178 research outputs found

    Top Quark Spin Polarization in ep Collision

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    We discuss the degree of spin polarization of single top quarks produced via WgWg fusion process in epep collision at TESLA+HERAp and CLIC+LHC energies s=1.6\sqrt{s}=1.6 and 5.3 TeV. For eb→tνˉeb \to t \bar{\nu} subprocess we show that the top quark spin is completely polarized when the spin basis is chosen in the direction of the incoming positron beam in the rest frame of top quark. A description on how to combine the cross sections of e+b→tνˉe^{+}b\to t\bar{\nu} and e+g→tbˉνˉe^{+}g\to t\bar{b}\bar{\nu} processes is given. e+e^{+}-beam direction is taken to be the favorite top quark spin decomposition axis in its rest frame and it is found to be comparable with the ones in pppp collision. It is argued that theoretical simplicity and experimental clearness are the advantage of epep collision.Comment: Revised version of Phys. Rev. D69 (2004)03401

    A compact representation of the 2 photon 3 gluon amplitude

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    A compact representation of the loop amplitude gamma gamma ggg -> 0 is presented. The result has been obtained by using helicity methods and sorting with respect to an irreducible function basis. We show how to convert spinor representations into a field strength representation of the amplitude. The amplitude defines a background contribution for Higgs boson searches at the LHC in the channel H -> gamma gamma + jet which was earlier extracted indirectly from the one-loop representation of the 5-gluon amplitude.Comment: 15 pages Latex, 6 eps files included, revised versio

    Recursive numerical calculus of one-loop tensor integrals

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    A numerical approach to compute tensor integrals in one-loop calculations is presented. The algorithm is based on a recursion relation which allows to express high rank tensor integrals as a function of lower rank ones. At each level of iteration only inverse square roots of Gram determinants appear. For the phase-space regions where Gram determinants are so small that numerical problems are expected, we give general prescriptions on how to construct reliable approximations to the exact result without performing Taylor expansions. Working in 4+epsilon dimensions does not require an analytic separation of ultraviolet and infrared/collinear divergences, and, apart from trivial integrals that we compute explicitly, no additional ones besides the standard set of scalar one-loop integrals are needed.Comment: Typo corrected in formula 79. 22 pages, Latex, 1 figure, uses axodraw.st

    Numerical integration of one-loop Feynman diagrams for N-photon amplitudes

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    In the calculation of cross sections for infrared-safe observables in high energy collisions at next-to-leading order, one approach is to perform all of the integrations, including the virtual loop integration numerically. One would use a subtraction scheme that removes infrared and collinear divergences from the integrand in a style similar to that used for real emission graphs. Then one would perform the loop integration by Monte Carlo integration along with the integrations over final state momenta. In this paper, we have explored how one can perform the numerical integration. We have studied the N-photon scattering amplitude with a massless electron loop in order to have a case with a singular integrand that is not, however, so singular as to require the subtractions. We report results for N = 4, N = 5 with left-handed couplings, and N=6.Comment: 30 pages including 5 figures. This is a revised version that is close to the published versio

    The t->WZb decay in the Standard Model: A Critical Reanalysis

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    We compute the t->WZb decay rate, in the Standard Model, at the leading order in perturbation theory, with special attention to the effects of the finite widths of the W and Z bosons. These effects are extremely important, since the t->WZb decay occurs near its kinematical threshold. They increase the value of the decay rate by orders of magnitude near threshold or allow it below the nominal threshold. We discuss a procedure to take into account the finite-width effects and compare the results with previous studies of this decay. Within the Standard Model, for a top quark mass in the range between 170 and 180 GeV, we find BR(t->WZb) ~ 2 x 10^{-6}, which makes the observation at the LHC very difficult if at all possible.Comment: 10 pages, 4 eps figures, LaTeX. Few references added and minor changes in the text. Results unchanged. Final version to appear on PL

    Recursion Rules for Scattering Amplitudes in Non-Abelian Gauge Theories

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    We present a functional derivation of recursion rules for scattering amplitudes in a non-Abelian gauge theory in a form valid to arbitrary loop order. The tree-level and one-loop recursion rules are explicitly displayed.Comment: 18 pages, RevTeX, 2 postscript figures, a reference added, minor typographical errors correcte
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