10,568 research outputs found

    Vector Boson Pair Production and Trilinear Gauge Boson Couplings - Results From the Tevatron

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    Direct measurements of vector boson pair production processes and trilinear gauge boson couplings have been conducted by the CDF and DO Collaborations. Preliminary results from searches for anomalous WW/WZ->muon-neutrino-jet-jet and WZ->e-e-e-neutrino production are presented. 95% CL anomalous coupling limits from previously published DO results are -0.20 < lambda < 0.20 (Delta kappa=0) and -0.30 < Delta kappa < 0.43 (lambda=0) for Lambda=2000 GeV where the WWgamma couplings are assumed to equal the WWZ couplings. Combined DO + LEP experiment anomalous coupling limits are presented for the first time. 95% CL limits are -0.16<lambda(gamma)< 0.10 (Delta kappa=0) and -0.15 < Delta kappa(gamma) < 0.41 (lambda=0) under the assumption that the couplings are related by the ``HISZ'' constraints. 95% CL anomalous ZZg and Zgg coupling limits from DO are |h(30)^Z|<0.36 (h(40)^Z=0) and |h(40)^Z|<0.05 (h(30)^Z=0) for Lambda=750 GeV. CDF reports the first observation of a ZZ event. Prospects for Run II are discussed.Comment: Submitted to the proceedings of ICHEP 98 XXIX International Conference on High Energy Physics, UBC, Vancouver, B.C., Canada, July 23-29, 1998. 6 page

    Exclusive electroproduction and off-diagonal parton distributions

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    Off-diagonal parton distributions occur in several hard exclusive reactions. They extend the study of hadron structure beyond what can be learned from ordinary distributions and have a particularly rich spin structure. The hard scattering subprocesses in electroproduction of mesons and of real photons satisfy helicity selection rules, which provide powerful tools to test leading-twist dominance at a given value of the hard scale.Comment: 7 pages, 3 figures, LaTeX2e. Talk given at the Fourth Workshop on Quantum Chromodynamics, Paris, France, 1-6 June 199

    Polarisation in deeply virtual meson production

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    We discuss two aspects of polarisation in hard exclusive meson production: the leading-twist selection rule for the meson helicity, and the different partial waves of a (pi pi)-pair which may or may not be due to the decay of a rho.Comment: 9 pages, 1 figure. Talk given at the Workshop on Exclusive and Semiexclusive Processes at High Momentum Transfer, Jefferson Lab, May 1999. Remark added on the transition from two gluons to a pion pai

    Exclusive hadronization in gamma* gamma -> pi pi

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    We perform a QCD analysis of exclusive two-meson production in gamma* gamma collisions in the kinematical domain of large photon virtuality Q^2 and small hadronic invariant mass W^2. In these kinematics the amplitude factorizes into a perturbative subprocess gamma* gamma -> q qbar and a two-meson distribution amplitude. This opens the way to study new hadronic matrix elements related with the physics of hadronization and confinement.Comment: 5 pages, 1 figure. Talk given at PHOTON 99, Freiburg, Germany, 23-27 May 1999. Eq. (8) correcte

    Massive Field-Theory Approach to Surface Critical Behavior in Three-Dimensional Systems

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    The massive field-theory approach for studying critical behavior in fixed space dimensions d<4d<4 is extended to systems with surfaces.This enables one to study surface critical behavior directly in dimensions d<4d<4 without having to resort to the ϵ\epsilon expansion. The approach is elaborated for the representative case of the semi-infinite |\bbox{\phi}|^4 nn-vector model with a boundary term {1/2} c_0\int_{\partial V}\bbox{\phi}^2 in the action. To make the theory uv finite in bulk dimensions 3d<43\le d<4, a renormalization of the surface enhancement c0c_0 is required in addition to the standard mass renormalization. Adequate normalization conditions for the renormalized theory are given. This theory involves two mass parameter: the usual bulk `mass' (inverse correlation length) mm, and the renormalized surface enhancement cc. Thus the surface renormalization factors depend on the renormalized coupling constant uu and the ratio c/mc/m. The special and ordinary surface transitions correspond to the limits m0m\to 0 with c/m0c/m\to 0 and c/mc/m\to\infty, respectively. It is shown that the surface-enhancement renormalization turns into an additive renormalization in the limit c/mc/m\to\infty. The renormalization factors and exponent functions with c/m=0c/m=0 and c/m=c/m=\infty that are needed to determine the surface critical exponents of the special and ordinary transitions are calculated to two-loop order. The associated series expansions are analyzed by Pad\'e-Borel summation techniques. The resulting numerical estimates for the surface critical exponents are in good agreement with recent Monte Carlo simulations. This also holds for the surface crossover exponent Φ\Phi.Comment: Revtex, 40 pages, 3 figures, and 8 pictograms (included in equations

    Boundary critical behavior at m-axial Lifshitz points for a boundary plane parallel to the modulation axes

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    The critical behavior of semi-infinite dd-dimensional systems with nn-component order parameter ϕ\bm{\phi} and short-range interactions is investigated at an mm-axial bulk Lifshitz point whose wave-vector instability is isotropic in an mm-dimensional subspace of Rd\mathbb{R}^d. The associated mm modulation axes are presumed to be parallel to the surface, where 0md10\le m\le d-1. An appropriate semi-infinite ϕ4|\bm{\phi}|^4 model representing the corresponding universality classes of surface critical behavior is introduced. It is shown that the usual O(n) symmetric boundary term ϕ2\propto \bm{\phi}^2 of the Hamiltonian must be supplemented by one of the form λ˚α=1m(ϕ/xα)2\mathring{\lambda} \sum_{\alpha=1}^m(\partial\bm{\phi}/\partial x_\alpha)^2 involving a dimensionless (renormalized) coupling constant λ\lambda. The implied boundary conditions are given, and the general form of the field-theoretic renormalization of the model below the upper critical dimension d(m)=4+m/2d^*(m)=4+{m}/{2} is clarified. Fixed points describing the ordinary, special, and extraordinary transitions are identified and shown to be located at a nontrivial value λ\lambda^* if ϵd(m)d>0\epsilon\equiv d^*(m)-d>0. The surface critical exponents of the ordinary transition are determined to second order in ϵ\epsilon. Extrapolations of these ϵ\epsilon expansions yield values of these exponents for d=3d=3 in good agreement with recent Monte Carlo results for the case of a uniaxial (m=1m=1) Lifshitz point. The scaling dimension of the surface energy density is shown to be given exactly by d+m(θ1)d+m (\theta-1), where θ=νl4/νl2\theta=\nu_{l4}/\nu_{l2} is the anisotropy exponent.Comment: revtex4, 31 pages with eps-files for figures, uses texdraw to generate some graphs; to appear in PRB; v2: some references and additional remarks added, labeling in figure 1 and some typos correcte

    The 26^{26}Al Gamma-ray Line from Massive-Star Regions

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    The measurement of gamma rays from the diffuse afterglow of radioactivity originating in massive-star nucleosynthesis is considered a laboratory for testing models, when specific stellar groups are investigated, at known distance and with well-constrained stellar population. Regions which have been exploited for such studies include Cygnus, Carina, Orion, and Scorpius-Centaurus. The Orion region hosts the Orion OB1 association and its subgroups at about 450~pc distance. We report the detection of 26^{26}Al gamma rays from this region with INTEGRAL/SPI.Comment: Contribution to Symposium "Nuclei in the Cosmos XIV", Niigata, Japan, Jun 2016; 3 pages, 2 figures; accepted for publication in JPS (Japan Physical Society) Conference Proceedings http://jpscp.jps.jp
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