1,744 research outputs found
Unfolding and unzipping of single-stranded DNA by stretching
We present a theoretical study of single-stranded DNA under stretching.
Within the proposed framework, the effects of basepairing on the mechanical
response of the molecule can be studied in combination with an arbitrary
underlying model of chain elasticity. In a generic case, we show that the
stretching curve of ssDNA exhibits two distinct features: the second-order
"unfolding" phase transition, and a sharp crossover, reminiscent of the
first-order "unzipping" transition in dsDNA. We apply the theory to the
particular cases of Worm-like Chain (WLC) and Freely-Joint Chain (FJC) models,
and discuss the universal and model--dependent features of the mechanical
response of ssDNA. In particular, we show that variation of the width of the
unzipping crossover with interaction strength is very sensitive to the
energetics of hairpin loops. This opens a new way of testing the elastic
properties of ssDNA.Comment: 7 pages, 4 figures, substantially revised versio
Holographic Grand Unification
We present a framework for grand unification in which the grand unified symmetry is broken spontaneously by strong gauge dynamics, and yet the physics at the unification scale is described by (weakly coupled) effective field theory. These theories are formulated, through the gauge/gravity correspondence, in truncated 5D warped spacetime with the UV and IR branes setting the Planck and unification scales, respectively. In most of these theories, the Higgs doublets arise as composite states of strong gauge dynamics, corresponding to degrees of freedom localized to the IR brane, and the observed hierarchies of quark and lepton masses and mixings are explained by the wavefunction profiles of these fields in the extra dimension. We present several realistic models in this framework. We focus on one in which the doublet-triplet splitting of the Higgs fields is realized within the dynamical sector by the pseudo-Goldstone mechanism, with the associated global symmetry corresponding to a bulk gauge symmetry in the 5D theory. Alternatively, the light Higgs doublets can arise as a result of dynamics on the IR brane, without being accompanied by their triplet partners. Gauge coupling unification and proton decay can be studied in these models using higher dimensional effective field theory. The framework also sets a stage for further studies of, e.g., proton decay, fermion masses, and supersymmetry breaking
Phase transition in a non-conserving driven diffusive system
An asymmetric exclusion process comprising positive particles, negative
particles and vacancies is introduced. The model is defined on a ring and the
dynamics does not conserve the number of particles. We solve the steady state
exactly and show that it can exhibit a continuous phase transition in which the
density of vacancies decreases to zero. The model has no absorbing state and
furnishes an example of a one-dimensional phase transition in a homogeneous
non-conserving system which does not belong to the absorbing state universality
classes
Exons, introns and DNA thermodynamics
The genes of eukaryotes are characterized by protein coding fragments, the
exons, interrupted by introns, i.e. stretches of DNA which do not carry any
useful information for the protein synthesis. We have analyzed the melting
behavior of randomly selected human cDNA sequences obtained from the genomic
DNA by removing all introns. A clear correspondence is observed between exons
and melting domains. This finding may provide new insights in the physical
mechanisms underlying the evolution of genes.Comment: 4 pages, 8 figures - Final version as published. See also Phys. Rev.
Focus 15, story 1
Comment on "Why is the DNA denaturation transition first order?"
In this comment we argue that while the conclusions in the original paper (Y.
Kafri, D. Mukamel and L. Peliti, Phys. Rev. Lett. 85, 4988 (2000)) are correct
for asymptotically long DNA chains, they do not apply to the chains used in
typical experiments. In the added last paragraph, we point out that for real
DNA the average distance between denatured loops is not of the order of the
persistence length of a single-stranded chain but much larger. This
corroborates our reasoning that the double helix between loops is quite rigid,
and thereby our conclusion.Comment: 1 page, REVTeX. Last paragraph adde
Higgs Boson Decays to Neutralinos in Low-Scale Gauge Mediation
We study the decays of a standard model-like MSSM Higgs boson to pairs of
neutralinos, each of which subsequently decays promptly to a photon and a
gravitino. Such decays can arise in supersymmetric scenarios where
supersymmetry breaking is mediated to us by gauge interactions with a
relatively light gauge messenger sector (M_{mess} < 100 TeV). This process
gives rise to a collider signal consisting of a pair of photons and missing
energy. In the present work we investigate the bounds on this scenario within
the minimal supersymmetric standard model from existing collider data. We also
study the prospects for discovering the Higgs boson through this decay mode
with upcoming data from the Tevatron and the LHC.Comment: 18 pages, 5 figures, added references and discussion of neutralino
couplings, same as journal versio
Propfan Test Assessment (PTA)
The objectives of the Propfan Test Assessment (PTA) Program were to validate in flight the structural integrity of large-scale propfan blades and to measure noise characteristics of the propfan in both near and far fields. All program objectives were met or exceeded, on schedule and under budget. A Gulfstream Aerospace Corporation GII aircraft was modified to provide a testbed for the 2.74m (9 ft) diameter Hamilton Standard SR-7 propfan which was driven by a 4475 kw (600 shp) turboshaft engine mounted on the left-hand wing of the aircraft. Flight research tests were performed for 20 combinations of speed and altitude within a flight envelope that extended to Mach numbers of 0.85 and altitudes of 12,192m (40,000 ft). Propfan blade stress, near-field noise on aircraft surfaces, and cabin noise were recorded. Primary variables were propfan power and tip speed, and the nacelle tilt angle. Extensive low altitude far-field noise tests were made to measure flyover and sideline noise and the lateral attenuation of noise. In coopertion with the FAA, tests were also made of flyover noise for the aircraft at 6100m (20,000 ft) and 10,668m (35,000 ft). A final series of tests were flown to evaluate an advanced cabin wall noise treatment that was produced under a separate program by NASA-Langley Research Center
Theoretical investigation of finite size effects at DNA melting
We investigated how the finiteness of the length of the sequence affects the
phase transition that takes place at DNA melting temperature. For this purpose,
we modified the Transfer Integral method to adapt it to the calculation of both
extensive (partition function, entropy, specific heat, etc) and non-extensive
(order parameter and correlation length) thermodynamic quantities of finite
sequences with open boundary conditions, and applied the modified procedure to
two different dynamical models. We showed that rounding of the transition
clearly takes place when the length of the sequence is decreased. We also
performed a finite-size scaling analysis of the two models and showed that the
singular part of the free energy can indeed be expressed in terms of an
homogeneous function. However, both the correlation length and the average
separation between paired bases diverge at the melting transition, so that it
is no longer clear to which of these two quantities the length of the system
should be compared. Moreover, Josephson's identity is satisfied for none of the
investigated models, so that the derivation of the characteristic exponents
which appear, for example, in the expression of the specific heat, requires
some care
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