1,514 research outputs found
Modelling DNA Origami Self-Assembly at the Domain Level
We present a modelling framework, and basic model parameterization, for the
study of DNA origami folding at the level of DNA domains. Our approach is
explicitly kinetic and does not assume a specific folding pathway. The binding
of each staple is associated with a free-energy change that depends on staple
sequence, the possibility of coaxial stacking with neighbouring domains, and
the entropic cost of constraining the scaffold by inserting staple crossovers.
A rigorous thermodynamic model is difficult to implement as a result of the
complex, multiply connected geometry of the scaffold: we present a solution to
this problem for planar origami. Coaxial stacking and entropic terms,
particularly when loop closure exponents are taken to be larger than those for
ideal chains, introduce interactions between staples. These cooperative
interactions lead to the prediction of sharp assembly transitions with notable
hysteresis that are consistent with experimental observations. We show that the
model reproduces the experimentally observed consequences of reducing staple
concentration, accelerated cooling and absent staples. We also present a
simpler methodology that gives consistent results and can be used to study a
wider range of systems including non-planar origami
Collective molecule formation in a degenerate Fermi gas via a Feshbach resonance
We model collisionless collective conversion of a degenerate Fermi gas into
bosonic molecules via a Feshbach resonance, treating the bosonic molecules as a
classical field and seeding the pairing amplitudes with random phases. A
dynamical instability of the Fermi sea against association into molecules
initiates the conversion. The model qualitatively reproduces several
experimental observations {[Regal et al., Nature {\bf 424}, 47 (2003)]}. We
predict that the initial temperature of the Fermi gas sets the limit for the
efficiency of atom-molecule conversion.Comment: 4 pages, 3 figures, 10+ references, accepted to PR
A Laboratory Investigation of Supersonic Clumpy Flows: Experimental Design and Theoretical Analysis
We present a design for high energy density laboratory experiments studying
the interaction of hypersonic shocks with a large number of inhomogeneities.
These ``clumpy'' flows are relevant to a wide variety of astrophysical
environments including the evolution of molecular clouds, outflows from young
stars, Planetary Nebulae and Active Galactic Nuclei. The experiment consists of
a strong shock (driven by a pulsed power machine or a high intensity laser)
impinging on a region of randomly placed plastic rods. We discuss the goals of
the specific design and how they are met by specific choices of target
components. An adaptive mesh refinement hydrodynamic code is used to analyze
the design and establish a predictive baseline for the experiments. The
simulations confirm the effectiveness of the design in terms of articulating
the differences between shocks propagating through smooth and clumpy
environments. In particular, we find significant differences between the shock
propagation speeds in a clumpy medium compared to a smooth one with the same
average density. The simulation results are of general interest for foams in
both inertial confinement fusion and laboratory astrophysics studies. Our
results highlight the danger of using average properties of inhomogeneous
astrophysical environments when comparing timescales for critical processes
such as shock crossing and gravitational collapse times.Comment: 7 pages, 6 figures. Submitted to the Astrophysical Journal. For
additional information, including simulation animations and the pdf and ps
files of the paper with embedded high-quality images, see
http://pas.rochester.edu/~wm
Perforated Sheets as the Porous Material for a Suction-flap Application
Two-dimensional tests were made of an NACA 0006 airfoil with area suction applied to a porous region on a 0.3-chord trailing-edge flap deflected 50 degrees. The lift with suction approached the value computed from thin-airfoil theory. The lift gains and suction quantity requirements were unaffected by the perforation patterns of the surface over a wide range of hole sizes and spacings
An Examination of the Feasibility of Linear Deterministic Sea Wave Prediction in Multi-Directional Seas Using Wave Profiling Radar: Theory, Simulation and Sea Trials
This is the final version of the article. Available from the American Meteorological Society via the DOI in this record.For a number of maritime tasks there is a short time period, typically only a few tens of seconds, where a critical event occurs which defines a limiting wave height for the whole operation. Examples are the recovery of fixed and rotary winged aircraft, cargo transfers, final pipe mating in fluid transfer operations and launch/recovery of small craft. The recovery of a 30 ton rescue submersible onto a mother ship in the NATO Submarine Rescue System is a prime example. In such applications short term Deterministic Sea Wave Prediction (DSWP) can play a vital role in extending the sea states under which the system can be safely deployed. DSWP also has great potential in conducting experimental sea wave research at full scale. This report explores the feasibility of using data from an experimental wave profiling radar in achieving DSWP. The report includes theory, simulation and field testing. Two forms of DSWP are employed, a fixed point system based upon a restricted set of wave directions, from which we obtain some success, and the other a fully two dimensional technique, which requires further development. The main finding is that using wave profiling radar for DSWP offers promise but requires improvements both to the spatial reliability and resolution of the wave profiling radar, and to the temporal resolution of its sweep before the technique can be considered to be viable as a usable tool.The authors acknowledge funding from the European Union FP7 and U.K. Ministry of Defence NSRS projects
Can the Renormalization Group Improved Effective Potential be used to estimate the Higgs Mass in the Conformal Limit of the Standard Model?
We consider the effective potential in the standard model with a single
Higgs doublet in the limit that the only mass scale present is
radiatively generated. Using a technique that has been shown to determine
completely in terms of the renormalization group (RG) functions when using the
Coleman-Weinberg (CW) renormalization scheme, we first sum leading-log (LL)
contributions to using the one loop RG functions, associated with five
couplings (the top quark Yukawa coupling , the quartic coupling of the Higgs
field , the SU(3) gauge coupling , and the couplings
and ). We then employ the two loop RG functions with the three couplings
, , to sum the next-to-leading-log (NLL) contributions to and
then the three to five loop RG functions with one coupling to sum all the
contributions to . In order to compute these sums, it is
necessary to convert those RG functions that have been originally computed
explicitly in the minimal subtraction (MS) scheme to their form in the CW
scheme. The Higgs mass can then be determined from the effective potential: the
result is decreases to at
order and at order. No reasonable
estimate of can be made at orders or . This is taken
to be an indication that this mechanism for spontaneous symmetry breaking is in
fact viable, though one in which there is slow convergence towards the actual
value of . The mass is argued to be an upper bound on
.Comment: 24 pages, 5 figures. Updated version contains new discussion,
references, figures, and corrects errors in reference
Cutaneous leiomyosarcoma arising in a smallpox scar
BACKGROUND: Cutaneous leiomyosarcoma (CLM) is a very rare smooth muscle tumour that accounts for about 2–3% of all superficial soft tissue sarcomas. Although the development of various malignancies in scar tissue is well known, we report the first case of a CLM developing in a small pox scar. CASE PRESENTATION: A 66-year-old man presented with a painless, slow-growing lump in a small pox scar on his left shoulder. Histological biopsies showed the lesion to be a primary, well-differentiated cutaneous leiomyosarcoma. A CT scan of the thorax was conducted, which showed no signs of metastases. The complete lesion was then surgically excised, and histopathological examination revealed a radically excised cutaneous type leiomyosarcoma After 13 months’ review the patient was doing well with no evidence of tumour recurrence. CONCLUSIONS: This is the first report of a CLM arising in a small pox scar. Although the extended time interval between scarring and malignant changes makes it difficult to advise strict follow-up for patients with small pox scars, one should be aware that atypical changes and/or symptoms occurring in a small pox scar could potentially mean malignant transformation
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