114 research outputs found

    An Elusive Z' Coupled to Beauty

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    By extending the standard gauge group to SU(3)_c \times SU(2)_L \times U(1)_Y \times U(1)_X with X charges carried only by the third family we accommodate the LEP measurement of R_b and predict a potentially measurable discrepancy in A_{FB}^{b} in e^+e^- scattering and that D^0\bar{D}^0 mixing may be near its experimental limit. The Z', which explicitly violates the GIM mechanism, can nevertheless be naturally consistent with FCNC constraints. Direct detection of the Z' is possible but challenging.Comment: 12 pages, plus 1 Postscript figure, uses revtex, Discussion of FCNC extende

    R_b and New Physics: A Comprehensive Analysis

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    We survey the implications for new physics of the discrepancy between the LEP measurement of RbR_b and its Standard Model prediction. Two broad classes of models are considered: (ii) those in which new Z\bbar b couplings arise at tree level, through ZZ or bb-quark mixing with new particles, and (iiii) those in which new scalars and fermions alter the Z \bbar b vertex at one loop. We keep our analysis as general as possible in order to systematically determine what kinds of features can produce corrections to RbR_b of the right sign and magnitude. We are able to identify several successful mechanisms, which include most of those which have been recently been proposed in the literature, as well as some earlier proposals (\eg\ supersymmetric models). By seeing how such models appear as special cases of our general treatment we are able to shed light on the reason for, and the robustness of, their ability to explain RbR_b.Comment: 60 pages, 8 figures, plain tex, uses epsf. Final version to appear in Phys. Rev. D; propgating sign error corrected in eqs. 78, 87, 88, 89, 98, and 107; results unchange

    Constraints on the Universal Varying Yukawa Couplings: from SM-like to Fermiophobic

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    Varying the Standard Model (SM) fermion Yukawa couplings universally by a generic positive scale factor (FYuF_{Yu}), we study the phenomenological fit to the current available experimental results for the Higgs boson search at hadron colliders. We point out that the Higgs production cross section and its decay branching ratio to γγ\gamma\gamma can be varied oppositely by FYuF_{Yu} to make their product almost invariant. Thus, our scenario and the SM Higgs are indistinguishable in the inclusive HγγH\to \gamma\gamma channel. The current measurements on direct Yukawa coupling strength in the Hbbˉ/ττH\to b\bar{b}/\tau\tau channel are not precise enough to fix the scale factor FYuF_{Yu}. The most promising is the vector-boson-fusion channel in which the CMS has already observed possible suppression effect on the Yukawa couplings. Further more, the global χ2\chi^2 fit of the experimental data can get the optimal value by introducing a suppression factor FYu1/2F_{Yu}\sim1/2 on the SM Yukawa couplings.Comment: 16 pages, 12 figures, 5 tables, update analysis is supplemente

    Constraints on the parameters of the Left Right Mirror Model

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    We study some phenomenological constraints on the parameters of a left right model with mirror fermions (LRMM) that solves the strong CP problem. In particular, we evaluate the contribution of mirror neutrinos to the invisible Z decay width (\Gamma_Z^{inv}), and we find that the present experimental value on \Gamma_Z^{inv}, can be used to place an upper bound on the Z-Z' mixing angle that is consistent with limits obtained previously from other low-energy observables. In this model the charged fermions that correspond to the standard model (SM) mix with its mirror counterparts. This mixing, simultaneously with the Z-Z' one, leads to modifications of the \Gamma(Z --> f \bar{f}) decay width. By comparing with LEP data, we obtain bounds on the standard-mirror lepton mixing angles. We also find that the bottom quark mixing parameters can be chosen to fit the experimental values of R_b, and the resulting values for the Z-Z' mixing angle do not agree with previous bounds. However, this disagreement disappears if one takes the more recent ALEPH data.Comment: 7 pages, 2 figures, REVTe

    Spontaneous Breaking of Flavor Symmetry and Naturalness of Nearly Degenerate Neutrino Masses and Bi-maximal Mixing

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    The gauge model with SO(3)FSO(3)_{F} flavor symmetry and three Higgs triplets is studied. We show how the intriguing nearly degenerate neutrino mass and bi-maximal mixing scenario comes out naturally after spontaneous breaking of the symmetry. The hierarchy between the neutrino mass-squared differences, which is needed for reconciling both solar and atmospheric neutrino data, is naturally resulted from an approximate permutation symmetry. The model can also lead to interesting phenomena on lepton-flavor violations via the SO(3)FSO(3)_{F} gauge interactions.Comment: 13 pages, latex, no figures, the version appearing in SCIENCE IN CHINA (Series A), Vol.35 No.9 (2000

    Resonant origin for density fluctuations deep within the Sun: helioseismology and magneto-gravity waves

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    We analyze helioseismic waves near the solar equator in the presence of magnetic fields deep within the solar radiative zone. We find that reasonable magnetic fields can significantly alter the shapes of the wave profiles for helioseismic g-modes. They can do so because the existence of density gradients allows g-modes to resonantly excite Alfven waves, causing mode energy to be funnelled along magnetic field lines, away from the solar equatorial plane. The resulting wave forms show comparatively sharp spikes in the density profile at radii where these resonances take place. We estimate how big these waves might be in the Sun, and perform a first search for observable consequences. We find the density excursions at the resonances to be too narrow to be ruled out by present-day analyses of p-wave helioseismic spectra, even if their amplitudes were to be larger than a few percent. (In contrast it has been shown in (Burgess et al. 2002) that such density excursions could affect solar neutrino fluxes in an important way.) Because solar p-waves are not strongly influenced by radiative-zone magnetic fields, standard analyses of helioseismic data should not be significantly altered. The influence of the magnetic field on the g-mode frequency spectrum could be used to probe sufficiently large radiative-zone magnetic fields should solar g-modes ever be definitively observed. Our results would have stronger implications if overstable solar g-modes should prove to have very large amplitudes, as has sometimes been argued.Comment: 18 pages, 6 figures; misprints correcte

    Signature for heavy Majorana neutrinos in hadronic collisions

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    The production and decay of new possible heavy Majorana neutrinos are analyzed in hadronic collisions. New bounds on the mixing of these particles with standard neutrinos are estimated according to a fundamental representation suggested by grand unified models. A clear signature for these Majorana neutrinos is given by same-sign dileptons plus a charged weak vector boson in the final state. We discuss the experimental possibilities for the future Large Hadron Collider (LHC) at CERN.Comment: Latex2e(epsfig), 12 pages, 8 figures, to appear Physical Review

    Constraint of the magnetic moment of the top quark

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    We derive a bound on the magnetic dipole moment of the top quark in the context of the effective Lagrangian approach by using the values of the ratio Rb=Γb/ΓhR_b = \Gamma_b/\Gamma_h, Rl=Γh/ΓlR_l = \Gamma_h/\Gamma_l and ΓZ\Gamma_Z. We found that the oblique corrections are more sensible than the vertex ones for this moment.Comment: 6 pages, 3 figures, RevTe

    Discovering a Light Higgs Boson with Light

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    We evaluate the prospects for detecting a non-standard light Higgs boson with a significant branching ratio to two photons, in Run II of the Fermilab Tevatron. We derive the reach for several channels: 2γ2\gamma inclusive, 2γ+12\gamma+1 jet and 2γ+22\gamma+2 jets. We present the expected Run II limits on the branching ratio of hγγh\to\gamma\gamma as a function of the Higgs mass, for the case of ``bosonic'', as well as ``topcolor'' Higgs bosons.Comment: 11 pages, LaTeX, 7 figures, 4 tables, uses aipproc2.sty, contributed to the Physics at Run II Workshop, analysis redone with optimized cuts and improved background estimate, references adde

    Is Vtb=1 ?

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    The strongest constraint on Vtb presently comes from the 3 x 3 unitarity of the CKM matrix, which fixes Vtb to be very close to one. If the unitarity is relaxed, current information from top production at Tevatron still leaves open the possibility that Vtb is sizably smaller than one. In minimal extensions of the standard model with extra heavy quarks, the unitarity constraints are much weaker and the EW precision parameters entail the strongest bounds on Vtb. We discuss the experimental perspectives of discovering and identifying such new physics models at the Tevatron and the LHC, through a precise measurement of Vtb from the single top cross sections and by the study of processes where the extra heavy quarks are produced.Comment: 19 pages, 8 figure
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