6,930 research outputs found

    Consequences of Leading-Logarithm Summation for the Radiative Breakdown of Standard-Model Electroweak Symmetry

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    In the empirically sensible limit in which QCD, t-quark Yukawa, and scalar-field-interaction coupling constants dominate all other Standard-Model coupling constants, we sum all leading-logarithm terms within the perturbative expansion for the effective potential that contribute to the extraction of the Higgs boson mass via radiative electroweak symmetry breaking. A Higgs boson mass of 216 GeV emerges from such terms, as well as a scalar-field-interaction coupling constant substantially larger than that anticipated from conventional spontaneous symmetry breaking. The sum of the effective potential's leading logarithms is shown to exhibit a local minimum in the limit ϕ→0\phi \to 0 if the QCD coupling constant is sufficiently strong, suggesting (in a multiphase scenario) that electroweak physics may provide the mechanism for choosing the asymptotically-free phase of QCD.Comment: latex using aip proceedings class. 8 page write-out of presentation at MRST 2003 Conference (Syracuse

    Improvement of Renormalization-Scale Uncertainties Within Empirical Determinations of the b-Quark Mass

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    Accurate determinations of the MS-bar b-quark mass mb(mb)m_b(m_b) from σ(e+e−→hadrons)\sigma(e^+e^-\to{\rm hadrons}) experimental data currently contain three comparable sources of uncertainty; the experimental uncertainty from moments of this cross-section, the uncertainty associated with αs(Mz)\alpha_s(M_z), and the theoretical uncertainty associated with the renormalization scale. Through resummation of all logarithmic terms explicitly determined in the perturbative series by the renormalization-group (RG) equation, it is shown that the renormalization-scale dependence is virtually eliminated as a source of theoretical uncertainty in mb(mb)m_b(m_b). This resummation also reduces the estimated effect of higher-loop perturbative contributions, further reducing the theoretical uncertainties in mb(mb)m_b(m_b). Furthermore, such resummation techniques improve the agreement between the values of the MS-bar b-quark mass extracted from the various moments of R(s)=σ(e+e−→hadrons)/σptR(s)=\sigma(e^+e^-\to{\rm hadrons})/\sigma_{pt} [σpt=4πα2/(3s)\sigma_{pt}=4\pi\alpha^2/(3s)], obviating the need to choose an optimummoment for determining mb(mb)m_b(m_b). Resummation techniques are also shown to reduce renormalization-scale dependence in the relation between b-quark MS-bar and pole mass and in the relation between the pole and 1S1S mass.Comment: 19 pages, latex2e, 6 eps figures contained in latex file. Errors corrected in equations (20)--(22

    Pade-Improved Estimate of Perturbative Contributions to Inclusive Semileptonic b→ub\to u Decays

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    Pade-approximant methods are used to estimate the three-loop perturbative contributions to the inclusive semileptonic b→ub \to u decay rate. These improved estimates of the decay rate reduce the theoretical uncertainty in the extraction of the CKM matrix element ∣Vub∣|V_{ub}| from the measured inclusive semileptonic branching ratio.Comment: 3 pages, latex, write-up of talk presented at DPF 200

    Higher Order Stability of a Radiatively Induced 220 GeV Higgs Mass

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    The effective potential for radiatively broken electroweak symmetry in the single Higgs doublet Standard Model is explored to four sequentially subleading logarithm-summation levels (5-loops) in the dominant Higgs self-interaction couplant λ\lambda. We augment these results with all contributing leading logarithms in the remaining large but sub-dominant Standard Model couplants (t-quark, QCD and SU(2)⊗U(1)SU(2)\otimes U(1) gauge couplants) as well as next to leading logarithm contributions from the largest of these, the t-quark and QCD couplants. Order-by-order stability is demonstrated for earlier leading logarithm predictions of an order 220 GeV Higgs boson mass in conjunction with fivefold enhancement of the value for λ\lambda over that anticipated from conventional spontaneous symmetry breaking.Comment: revtex, 6 pages. Analysis and text is expanded in revised versio

    On the Standard Approach to Renormalization Group Improvement

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    Two approaches to renormalization-group improvement are examined: the substitution of the solutions of running couplings, masses and fields into perturbatively computed quantities is compared with the systematic sum of all the leading log (LL), next-to-leading log (NLL) etc. contributions to radiatively corrected processes, with n-loop expressions for the running quantities being responsible for summing N^{n}LL contributions. A detailed comparison of these procedures is made in the context of the effective potential V in the 4-dimensional O(4) massless λϕ4\lambda \phi^{4} model, showing the distinction between these procedures at two-loop order when considering the NLL contributions to the effective potential V.Comment: 6 page

    Impeded Growth of Magnetic Flux Bubbles in the Intermediate State Pattern of Type I Superconductors

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    Normal state bubble patterns in Type I superconducting Indium and Lead slabs are studied by the high resolution magneto-optical imaging technique. The size of bubbles is found to be almost independent of the long-range interaction between the normal state domains. Under bubble diameter and slab thickness proper scaling, the results gather onto a single master curve. On this basis, in the framework of the "current-loop" model [R.E. Goldstein, D.P. Jackson and A.T. Dorsey, Phys. Rev. Lett. 76, 3818 (1996)], we calculate the equilibrium diameter of an isolated bubble resulting from the competition between the Biot-and-Savart interaction of the Meissner current encircling the bubble and the superconductor-normal interface energy. A good quantitative agreement with the master curve is found over two decades of the magnetic Bond number. The isolation of each bubble in the superconducting matrix and the existence of a positive interface energy are shown to preclude any continuous size variation of the bubbles after their formation, contrary to the prediction of mean-field models.Comment: \'{e}quipe Nanostructures Quantique
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