25,100 research outputs found
The C-Theorem and Chiral Symmetry Breaking in Asymptotically Free Vectorlike Gauge Theories
We confront Cardy's suggested c-function for four-dimensional field theories
with the spontaneous breaking of chiral symmetries in asymptotically free
vectorlike gauge theories with fermions transforming according to different
representations under the gauge group. Assuming that the infrared limit of the
c-function is determined by the dimension of the associated Goldstone manifold,
we find that this c-function always decreases between the ultraviolet and
infrared fixed points.Comment: 8 pages, no figures, a few references adde
Operator Relations for SU(3) Breaking Contributions to K and K* Distribution Amplitudes
We derive constraints on the asymmetry a1 of the momentum fractions carried
by quark and antiquark in K and K* mesons in leading twist. These constraints
follow from exact operator identities and relate a1 to SU(3) breaking
quark-antiquark-gluon matrix elements which we determine from QCD sum rules.
Comparing our results to determinations of a1 from QCD sum rules based on
correlation functions of quark currents, we find that, for a1^\parallel(K*) the
central values agree well and come with moderate errors, whereas for a1(K) and
a1^\perp(K*) the results from operator relations are consistent with those from
quark current sum rules, but come with larger uncertainties. The consistency of
results confirms that the QCD sum rule method is indeed suitable for the
calculation of a1. We conclude that the presently most accurate predictions for
a1 come from the direct determination from QCD sum rules based on correlation
functions of quark currents and are given by: a1(K) = 0.06\pm 0.03,
a1^\parallel(K*) = 0.03\pm 0.02, a1^\perp(K*) = 0.04\pm 0.03.Comment: 21 page
Tungsten thermal neutron dosimeter
Tungsten-185 activity, which is produced by neutron activation of tungsten-184, determines thermal neutron flux. Radiochemical separation methods and counting techniques for irradiated tungsten provide accurate determination of the radiation exposure
BFKL at NNLO
We present a recent determination of an approximate expression for the
O(alpha_s^3) contribution chi_2 to the kernel of the BFKL equation. This
includes all collinear and anticollinear singular contributions and is derived
using duality relations between the GLAP and BFKL kernels.Comment: 8 pages. Talk presented at 12th International Conference on Elastic
and Diffractive Scattering: Forward Physics and QCD, Hamburg, DESY, Germany,
21-25 May 200
Reweighting of the form factors in exclusive B --> X ell nu decays
A form factor reweighting technique has been elaborated to permit relatively
easy comparisons between different form factor models applied to exclusive B
--> X l nu decays. The software tool developped for this purpose is described.
It can be used with any event generator, three of which were used in this work:
ISGW2, PHSP and FLATQ2, a new powerful generator. The software tool allows an
easy and reliable implementation of any form factor model. The tool has been
fully validated with the ISGW2 form factor hypothesis. The results of our
present studies indicate that the combined use of the FLATQ2 generator and the
form factor reweighting tool should play a very important role in future
exclusive |Vub| measurements, with largely reduced errors.Comment: accepted for publication by EPJ
Lattice deformations at martensite-martensite interfaces in Ni-Al
The atomic configurations at macrotwin interfaces between microtwinned martensite plates in material are investigated using high resolution transmission electron microscopy (HRTEM). The observed structures are interpreted in view of possible formation mechanisms of these interfaces. A distinction is made between cases in which the microtwins, originating from mutually perpendicular \{110\} austenite planes, enclose a final angle larger or smaller than , measured over the boundary. Two different configurations, one with crossing microtwins and the other with ending microtwins producing a step configuration are described. The latter is related with the existence of microtwin sequences with changing variant widths. Although both features appear irrespective of the material’s preparation technique, rapid solidification seems to prefer the step configuration. Depending on the actual case, tapering, bending and tip splitting of the small microtwin variants is observed. Sever lattice deformations and reorientations occur in a region of 5 – 10 nm around the interface while sequences of single plane ledges gradually bending the microtwins are found up to 50 nm away form the interface. These structures and deformations are interpreted in view of the need to accommodate any remaining stresses
Incompatible sets of gradients and metastability
We give a mathematical analysis of a concept of metastability induced by
incompatibility. The physical setting is a single parent phase, just about to
undergo transformation to a product phase of lower energy density. Under
certain conditions of incompatibility of the energy wells of this energy
density, we show that the parent phase is metastable in a strong sense, namely
it is a local minimizer of the free energy in an neighbourhood of its
deformation. The reason behind this result is that, due to the incompatibility
of the energy wells, a small nucleus of the product phase is necessarily
accompanied by a stressed transition layer whose energetic cost exceeds the
energy lowering capacity of the nucleus. We define and characterize
incompatible sets of matrices, in terms of which the transition layer estimate
at the heart of the proof of metastability is expressed. Finally we discuss
connections with experiment and place this concept of metastability in the
wider context of recent theoretical and experimental research on metastability
and hysteresis.Comment: Archive for Rational Mechanics and Analysis, to appea
Heavy to Light Meson Exclusive Semileptonic Decays in Effective Field Theory of Heavy Quark
We present a general study on exclusive semileptonic decays of heavy (B, D,
B_s) to light (pi, rho, K, K^*) mesons in the framework of effective field
theory of heavy quark. Transition matrix elements of these decays can be
systematically characterized by a set of wave functions which are independent
of the heavy quark mass except for the implicit scale dependence. Form factors
for all these decays are calculated consistently within the effective theory
framework using the light cone sum rule method at the leading order of 1/m_Q
expansion. The branching ratios of these decays are evaluated, and the heavy
and light flavor symmetry breaking effects are investigated. We also give
comparison of our results and the predictions from other approaches, among
which are the relations proposed recently in the framework of large energy
effective theory.Comment: 18 pages, ReVtex, 5 figures, added references and comparison of
results, and corrected signs in some formula
Twist-3 Distribution Amplitudes of K* and phi Mesons
We present a systematic study of twist-3 light-cone distribution amplitudes
of and mesons in QCD. The structure of SU(3)-breaking corrections
is studied in detail. Non-perturbative input parameters are estimated from QCD
sum rules. As a by-product, we update the parameters describing the twist-3
distribution amplitudes of the meson. We also review and update
predictions for the twist-2 distribution amplitudes of , and
.Comment: 27 page
All work and no pay: consequences of unpaid work experience in the creative industries
This research note evaluates the benefits and pitfalls of unpaid work as an entry route into employment in the creative industries and investigates the consequences of this practice for those who already work in the sector. Based on a qualitative study of perspectives of stakeholders in unpaid work, this article argues that the social capital thesis, often used as a rationale for unpaid work, inadequately explains the practice of unpaid work experience, primarily because it does not take cognisance of the consequences of this practice for other people working in the sector. The study also highlights methodological issues that need to be considered in the future. As well as the importance of a plurality of stakeholder perspectives, the study emphasizes the need to consider the perspectives of those who are excluded from unpaid work and those who are potentially displaced by it
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