496 research outputs found
The Adler -function for SQCD regularized by higher covariant derivatives in the three-loop approximation
We calculate the Adler -function for SQCD in the three-loop
approximation using the higher covariant derivative regularization and the
NSVZ-like subtraction scheme. The recently formulated all-order relation
between the Adler function and the anomalous dimension of the matter
superfields defined in terms of the bare coupling constant is first considered
and generalized to the case of an arbitrary representation for the chiral
matter superfields. The correctness of this all-order relation is explicitly
verified at the three-loop level. The special renormalization scheme in which
this all-order relation remains valid for the -function and the anomalous
dimension defined in terms of the renormalized coupling constant is constructed
in the case of using the higher derivative regularization. The analytic
expression for the Adler function for SQCD is found in this scheme
to the order . The problem of scheme-dependence of the
-function and the NSVZ-like equation is briefly discussed.Comment: 25 pages, 2 figures; the version accepted for publication in Nuclear
Physics
Strong enhancement of spin fluctuations in the low-temperature-tetragonal phase of antiferromagnetically ordered La_{2-x-y}Eu_ySr_xCuO_4
Measurements of the static magnetization, susceptibility and ESR of Gd spin
probes have been performed to study the properties of antiferromagnetically
ordered La_{2-x-y}Eu_ySr_xCuO_4 (x less or equal 0.02) with the low temperature
tetragonal structure. According to the static magnetic measurements the CuO_2
planes are magnetically decoupled in this structural phase. The ESR study
reveals strong magnetic fluctuations at the ESR frequency which are not present
in the orthorhombic phase. It is argued that this drastic enhancement of the
spin fluctuations is due to a considerable weakening of the interlayer exchange
and a pronounced influence of hole motion on the antiferromagnetic properties
of lightly hole doped La_2CuO_4. No evidence for the stripe phase formation at
small hole doping is obtained in the present study.Comment: 10 pages, LaTeX, 3 EPS figures; to be published in Journal of
Physics: Condensed Matte
Next-to-next-to-leading order fits to CCFR'97 data and infrared renormalons
We briefly summarize the outcomes of our recent improved fits to the
experimental data of CCFR collaboration for structure function of deep-inelastic scattering at the next-to-next-to-leading order. Special
attention is paid to the extraction of and the parameter of the
infrared renormalon model for -correction at different orders of
perturbation theory. The results can be of interest for planning similar
studies using possible future data of Neutrino Factories.Comment: 3 pages, presented at WG3 of 4th NuFact'02 Workshop, London 1-6 July,
200
The order and corrections to the decay width of the neutral Higgs boson to the pair
We present the analytical expressions for the contributions of the order
and corrections to the
decay width of the Standard Model Higgs boson into the -pair. The
numerical value of the mixed QED and QCD correction of order
is comparable with the previously calculated
terms in the perturbative series for .Comment: LaTeX 5 pages, accepted for publication in Pisma Zh. Eksp. Teor. Fiz.
v 66, N5 (1997
Reconsidered estimates of the 10th order QED contributions to the muon anomaly
The problem of estimating the 10th order QED corrections to the muon
anomalous magnetic moment is reconsidered. The incorporation of the recently
improved contributions to the and - corrections to
within the renormalization-group inspired scheme-invariant approach
leads to the estimate . It is in good
agreement with the estimate , obtained
by Kinoshita and Nio from the numerical calculations of 2958 10-th order
diagrams, which are considered to be more important than the still uncalculated
6122 10th-order -dependent vertex graphs, and 12672 5-loop
diagrams, responsible for the mass-independent constant contribution both to
and . This confirms Kinoshita and Nio guess about dominance of
the 10-th order diagrams calculated by them. Comparisons with other estimates
of the - contributions to , which exist in the literature,
are presented.Comment: 19 pages, LaTeX, some misprints in the text and literature corrected.
Results unchaged, to appear in Phys.Rev.
Non-singlet QCD analysis of F_2(x,Q^2) up to NNLO
The significance of NNLO (3-loop) QCD contributions to the flavor non-singlet
sector of F_2^ep and F_2^ed has been studied as compared to uncertainties
(different factorization schemes, higher twist and QED contributions) of
standard NLO (and LO) QCD analyses. The latter effects turn out to be
comparable in size to the NNLO contributions. Therefore the minute NNLO effects
are not observable with presently available data on non-singlet structure
functions.Comment: 14 pages, 2 figure
Magnetism of the LTT phase of Eu doped La_{2-x}Sr_xCuO_4
The ESR signal of Gd spin probes (0.5 at %) as well as the static normal
state susceptibility of Eu (J(Eu^{3+})=0) doped La_{2-x-y}Sr_xEu_yCuO_4 reveal
pronounced changes of the Cu magnetism at the structural transition from the
orthorhombic to the low temperature tetragonal phase for all
non-superconducting compositions. Both a jumplike decrease of \chi as well as
the ESR data show an increase of the in-plane magnetic correlation length in
the LTT phase. From the Gd^{3+} ESR linewidth we find that for specific Eu and
Sr concentrations in the LTT phase the correlation length increases up to more
than 100 lattice constants and the fluctuation frequency of the CuO_2 spin
system slows down to 10^{10}- 10^{11}sec^{-1}. However, there is no static
order above T ~ 8K in contrast to the LTT phase of Nd doped La_{2-x}Sr_xCuO_4
with pinned stripe correlations.Comment: 7 pages, RevTex, 3 eps figures. To appear in the Proceedings of the
International Conference "Stripes, Lattice Instabilities and High Tc
Superconductivity", (Rome, Dec. 1996
Self-Consistency Requirements of the Renormalization Group for Setting the Renormalization Scale
In conventional treatments, predictions from fixed-order perturbative QCD
calculations cannot be fixed with certainty due to ambiguities in the choice of
the renormalization scale as well as the renormalization scheme. In this paper
we present a general discussion of the constraints of the renormalization group
(RG) invariance on the choice of the renormalization scale. We adopt the RG
based equations, which incorporate the scheme parameters, for a general
exposition of RG invariance, since they simultaneously express the invariance
of physical observables under both the variation of the renormalization scale
and the renormalization scheme parameters. We then discuss the self-consistency
requirements of the RG, such as reflexivity, symmetry, and transitivity, which
must be satisfied by the scale-setting method. The Principle of Minimal
Sensitivity (PMS) requires the slope of the approximant of an observable to
vanish at the renormalization point. This criterion provides a
scheme-independent estimation, but it violates the symmetry and transitivity
properties of the RG and does not reproduce the Gell-Mann-Low scale for QED
observables. The Principle of Maximum Conformality (PMC) satisfies all of the
deductions of the RG invariance - reflectivity, symmetry, and transitivity.
Using the PMC, all non-conformal -terms (
stands for an arbitrary renormalization scheme) in the perturbative expansion
series are summed into the running coupling, and one obtains a unique,
scale-fixed, scheme-independent prediction at any finite order. The PMC scales
and the resulting finite-order PMC predictions are both to high accuracy
independent of the choice of initial renormalization scale, consistent with RG
invariance. [...More in the text...]Comment: 15 pages, 4 figures. References updated. To be published in
Phys.Rev.
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