5,271 research outputs found

    Tau polarization effects in the CNGS tau-neutrino appearance experiments

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    We studied tau polarization effects on the decay distributions of tau produced in the CNGS tau-neutrino appearance experiments. We show that energy and angular distributions for the decay products in the laboratory frame are significantly affected by the tau polarization. Rather strong azimuthal asymmetry about the tau momentum axis is predicted, which may have observable consequences in experiments even with small statistics.Comment: 5 pages, 6 eps figures, espcrc2.sty; Proceedings of the 4th International Workshop on Neutrino-Nucleus Interactions in the Few GeV Region (NuInt05), September 26-29, 2005, Okayama, Japa

    Recent measurements of CP violation at the B factories

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    Recent measurements of time dependent CP asymmetries at the B factories have led to substantial progress in our understanding of CP violation. In this article, I review some of these experimental results and discuss their implications in the Standard Model and their sensitivity to New Physics.Comment: 17 pages, 13 figures, to be published in Modern Physics Letters A. To be published in Modern Physics Letters

    Probing the CP nature of the Higgs bosons by top-pair production at photon linear colliders

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    We study effects of heavy Higgs bosons on the top-pair production process at photon linear colliders. The interference patterns between the resonant Higgs-production amplitudes and the continuum QED amplitudes are examined. The patterns tell us not only the CP nature of the Higgs bosons but also the phase of the γγ\gamma\gamma--Higgs vertex which gives new information about the Higgs couplings to new charged particles. We point out that it is necessary to use circularly polarized photon beams to produce efficiently heavy Higgs bosons whose masses exceed the electron beam energy, and show that the above interference patterns of the production amplitudes can be studied by observing top decay angular distributions. Analytic expressions for the helicity amplitudes for the sequential process γγttˉ(bW+)(bˉW)(bf1fˉ2)(bˉf3fˉ4)\gamma\gamma \to t \bar{t} \to (bW^+) (\bar{b}W^-) \to (b f_1 \bar{f}_2) (\bar{b} f_3 \bar{f}_4) are presented in terms of the generic γγttˉ\gamma\gamma \to t \bar{t} production amplitudes.Comment: 34 pages, LATEX file with 6 PS figures, comments adde

    Pseudoscalar Form Factors in Tau-Neutrino Nucleon Scattering

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    We investigate the pseudoscalar transition form factors of nucleon for quasi-elastic scattering and \Delta resonance production in tau-neutrino nucleon scattering via the charged current interactions. Although the pseudoscalar form factors play an important role for the \tau production in neutrino-nucleon scattering, these are not known well. In this article, we examine their effects in quasi-elastic scattering and \Delta resonance production and find that the cross section, Q^{2} distribution, and spin polarization of the produced \tau^{\pm} leptons are quite sensitive to the pseudoscalar form factors.Comment: 7 pages, 3 figure

    Polarization effects in tau production by neutrino

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    We studied polarization effects in tau production by neutrino-nucleon scattering. Quasi-elastic scattering, DeltaDelta resonance production and deep inelastic scattering processes are taken into account for the CERN-to-Gran Sasso projects. We show that the tau produced by neutrino has high degree of polarization, and its spin direction depends non-trivially on the energy and the scattering angle of tau in the laboratory frame.Comment: 6 pages, 5 eps figures, espcrc2.sty; Proceedings of the 3rd International Workshop on Neutrino-Nucleus Interactions in the Few GeV Region (NuInt04), March 17-21, 2004, Gran Sasso, Italy; minor changes, typos in Eq. (6) correcte

    Analyticity, crossing and the absorptive parts of the one-loop contributions to the quark-quark-gluon gauge boson four-point function

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    Starting from the known one-loop result for the e+ee^{+}e^{-}-annihilation process e+eγ,Zqqˉge^{+}e^{-}\stackrel{\gamma,Z} {\longrightarrow} q\bar{q}g with massless quarks we employ analyticity and crossing to determine the absorptive parts of the corresponding one-loop contributions in Deep Inelastic Scattering (DIS) and in the Drell-Yan process (DY). Whereas the O(αs2){\cal O}(\alpha_s^2) absorptive parts generate a non-measurable phase factor in the e+ee^{+}e^{-}-annihilation channel one obtains measurable phase effects from the one-loop contributions in the deep inelastic and in the Drell-Yan case. We compare our results with the results of previous calculations where the absorptive parts in DIS and in the DY process were calculated directly in the respective channels. We also present some new results on the dispersive and absorptive contributions of the triangle anomaly graph to the DIS process.Comment: 23 pages, 5 figures, typos corrected. Version to appear in Phys. Rev.

    Monte Carlo integration on GPU

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    We use a graphics processing unit (GPU) for fast computations of Monte Carlo integrations. Two widely used Monte Carlo integration programs, VEGAS and BASES, are parallelized on GPU. By using W+W^{+} plus multi-gluon production processes at LHC, we test integrated cross sections and execution time for programs in FORTRAN and C on CPU and those on GPU. Integrated results agree with each other within statistical errors. Execution time of programs on GPU run about 50 times faster than those in C, and more than 60 times faster than the original FORTRAN programs.Comment: 6 pages, 2 figure

    Light bottom squark and gluino confront electroweak precision measurements

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    We address the compatibility of a light sbottom (mass 2\sim 5.5 \gev) and a light gluino (mass 12\sim 16 \gev) with electroweak precision measurements. Such light particles have been suggested to explain the observed excess in the bb quark production cross section at the Tevatron. The electroweak observables may be affected by the sbottom and gluino through the SUSY-QCD corrections to the ZbbZbb vertex. We examine, in addition to the SUSY-QCD corrections, the electroweak corrections to the gauge boson propagators from the stop which are allowed to be light from the SU(2)L_L symmetry. We find that this scenario is strongly disfavored from electroweak precision measurements unless the heavier sbottom mass eigenstate is lighter than 180\gev and the left-right mixing in the stop sector is sufficiently large. This implies that one of the stops should be lighter than about 98\gev.Comment: 4 pages, revtex, 2 figures. Reference added, version to appear in Phys.Rev.Let

    Probing the Weak Boson Sector in γeZe\gamma e\rightarrow Ze

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    We study possible deviations from the standard model in the reaction γeZe\gamma e\rightarrow Ze at a 500 GeV e+ee^+e^- collider. As a photon source we use a laser backscattered photon beam. We investigate the most general γZγ\gamma Z\gamma and γZZ\gamma ZZ vertices including operators up to energy-dimension-six which are Lorentz invariant. These vertices require four extra parameters; two are CP-conserving, h1γh^\gamma_1 and h1Zh^Z_1, and two are CP-violating, h2γh^\gamma_2 and h2Zh^Z_2. We present analytical expressions of the helicity amplitudes for the process γeZe\gamma e\rightarrow Ze for arbitrary values of anomalous couplings. Assuming Standard Model values are actually measured we present the allowed region in the (h1γ,h1Zh^\gamma_1,h^Z_1) plane at the 90\% confidence level. We then show how the angular correlation of the ZZ decay products can be used to extract detailed information on the anomalous (especially CP-violating) γZγ\gamma Z\gamma and γZZ\gamma ZZ couplings.Comment: Latex, 25 pages, 12 figures (not included). One compressed postscript file including all the figures available at ftp://ftp.kek.jp/kek/preprints/TH/TH-420/kekth420.ps.g

    Calculation of HELAS amplitudes for QCD processes using graphics processing unit (GPU)

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    We use a graphics processing unit (GPU) for fast calculations of helicity amplitudes of quark and gluon scattering processes in massless QCD. New HEGET ({\bf H}ELAS {\bf E}valuation with {\bf G}PU {\bf E}nhanced {\bf T}echnology) codes for gluon self-interactions are introduced, and a C++ program to convert the MadGraph generated FORTRAN codes into HEGET codes in CUDA (a C-platform for general purpose computing on GPU) is created. Because of the proliferation of the number of Feynman diagrams and the number of independent color amplitudes, the maximum number of final state jets we can evaluate on a GPU is limited to 4 for pure gluon processes (gg4ggg\to 4g), or 5 for processes with one or more quark lines such as qqˉ5gq\bar{q}\to 5g and qqqq+3gqq\to qq+3g. Compared with the usual CPU-based programs, we obtain 60-100 times better performance on the GPU, except for 5-jet production processes and the gg4ggg\to 4g processes for which the GPU gain over the CPU is about 20
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