287 research outputs found
Statistical mechanics of base stacking and pairing in DNA melting
We propose a statistical mechanics model for DNA melting in which base
stacking and pairing are explicitly introduced as distinct degrees of freedom.
Unlike previous approaches, this model describes thermal denaturation of DNA
secondary structure in the whole experimentally accessible temperature range.
Base pairing is described through a zipper model, base stacking through an
Ising model. We present experimental data on the unstacking transition,
obtained exploiting the observation that at moderately low pH this transition
is moved down to experimentally accessible temperatures. These measurements
confirm that the Ising model approach is indeed a good description of base
stacking. On the other hand, comparison with the experiments points to the
limitations of the simple zipper model description of base pairing.Comment: 13 pages with figure
Chiral Extrapolation of Lattice Data for Heavy Baryons
The masses of heavy baryons containing a b quark have been calculated
numerically in lattice QCD with pion masses which are much larger than its
physical value. In the present work we extrapolate these lattice data to the
physical mass of the pion by applying the effective chiral Lagrangian for heavy
baryons, which is invariant under chiral symmetry when the light quark masses
go to zero and heavy quark symmetry when the heavy quark masses go to infinity.
A phenomenological functional form with three parameters, which has the correct
behavior in the chiral limit and appropriate behavior when the pion mass is
large, is proposed to extrapolate the lattice data. It is found that the
extrapolation deviates noticably from the naive linear extrapolation when the
pion mass is smaller than about 500MeV. The mass differences between Sigma_b
and Sigma_b^* and between Sigma_b^{(*)} and Lambda_b are also presented.
Uncertainties arising from both lattice data and our model parameters are
discussed in detail. We also give a comparision of the results in our model
with those obtained in the naive linear extrapolations.Comment: 29 pages, 9 figure
Chiral extrapolation of lattice data for the hyperfine splittings of heavy mesons
Hyperfine splittings between the heavy vector (D*, B*) and pseudoscalar (D,
B) mesons have been calculated numerically in lattice QCD, where the pion mass
(which is related to the light quark mass) is much larger than its physical
value. Naive linear chiral extrapolations of the lattice data to the physical
mass of the pion lead to hyperfine splittings which are smaller than
experimental data. In order to extrapolate these lattice data to the physical
mass of the pion more reasonably, we apply the effective chiral perturbation
theory for heavy mesons, which is invariant under chiral symmetry when the
light quark masses go to zero and heavy quark symmetry when the heavy quark
masses go to infinity. This leads to a phenomenological functional form with
three parameters to extrapolate the lattice data. It is found that the
extrapolated hyperfine splittings are even smaller than those obtained using
linear extrapolation. We conclude that the source of the discrepancy between
lattice data for hyperfine splittings and experiment must lie in non-chiral
physics.Comment: 27 pages, 6 figure
Impurity (Fe, Cl, and P)-induced grain boundary and secondary phases in commercially pure titanium (CP-Ti)
A detailed transmission electron microscopy (TEM) study has been made of the microstructures of two as-sintered CP-Ti materials. We show that iron content at the impurity level of 1280 ppm, less than the limit (2000 ppm) for CP-Ti ASTM Grade 1, is sufficient to lead to the formation of a grain boundary (GB) β-Ti phase in the as-sintered microstructure due to segregation. The Fe-stabilized GB β-Ti phase contains ∼7 at. pct Fe and ∼1.5 at. pct Cl. In addition, nano-precipitates of ω-Ti exist in the Fe-stabilized GB β-Ti phase. A phosphorus (P)-enriched Ti-P-based phase was also identified, which has a tetragonal crystal structure with lattice parameters of (a = b = 8.0 ± 0.2 Å and c = 2.7 ± 0.2 Å) and is new to the existing database for Ti-P-enriched phases. As-sintered CP-Ti materials are thus not necessarily a single α-Ti phase material. These impurity-induced phases may exert potential impacts on the properties of sintered CP-Ti
Behavior of metakaolin-based geopolymers incorporating sewage sludge ash (SSA)
[EN] In recent years, geopolymers have become a widely researched binding material. There are technological and environmental advantages tousing this type of binder instead of Portland cement. In this study, binary systems of geopolymers were produced by using mixtures of metakaolin (MK) ,a well-known aluminosilicate raw material, and a residue from sewage sludge incineration: sewage sludge ash (SSA). This ash was used to partially replace the metakaolin in proportions of 0 20%. The mixtures were activated
with alkaline solutions and they were cured by using two different conditions: at room temperature (25 °C) and in a thermal bath (65 °C). The samples were assessed by X-ray diffraction, scanning electron microscopy (pastes) and compressive strength (mortars). The results from these studies showed zeolite formation (faujasite) in geopolymers cured in the thermal bath, which caused a decrease in the compressive strength of the alkali-activated mortars.Replacement of MK with SSA caused a lower reduction in the compressive strength of mortars cured at 65 °C. However, at room temperature, similar
mechanical strength was observed for the MK and MK-SSA systems. These results demonstrated that SSA is a suitable mineral precursor for partial replacement of MK in geopolymer production.The authors acknowledge Santander Universidades for the grant to Lucia Reig (program: Becas lberoamerica Jovenes Profesores Investigadores Espana 2014), CAPES (CAPES/DGU no 266/12), CNPq (no. 14/2013 processo 478057/2013-0) Scanning electron microscopy service of FEIS/UNESP and CNPq (processo 309015/2015-4).Istuque, D.; Reig Cerdá, L.; Moraes, J.; Akasaki, JL.; Borrachero Rosado, MV.; Soriano Martínez, L.; Paya Bernabeu, JJ.... (2016). Behavior of metakaolin-based geopolymers incorporating sewage sludge ash (SSA). Materials Letters. 180:192-195. https://doi.org/10.1016/j.matlet.2016.05.137S19219518
Counting function fluctuations and extreme value threshold in multifractal patterns: the case study of an ideal noise
To understand the sample-to-sample fluctuations in disorder-generated
multifractal patterns we investigate analytically as well as numerically the
statistics of high values of the simplest model - the ideal periodic
Gaussian noise. By employing the thermodynamic formalism we predict the
characteristic scale and the precise scaling form of the distribution of number
of points above a given level. We demonstrate that the powerlaw forward tail of
the probability density, with exponent controlled by the level, results in an
important difference between the mean and the typical values of the counting
function. This can be further used to determine the typical threshold of
extreme values in the pattern which turns out to be given by
with . Such observation provides a
rather compelling explanation of the mechanism behind universality of .
Revealed mechanisms are conjectured to retain their qualitative validity for a
broad class of disorder-generated multifractal fields. In particular, we
predict that the typical value of the maximum of intensity is to be
given by , where is the
corresponding singularity spectrum vanishing at . For the
noise we also derive exact as well as well-controlled approximate
formulas for the mean and the variance of the counting function without
recourse to the thermodynamic formalism.Comment: 28 pages; 7 figures, published version with a few misprints
corrected, editing done and references adde
Effective Field Theories
These lectures introduce some of the basic notions of effective field
theories, as used in particle physics. The topics discussed are the and weak interactions, and chiral perturbation theory as
applied to mesons, baryons, and hadrons containing heavy quarks.Comment: Lectures on Effective Field Theories at the Lake Louise Winter
Institute, February 1995. 42 pages, 16 figures; extended version in
hep-ph/960622
Production of singlet P-wave and states
No spin-singlet quarkonium state has yet been observed. In this
paper we discuss the production of the singlet P-wave and
states and . We consider two possibilities. In the first the
states are produced via the electromagnetic cascades \ups(3S) \to
\eta_b(2S) + \gamma \to h_b + \gamma \gamma \to \eta_b +\gamma\gamma\gamma
and . A more promising process consists of single pion
transition to the state followed by the radiative transition to the
state: \ups(3S)\to h_b + \pi^0 \to \eta_b + \pi^0 +\gamma and . For a million \ups(3S) or
's produced we expect these processes to produce several hundred events.Comment: 13 pages, LaTeX, 1 figure, to be published Phys. Rev. D. Some
equation numbers and one table number correcte
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