1,263 research outputs found
Perturbative nonequilibrium dynamics of phase transitions in an expanding universe
A complete set of Feynman rules is derived, which permits a perturbative
description of the nonequilibrium dynamics of a symmetry-breaking phase
transition in theory in an expanding universe. In contrast to a
naive expansion in powers of the coupling constant, this approximation scheme
provides for (a) a description of the nonequilibrium state in terms of its own
finite-width quasiparticle excitations, thus correctly incorporating
dissipative effects in low-order calculations, and (b) the emergence from a
symmetric initial state of a final state exhibiting the properties of
spontaneous symmetry breaking, while maintaining the constraint . Earlier work on dissipative perturbation theory and spontaneous symmetry
breaking in Minkowski spacetime is reviewed. The central problem addressed is
the construction of a perturbative approximation scheme which treats the
initial symmetric state in terms of the field , while the state that
emerges at later times is treated in terms of a field , linearly related
to . The connection between early and late times involves an infinite
sequence of composite propagators. Explicit one-loop calculations are given of
the gap equations that determine quasiparticle masses and of the equation of
motion for and the renormalization of these equations is
described. The perturbation series needed to describe the symmetric and
broken-symmetry states are not equivalent, and this leads to ambiguities
intrinsic to any perturbative approach. These ambiguities are discussed in
detail and a systematic procedure for matching the two approximations is
described.Comment: 22 pages, using RevTeX. 6 figures. Submitted to Physical Review
Nonequilibrium perturbation theory for complex scalar fields
Real-time perturbation theory is formulated for complex scalar fields away
from thermal equilibrium in such a way that dissipative effects arising from
the absorptive parts of loop diagrams are approximately resummed into the
unperturbed propagators. Low order calculations of physical quantities then
involve quasiparticle occupation numbers which evolve with the changing state
of the field system, in contrast to standard perturbation theory, where these
occupation numbers are frozen at their initial values. The evolution equation
of the occupation numbers can be cast approximately in the form of a Boltzmann
equation. Particular attention is given to the effects of a non-zero chemical
potential, and it is found that the thermal masses and decay widths of
quasiparticle modes are different for particles and antiparticles.Comment: 15 pages using RevTeX; 2 figures in 1 Postscript file; Submitted to
Phys. Rev.
Scaling in high-temperature superconductors
A Hartree approximation is used to study the interplay of two kinds of
scaling which arise in high-temperature superconductors, namely critical-point
scaling and that due to the confinement of electron pairs to their lowest
Landau level in the presence of an applied magnetic field. In the neighbourhood
of the zero-field critical point, thermodynamic functions scale with the
scaling variable , which differs from the variable
suggested by the gaussian approximation.
Lowest-Landau-level (LLL) scaling occurs in a region of high field surrounding
the upper critical field line but not in the vicinity of the zero-field
transition. For YBaCuO in particular, a field of at least 10 T is needed to
observe LLL scaling. These results are consistent with a range of recent
experimental measurements of the magnetization, transport properties and,
especially, the specific heat of high- materials.Comment: 22 pages + 1 figure appended as postscript fil
The Relationship Between Gall Bladder Disease and Coronary Artery Disease
Abstract Not Provided
The parasitic worm-derived immunomodulator, ES-62 and its drug-like small molecule analogues exhibit therapeutic potential in a model of chronic asthma
Chronic asthma is associated with persistent lung inflammation and long-term remodelling of the airways that have proved refractory to conventional treatments such as steroids, despite their efficacy in controlling acute airway contraction and bronchial inflammation. As its recent dramatic increase in industrialised countries has not been mirrored in developing regions, it has been suggested that helminth infection may protect humans against developing asthma. Consistent with this, ES-62, an immunomodulator secreted by the parasitic worm Acanthocheilonema viteae, can prevent pathology associated with chronic asthma (cellular infiltration of the lungs, particularly neutrophils and mast cells, mucus hyper-production and airway thickening) in an experimental mouse model. Importantly, ES-62 can act even after airway remodelling has been established, arresting pathogenesis and ameliorating the inflammatory flares resulting from repeated exposure to allergen that are a debilitating feature of severe chronic asthma. Moreover, two chemical analogues of ES-62, 11a and 12b mimic its therapeutic actions in restoring levels of regulatory B cells and suppressing neutrophil and mast cell responses. These studies therefore provide a platform for developing ES-62-based drugs, with compounds 11a and 12b representing the first step in the development of a novel class of drugs to combat the hitherto intractable disorder of chronic asthma
WD0837+185:the formation and evolution of an extreme mass ratio white dwarf-brown dwarf binary in Praesepe
There is a striking and unexplained dearth of brown dwarf companions in close
orbits (< 3AU) around stars more massive than the Sun, in stark contrast to the
frequency of stellar and planetary companions. Although rare and relatively
short-lived, these systems leave detectable evolutionary end points in the form
of white dwarf - brown dwarf binaries and these remnants can offer unique
insights into the births and deaths of their parent systems. We present the
discovery of a close (orbital separation ~ 0.006 AU) substellar companion to a
massive white dwarf member of the Praesepe star cluster. Using the cluster age
and the mass of the white dwarf we constrain the mass of the white dwarf
progenitor star to lie in the range 3.5 - 3.7 Msun (B9). The high mass of the
white dwarf means the substellar companion must have been engulfed by the B
star's envelope while it was on the late asymptotic giant branch (AGB). Hence,
the initial separation of the system was ~2 AU, with common envelope evolution
reducing the separation to its current value. The initial and final orbital
separations allow us to constrain the combination of the common envelope
efficiency (alpha) and binding energy parameters (lambda) for the AGB star to
alpha lambda ~3. We examine the various formation scenarios and conclude that
the substellar object was most likely to have been captured by the white dwarf
progenitor early in the life of the cluster, rather than forming in situ.Comment: Accepted for publication in ApJ
Online learning in chemistry: Design, development, accessibility, and evaluation
Online learning has played an integral role in delivering large-cohort chemistry courses in undergraduate degree programs. This study includes describing how a first-year chemistry course transitioned from traditional face-to-face teaching to blended learning using the Resource-Based Learning framework (Hannafin & Hill, 2007; Reyes et al., 2022a). Using this framework, different types of online learning resources were curated to deliver chemistry content. A variety of learning activities were also developed to enhance these resources guided by Laurillard’s Conversational Framework (Laurillard, 2002). Considering that accessibility is a critical aspect to improve students’ learning experience, the Universal Design for Learning (UDL) framework was integrated into the learning design of first-year chemistry (Rose & Meyer, 2002; Reyes et al., 2022b). The perceived utility of online learning resources enhanced with UDL-based features was evaluated through students’ responses to surveys, interviews, and focus groups. Furthermore, learning analytics using temporal, sequence, and process mining analytical techniques were employed on students’ trace data to evaluate course learning design and to understand students’ engagement with learning resources and activities included in the course. Results of this study show the importance of careful development and implementation of learning design of the online learning component of chemistry courses, to enhance the students’ learning experiences.
REFERENCES
Hannafin, M. J., & Hill, J. (2007). Resource-based learning. In M. Spector, M. D. Merrill, J. van Merrienboer, & M. P. Driscoll (Eds.), Handbook of research on educational communications and technology. Erlbaum.
Laurillard, D. (2002). Rethinking university teaching: A conversational framework for the effective use of learning technologies (2nd ed.). RoutledgeFalmer.
Reyes, C.T., Kyne, S. H., Lawrie, G. A., & Thompson, C. D. (2022a). Implementing blended first-year chemistry in a developing country using online resources. Online Learning, 26(1), 174–202. https://doi.org/10.24059/olj.v26i1.2508
Reyes, C.T., Lawrie, G. A., Thompson, C. D., & Kyne, S. H. (2022b). “Every little thing that could possibly be provided helps”: analysis of online first-year chemistry resources using the universal design for learning framework. Chemistry Education Research and Practice. https://doi.org/10.1039/d1rp00171j
Rose D. H. & Meyer A. (2002). Teaching every student in the digital age: Universal Design for Learning. Alexandria, VA: ASCD
Chaotic Symmetry Breaking and Dissipative Two-Field Dynamics
The dynamical symmetry breaking in a two-field model is studied by
numerically solving the coupled effective field equations. These are
dissipative equations of motion that can exhibit strong chaotic dynamics. By
choosing very general model parameters leading to symmetry breaking along one
of the field directions, the symmetry broken vacua make the role of transitory
strange attractors and the field trajectories in phase space are strongly
chaotic. Chaos is quantified by means of the determination of the fractal
dimension, which gives an invariant measure for chaotic behavior. Discussions
concerning chaos and dissipation in the model and possible applications to
related problems are given.Comment: 18 pages, 2 .eps figures (uses epsf), Revtex. A much larger version,
more comments, refs. and results. Version in press Physical Review
Critical-point scaling function for the specific heat of a Ginzburg-Landau superconductor
If the zero-field transition in high temperature superconductors such as
YBa_2Cu_3O_7-\delta is a critical point in the universality class of the
3-dimensional XY model, then the general theory of critical phenomena predicts
the existence of a critical region in which thermodynamic functions have a
characteristic scaling form. We report the first attempt to calculate the
universal scaling function associated with the specific heat, for which
experimental data have become available in recent years. Scaling behaviour is
extracted from a renormalization-group analysis, and the 1/N expansion is
adopted as a means of approximation. The estimated scaling function is
qualitatively similar to that observed experimentally, and also to the
lowest-Landau-level scaling function used by some authors to provide an
alternative interpretation of the same data. Unfortunately, the 1/N expansion
is not sufficiently reliable at small values of N for a quantitative fit to be
feasible.Comment: 20 pages; 4 figure
Relaxation and Kinetics in Scalar Field Theories
A new approach to the dynamics of relaxation and kinetics of thermalization
in a scalar field theory is presented that incorporates the relevant time
scales through the resummation of hard thermal loops. An alternative derivation
of the kinetic equations for the ``quasiparticle'' distribution functions is
obtained that allows a clear understanding of the different ``coarse graining''
approximations usually involved in a kinetic description. This method leads to
a systematic perturbative expansion to obtain the kinetic equations including
hard-thermal loop resummation and to an improvement including renormalization,
off-shell effects and contributions that change chemical equilibrium on short
time scales. As a byproduct of these methods we establish the relation between
the relaxation time scale in the linearized equation of motion of the
quasiparticles and the thermalization time scale of the quasiparticle
distribution function in the ``relaxation time approximation''. Hard thermal
loop resummation dramatically modifies the scattering rate for long wavelength
modes as compared to the usual (semi) classical estimate. Relaxation and
kinetics are studied both in the unbroken and broken symmetry phases of the
theory. The broken symmetry phase also provides the setting to obtain the
contribution to the kinetic equations from processes that involve decay of a
heavy scalar into light scalar particles in the medium.Comment: 28 pages, revtex 3.0, two figures available upon reques
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