76 research outputs found
Computation of quark mass anomalous dimension at O(1/N_f^2) in quantum chromodynamics
We present the formalism to calculate d-dimensional critical exponents in QCD
in the large N_f expansion where N_f is the number of quark flavours. It relies
in part on demonstrating that at the d-dimensional fixed point of QCD the
critical theory is equivalent to a non-abelian version of the Thirring model.
We describe the techniques used to compute critical two and three loop Feynman
diagrams and as an application determine the quark wave function, eta, and mass
renormalization critical exponents at O(1/N_f^2) in d-dimensions. Their values
when expressed in relation to four dimensional perturbation theory are in exact
agreement with the known four loop MSbar results. Moreover, new coefficients in
these renormalization group functions are determined to six loops and
O(1/N_f^2). The computation of the exponents in the Schwinger Dyson approach is
also provided and an expression for eta in arbitrary covariant gauge is given.Comment: 41 latex pages, 17 postscript figure
Determination of the anomalous dimension of gluonic operators in deep inelastic scattering at O(1/N_f)
Using large N_f methods we compute the anomalous dimension of the
predominantly gluonic flavour singlet twist-2 composite operator which arises
in the operator product expansion used in deep inelastic scattering. We obtain
a d-dimensional expression for it which depends on the operator moment n. Its
expansion in powers of epsilon = (4-d)/2 agrees with the explicit exact three
loop MSbar results available for n less than or equal to 8 and allows us to
determine some new information on the explicit n-dependence of the three and
higher order coefficients. In particular the n-dependence of the three loop
anomalous dimension gamma_{gg}(a) is determined in the C_2(G) sector at
O(1/N_f).Comment: 26 latex pages, 7 postscript figure
Four loop wave function renormalization in the non-abelian Thirring model
We compute the anomalous dimension of the fermion field with N_f flavours in
the fundamental representation of a general Lie colour group in the non-abelian
Thirring model at four loops. The implications on the renormalization of the
two point Green's function through the loss of multiplicative renormalizability
of the model in dimensional regularization due to the appearance of evanescent
four fermi operators are considered at length. We observe the appearance of one
new colour group Casimir, d_F^{abcd} d_F^{abcd}, in the final four loop result
and discuss its consequences for the relation of the Knizhnik-Zamolodchikov
critical exponents in the Wess Zumino Witten Novikov model to the non-abelian
Thirring model. Renormalization scheme changes are also considered to ensure
that the underlying Fierz symmetry broken by dimensional regularization is
restored.Comment: 25 latex pages with 9 postscript figure
Anomalous dimensions of operators in polarized deep inelastic scattering at O(1/N_f)
Critical exponents are computed for a variety of twist-2 composite operators,
which occur in polarized and unpolarized deep inelastic scattering, at leading
order in the 1/N_f expansion. The resulting d-dimensional expressions, which
depend on the moment of the operator, are in agreement with recent explicit two
and three loop perturbative calculations. An interesting aspect of the critical
point approach which is used, is that the anomalous dimensions of the flavour
singlet eigenoperators, which diagonalize the perturbative mixing matrix, are
computed directly. We also elucidate the treatment of gamma^5 at the fixed
point which is important in simplifying the calculation for polarized
operators. Finally, the anomalous dimension of the singlet axial current is
determined at O(1/N_f) by considering the renormalization of the anomaly in
operator form.Comment: 22 latex pages plus 7 figure
Jockeying for position: the construction of masculine identities
In this paper we examine the construction of masculine identities within a real-life social situation. Using data from an extensive series of interviews with small groups of sixth-form (17-18-year-old) students attending a UK-based, single-sex independent school, the analysis looks at the action orientation of different constructions of identity. More specifically, it focuses upon how the identity talk of one particular group of students were oriented towards managing their subordinate status within the school. In a number of instances the identity of the `new man' was adopted as a strategy of resistance. However, it was found that the more common strategy involved buying back into values embodied within a more traditional definition of masculinity
Predictive powers of chiral perturbation theory in Compton scattering off protons
We study low-energy nucleon Compton scattering in the framework of baryon
chiral perturbation theory (BPT) with pion, nucleon, and (1232)
degrees of freedom, up to and including the next-to-next-to-leading order
(NNLO). We include the effects of order , and , with
MeV the -resonance excitation energy. These are
all "predictive" powers in the sense that no unknown low-energy constants enter
until at least one order higher (i.e, ). Estimating the theoretical
uncertainty on the basis of natural size for effects, we find that
uncertainty of such a NNLO result is comparable to the uncertainty of the
present experimental data for low-energy Compton scattering. We find an
excellent agreement with the experimental cross section data up to at least the
pion-production threshold. Nevertheless, for the proton's magnetic
polarizability we obtain a value of fm, in
significant disagreement with the current PDG value. Unlike the previous
PT studies of Compton scattering, we perform the calculations in a
manifestly Lorentz-covariant fashion, refraining from the heavy-baryon (HB)
expansion. The difference between the lowest order HBPT and BPT
results for polarizabilities is found to be appreciable. We discuss the chiral
behavior of proton polarizabilities in both HBPT and BPT with the
hope to confront it with lattice QCD calculations in a near future. In studying
some of the polarized observables, we identify the regime where their naive
low-energy expansion begins to break down, thus addressing the forthcoming
precision measurements at the HIGS facility.Comment: 24 pages, 9 figures, RevTeX4, revised version published in EPJ
Integrated motor drives: state of the art and future trends
With increased need for high power density, high efficiency and high temperature capabilities in Aerospace and Automotive applications, Integrated Motor Drives (IMD) offers a potential solution. However, close physical integration of the converter and the machine may also lead to an increase in components temperature. This requires careful mechanical, structural and thermal analysis; and design of the IMD system.
This paper reviews existing IMD technologies and their thermal effects on the IMD system. The effects of the power electronics (PE) position on the IMD system and its respective thermal management concepts are also investigated. The challenges faced in designing and manufacturing of an IMD along with the mechanical and structural impacts of close physical integration is also discussed and potential solutions are provided. Potential converter topologies for an IMD like the Matrix converter, 2-level Bridge, 3-level NPC and Multiphase full bridge converters are also reviewed. Wide band gap devices like SiC and GaN and their packaging in power modules for IMDs are also discussed. Power modules components and packaging technologies are also presented
Efeito da idade da matriz sobre o desempenho e rendimento de carcaça de frangos de corte
Diving into the vertical dimension of elasmobranch movement ecology
Knowledge of the three-dimensional movement patterns of elasmobranchs is vital to understand their ecological roles and exposure to anthropogenic pressures. To date, comparative studies among species at global scales have mostly focused on horizontal movements. Our study addresses the knowledge gap of vertical movements by compiling the first global synthesis of vertical habitat use by elasmobranchs from data obtained by deployment of 989 biotelemetry tags on 38 elasmobranch species. Elasmobranchs displayed high intra- and interspecific variability in vertical movement patterns. Substantial vertical overlap was observed for many epipelagic elasmobranchs, indicating an increased likelihood to display spatial overlap, biologically interact, and share similar risk to anthropogenic threats that vary on a vertical gradient. We highlight the critical next steps toward incorporating vertical movement into global management and monitoring strategies for elasmobranchs, emphasizing the need to address geographic and taxonomic biases in deployments and to concurrently consider both horizontal and vertical movements
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