 73 GV/cm, -17 GV/cm, and
-18 GV/cm respectively. As a byproduct of this discussion, we also explain how
to make estimates of the size of the effective electric field acting in a
molecule, using commercially available, nonrelativistic molecular structure
software.Comment: 25 pages, 3 figures, submitted to Physical Review
Coupled-Bunch Beam Breakup due to Resistive-Wall Wake
The coupled-bunch beam breakup problem excited by the resistive wall wake is
formulated. An approximate analytic method of finding the asymptotic behavior
of the transverse bunch displacement is developed and solved.Comment: 8 page
Cold collisions of OH and Rb. I: the free collision
We have calculated elastic and state-resolved inelastic cross sections for
cold and ultracold collisions in the Rb() + OH() system,
including fine-structure and hyperfine effects. We have developed a new set of
five potential energy surfaces for Rb-OH() from high-level {\em ab
initio} electronic structure calculations, which exhibit conical intersections
between covalent and ion-pair states. The surfaces are transformed to a
quasidiabatic representation. The collision problem is expanded in a set of
channels suitable for handling the system in the presence of electric and/or
magnetic fields, although we consider the zero-field limit in this work.
Because of the large number of scattering channels involved, we propose and
make use of suitable approximations. To account for the hyperfine structure of
both collision partners in the short-range region we develop a
frame-transformation procedure which includes most of the hyperfine
Hamiltonian. Scattering cross sections on the order of cm are
predicted for temperatures typical of Stark decelerators. We also conclude that
spin orientation of the partners is completely disrupted during the collision.
Implications for both sympathetic cooling of OH molecules in an environment of
ultracold Rb atoms and experimental observability of the collisions are
discussed.Comment: 20 pages, 16 figure
Repulsive Forces Between Looping Chromosomes Induce Entropy-Driven Segregation
One striking feature of chromatin organization is that chromosomes are compartmentalized into distinct territories during interphase, the degree of intermingling being much smaller than expected for linear chains. A growing body of evidence indicates that the formation of loops plays a dominant role in transcriptional regulation as well as the entropic organization of interphase chromosomes. Using a recently proposed model, we quantitatively determine the entropic forces between chromosomes. This Dynamic Loop Model assumes that loops form solely on the basis of diffusional motion without invoking other long-range interactions. We find that introducing loops into the structure of chromatin results in a multi-fold higher repulsion between chromosomes compared to linear chains. Strong effects are observed for the tendency of a non-random alignment; the overlap volume between chromosomes decays fast with increasing loop number. Our results suggest that the formation of chromatin loops imposes both compartmentalization as well as order on the system without requiring additional energy-consuming processes
Two accreting protoplanets around the young star PDS 70
Newly forming proto-planets are expected to create cavities and substructures
in young, gas-rich proto-planetary disks, but they are difficult to detect as
they could be confused with disk features affected by advanced image-analysis
techniques. Recently, a planet was discovered inside the gap of the
transitional disk of the T-Tauri star PDS 70. Here we report on the detection
of strong H-alpha emission from two distinct locations in the PDS 70 system,
one corresponding to the previously discovered planet PDS 70 b, which confirms
the earlier H detection, and another located close to the outer-edge of
the gap, coinciding with a previously identified bright dust spot in the disk
and with a small opening in a ring of molecular emission. We identify this
second H peak as a second proto-planet in the PDS 70 system. The
H emission spectra of both proto-planets indicate ongoing accretion
onto the proto-planets, which appear to be near a 2:1 mean motion resonance.
Our observations show that adaptive-optics-assisted, medium-resolution,
integral-field spectroscopy with MUSE targeting accretion signatures will be a
powerful way to trace ongoing planet formation in transitional disks at
different stages of their evolution. Finding more young planetary systems in
mean motion resonance would give credibility to the Grand Tack hypothesis in
which Jupiter and Saturn migrated in a resonance orbit during the early
formation period of our Solar System.Comment: Nature Astronomy, June 3, 2019; 15 pages, 3 Figs, 1 Tabl
Analysis and Prediction of Deforming 3D Shapes using Oriented Bounding Boxes and LSTM Autoencoders
For sequences of complex 3D shapes in time we present a general approach to
detect patterns for their analysis and to predict the deformation by making use
of structural components of the complex shape. We incorporate long short-term
memory (LSTM) layers into an autoencoder to create low dimensional
representations that allow the detection of patterns in the data and
additionally detect the temporal dynamics in the deformation behavior. This is
achieved with two decoders, one for reconstruction and one for prediction of
future time steps of the sequence. In a preprocessing step the components of
the studied object are converted to oriented bounding boxes which capture the
impact of plastic deformation and allow reducing the dimensionality of the data
describing the structure. The architecture is tested on the results of 196 car
crash simulations of a model with 133 different components, where material
properties are varied. In the latent representation we can detect patterns in
the plastic deformation for the different components. The predicted bounding
boxes give an estimate of the final simulation result and their quality is
improved in comparison to different baselines
Prospects for an electron electric dipole moment search in metastable ThO and ThF
The observation of an electron electric dipole moment (eEDM) would have major
ramifications for the standard model of physics. Polar molecules offer a
near-ideal laboratory for such searches due to the large effective electric
field (), on order of tens of GV/cm that can be easily
oriented in the lab frame. We present an improved method for simply and
accurately determining , in a heavy polar molecule, allowing
for a quick determination of candidates for an eEDM experiment. We apply this
method to ThO and ThF, both of which possess metastable
electronic states. The values of in ThO and ThF
are estimated to be 104 GV/cm and 90 GV/cm respectively, and are therefore two
of the best known candidates for the eEDM search.Comment: Two column format submitted to PR
Effective s- and p-Wave Contact Interactions in Trapped Degenerate Fermi Gases
The structure and stability of dilute degenerate Fermi gases trapped in an
external potential is discussed with special emphasis on the influence of s-
and p-wave interactions. In a first step an Effective Contact Interaction for
all partial waves is derived, which reproduces the energy spectrum of the full
potential within a mean-field model space. Using the s- and p-wave part the
energy density of the multi-component Fermi gas is calculated in Thomas-Fermi
approximation. On this basis the stability of the one- and two-component Fermi
gas against mean-field induced collapse is investigated. Explicit stability
conditions in terms of density and total particle number are given. For the
single-component system attractive p-wave interactions limit the density of the
gas. In the two-component case a subtle competition of s- and p-wave
interactions occurs and gives rise to a rich variety of phenomena. A repulsive
p-wave part, for example, can stabilize a two-component system that would
otherwise collapse due to an attractive s-wave interaction. It is concluded
that the p-wave interaction may have important influence on the structure of
degenerate Fermi gases and should not be discarded from the outset.Comment: 18 pages, 11 figures (using RevTEX4
Cooper Pairing in Ultracold K-40 Using Feshbach Resonances
We point out that the fermionic isotope K-40 is a likely candidate for the
formation of Cooper pairs in an ultracold atomic gas. Specifically, in an
optical trap that simultaneously traps the spin states |9/2,-9/2> and
|9/2,-7/2>, there exists a broad magnetic field Feshbach resonance at B = 196
gauss that can provide the required strong attractive interaction between
atoms. An additional resonance, at B = 191 gauss, could generate p-wave pairing
between identical |9/2,-7/2> atoms. A Cooper-paired degenerate Fermi gas could
thus be constructed with existing ultracold atom technology.Comment: 4 pages, 2 figs, submitted to Phys. Rev.
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