211 research outputs found
Resizing section of the tubular billet of steel andalloys D16 with differentiated induction heating
Running mass of the b-quark in QCD and SUSY QCD
The running mass of the b-quark defined in DRbar-scheme is one of the
important parameters of SUSY QCD. To find its value it should be related to
some known experimental input. In this paper the b-quark running mass defined
in nonsupersymmetric QCD is chosen for determination of corresponding parameter
in SUSY QCD. The relation between these two quantities is found by considering
five-flavor QCD as an effective theory obtained from its supersymmetric
extension. A numerical analysis of the calculated two-loop relation and its
impact on the MSSM spectrum is discussed. Since for nonsupersymmetric models
MSbar-scheme is more natural than DRbar, we also propose a new procedure that
allows one to calculate relations between MSbar- and DRbar-parameters.
Unphysical epsilon-scalars that give rise to the difference between mentioned
schemes are assumed to be heavy and decoupled in the same way as physical
degrees of freedom. By means of this method it is possible to ``catch two
rabbits'', i.e., decouple heavy particles and turn from DRbar to MSbar, at the
same time. Explicit two-loop example of DRbar -> MSbar transition is given in
the context of QCD. The advantages and disadvantages of the method are briefly
discussed.Comment: 33 pages, 6 figures, 1 table, typos corrected, added references
Spin-polarized tunneling through randomly transparent magnetic junctions: Reentrant magnetoresistance approaching the Julliere limit
Electron conductance in planar magnetic tunnel junctions with long-range
barrier disorder is studied within Glauber-eikonal approximation enabling exact
disorder ensemble averaging by means of the Holtsmark-Markov method. This
allows us to address a hitherto unexplored regime of the tunneling
magnetoresistance effect characterized by the crossover from
momentum-conserving to random tunneling as a function of the defect
concentration. We demonstrate that such a crossover results in a reentrant
magnetoresistance: It goes through a pronounced minimum before reaching
disorder- and geometry-independent Julliere's value at high defect
concentrations.Comment: 7 pages, 5 figures, derivation of Eq. (39) added, errors in Ref. 7
correcte
Two-loop QCD corrections to the vector form factors for the heavy-quark photo-production
We review on the calculation of the heavy-quark photo-production vector form
factors, with the full dependence on the mass of the heavy-quark. The Feynman
diagrams are evaluated within the dimensional regularization scheme and
expressed in Laurent series of , where is the space-time
dimension. The coefficients of the expansion are expressed in terms of harmonic
polylogarithms. The numerical evaluation of the analytical formulas and the
threshold limit of our results are presented.Comment: Talk presented at the 11th International QCD Conference: QCD04,
Montpellier, France, 5-10 July 2004; 4 pages, 4 figure
Geometric approach to asymptotic expansion of Feynman integrals
We present an algorithm that reveals relevant contributions in
non-threshold-type asymptotic expansion of Feynman integrals about a small
parameter. It is shown that the problem reduces to finding a convex hull of a
set of points in a multidimensional vector space.Comment: 6 pages, 2 figure
Dithienosilole-based all-conjugated block copolymers synthesized by a combination of quasiliving Kumada and Negishi catalyst-transfer polycondensations
Herein, we present a quasi-living Negishi-type catalyst-transfer polycondensation of a zinc–organic DTSbased monomer which provides an access to narrowly distributed poly(4,4-bis(2-ethylhexyl)dithieno[3,2-b:20,30-d]silole (PDTS) with controlled molecular weight. The synthesis of well-defined all-conjugated diblock copolymers containing a PDTS block was accomplished by a combination of Kumada and Negishi catalyst-transfer polycondensations (KCTP and NCTP, respectively). Particularly, it was shown that living P3HT chains obtained by KCTP of magnesium–organic thiophene-based monomer efficiently initiate NCTP of zinc–organic DTS-based monomer. The purity of the DTS-based monomer was found to be a crucial factor for achieving a clean chain-growth polymerization process. A combination of physico-chemical methods was used to prove the success of the block copolymerization
QED vertex form factors at two loops
We present the closed analytic expression of the form factors of the two-loop
QED vertex amplitude for on-shell electrons of finite mass and arbitrary
momentum transfer . The calculation is carried out within the
continuous -dimensional regularization scheme, with a single continuous
parameter , the dimension of the space-time, which regularizes at the same
time UltraViolet (UV) and InfraRed (IR) divergences. The results are expressed
in terms of 1-dimensional harmonic polylogarithms of maximum weight 4.Comment: 53 pages, 3 figure
Vertex diagrams for the QED form factors at the 2-loop level
We carry out a systematic investigation of all the 2-loop integrals occurring
in the electron vertex in QED in the continuous -dimensional regularization
scheme, for on-shell electrons, momentum transfer and finite squared
electron mass . We identify all the Master Integrals (MI's) of the
problem and write the differential equations in which they satisfy. The
equations are expanded in powers of and solved by the
Euler's method of the variation of the constants. As a result, we obtain the
coefficients of the Laurent expansion in of the MI's up to zeroth
order expressed in close analytic form in terms of Harmonic Polylogarithms.Comment: A few misprints have been corrected. The results are now available at
http://pheno.physik.uni-freiburg.de/~bhabha, as FORM input file
Master Integrals for the 2-loop QCD virtual corrections to the Forward-Backward Asymmetry
We present the Master Integrals needed for the calculation of the two-loop
QCD corrections to the forward-backward asymmetry of a quark-antiquark pair
produced in electron-positron annihilation events. The abelian diagrams
entering in the evaluation of the vector form factors were calculated in a
previous paper. We consider here the non-abelian diagrams and the diagrams
entering in the computation of the axial form factors, for arbitrary space-like
momentum transfer Q^2 and finite heavy quark mass m. Both the UV and IR
divergences are regularized in the continuous D-dimensional scheme. The Master
Integrals are Laurent-expanded around D=4 and evaluated by the differential
equation method; the coefficients of the expansions are expressed as
1-dimensional harmonic polylogarithms of maximum weight 4.Comment: 38 pages, 6 figures, typos corrected, version accepted by Nucl. Phys.
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