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 $\epsilon=(4-D)/2$, where $D$ 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 $m$ and arbitrary momentum transfer $S=-Q^2$. The calculation is carried out within the continuous $D$-dimensional regularization scheme, with a single continuous parameter $D$, 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 $D$-dimensional regularization scheme, for on-shell electrons, momentum transfer $t=-Q^2$ and finite squared electron mass $m_e^2=a$. We identify all the Master Integrals (MI's) of the problem and write the differential equations in $Q^2$ which they satisfy. The equations are expanded in powers of $\epsilon = (4-D)/2$ 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 $\epsilon$ 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.