132 research outputs found

    Computation of Gr\"obner Bases for Two-Loop Propagator Type Integrals

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    The Gr\"obner basis technique for calculating Feynman diagrams proposed in [O.V. Tarasov, Acta Physica Polonica, v. B29 (1998) 2655] is applied to the two-loop propagator type integrals with arbitrary masses and momentum. We describe the derivation of Gr\"obner bases for all integrals with 1PI topologies and present elements of the Gr\"obner bases.Comment: 4 pages, LaTeX, to appear in the Proceedings of ACAT-03, Tsukuba, Japa

    Massive two-loop Bhabha scattering -- the factorizable subset

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    The experimental precision that will be reached at the next generation of colliders makes it indispensable to improve theoretical predictions significantly. Bhabha scattering (e^+ e^- \to e^+ e^-) is one of the prime processes calling for a better theoretical precision, in particular for non-zero electron masses. We present a first subset of the full two-loop calculation, namely the factorizable subset. Our calculation is based on DIANA. We reduce tensor integrals to scalar integrals in shifted (increased) dimensions and additional powers of various propagators, so-called dots-on-lines. Recurrence relations remove those dots-on-lines as well as genuine dots-on-lines (originating from mass renormalization) and reduce the dimension of the integrals to the generic d = 4 - 2 \epsilon dimensions. The resulting master integrals have to be expanded to O(ϵ){\it O}(\epsilon) to ensure proper treatment of all finite terms.Comment: 5 pages, Talk presented by A.W. at RADCOR and Loops and Legs 2002 in Banz, Germany, to appear in the proceeding

    DIANA and selected applications

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    New developments concerning the extension of the Feynman diagram analyzer DIANA are presented. We discuss new graphics facilities, different approaches to automation of momenta distribution and parallel processing facilities. Furthermore applications to ttˉt\bar t production and Bhabha scattering are shortly discussed.Comment: Latex, 5 pages, 4 eps figures, uses included npb.sty, presented by at RADCOR and Loops and Legs 2002 8-13 September 2002, in Kloster Banz, German

    FIRCLA, one-loop correction to e+ e- to nu anti-nu H and basis of Feynman integrals in higher dimensions

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    An approach for an effective computer evaluation of one-loop multi-leg diagrams is proposed. It's main feature is the combined use of several systems - DIANA, FORM and MAPLE. As an application we consider the one-loop correction to Higgs production in e+ e- to nu anti-nu H, which is important for future e+ e- colliders. To improve the stability of numerical evaluations a non-standard basis of integrals is introduced by transforming integrals to higher dimensions.Comment: 6 pages 1 figure, reference to G. Belanger et al. adde

    Non-renormalization of the full <VVA> correlator at two-loop order

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    By explicit calculation of the two-loop QCD corrections we show that for singlet axial and vector currents the full off-shell correlation function in the limit of massless fermions is proportional to the one-loop result, when calculated in the MS-bar scheme. By the same finite renormalization which is needed to make the one-loop anomaly exact to all orders, we arrive at the conclusion that two-loop corrections are absent altogether, for the complete correlator not only its anomalous part. In accordance with the one-loop nature of the correlator, one possible amplitude, which seems to be missing by accident at the one-loop level, also does not show up at the two-loop level.Comment: 6 pages, 1 figur

    Algebraic reduction of one-loop Feynman graph amplitudes

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    An algorithm for the reduction of one-loop n-point tensor integrals to basic integrals is proposed. We transform tensor integrals to scalar integrals with shifted dimension and reduce these by recurrence relations to integrals in generic dimension. Also the integration-by-parts method is used to reduce indices (powers of scalar propagators) of the scalar diagrams. The obtained recurrence relations for one-loop integrals are explicitly evaluated for 5- and 6-point functions. In the latter case the corresponding Gram determinant vanishes identically for d=4, which greatly simplifies the application of the recurrence relations.Comment: 18 pages, 1 figure, added references, expanded introduction, improved tex

    Explicit Results for the Anomalous Three Point Function and Non-Renormalization Theorems

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    Two-loop corrections for the correlator of the singlet axial and vector currents in QCD are calculated in the chiral limit for arbitrary momenta. Explicit calculations confirm the non-renormalization theorems derived recently by Vainshtein and Knecht et.al. We find that as in the one-loop case also at the two loops the correlator has only 3 independent form-factors instead of 4. From the explicit results we observe that the two-loop correction to the correlator is equal to the one-loop result times the constant factor C_2(R) alpha_s/pi in the MSbar scheme. This holds for the full correlator, for the anomalous longitudinal as well as for the non- anomalous thansversal amplitudes. The finite overall alpha_s dependent constant has to be normalized away by renormalizing the axial current according to Witten's algebraic/geometrical constraint on the anomalous Ward identity. Our observations, together with known facts, suggest that in perturbation theory the correlator is proportional to the one-loop term to all orders and that the non- renormalization theorem of the Adler-Bell-Jackiw anomaly carries over to the full correlator.Comment: 10 pages, 2 Postscript figures, uses axodraw.st

    Decoupling of heavy quarks in HQET

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    Decoupling of c-quark loops in b-quark HQET is considered. The decoupling coefficients for the HQET heavy-quark field and the heavy-light quark current are calculated with the three-loop accuracy. The last result can be used to improve the accuracy of extracting f_B from HQET lattice simulations (without c-quark loops). The decoupling coefficient for the flavour-nonsinglet QCD current with n antisymmetrized gamma-matrices is also obtained at three loops; the result for the tensor current (n=2) is new.Comment: JHEP3 documentclass; the results in a computer-readable form can be found at http://www-ttp.physik.uni-karlsruhe.de/Progdata/ttp06/ttp06-25/ V2: a few typos corrected, a few minor text improvements, a few references added; V3: several typos in formulas fixe
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