35,011 research outputs found

    Influence of Light and Heavy Thresholds on SUSY Unification

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    In this paper we study and compare susy unification using two different approaches in order to take into account the effect of light particle thresholds on the evolution of gauge couplings: the step--function approximation, on the one hand, and a mass dependent procedure, which gives a more accurate description of the dependence of the results on the masses, on the other. We also include the effect of heavy thresholds, when SU(5)SU(5) is chosen as the unifying group. We find that the mass--dependent procedure excludes scenarios where all susy masses are below 1  TeV1\;TeV, and favors a value of α3(mZ)\alpha_3(m_Z) near its upper experimental bound, contrary to the results obtained with the step--function approximation. We underline the dependence of the results on the procedure chosen to deal with light thresholds.Comment: 18 pages,LAEFF-93/014,REVTEX-2.1, 5 figures not included, available upon request (include FAX number)

    Semi-perturbative unification with extra vector-like families

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    We make a comprehensive analysis of an extended supersymmetric model(ESSM) obtained by adding a pair of vector-like families to the minimal supersymmetric standard model and having specific forms of 5 x 5 fermion mass matrices. The singlet Higgs couplings which link the ordinary to vector-like generations do not have the renormalization effects of the gauge interactions and hence the "quasi-infrared fixed point" near the scale of the top quark mass. The two-loop Yukawa effects on gauge couplings lead to an unified coupling αX\alpha_X around 0.2 with an unification scale M_X of 10^{16.9} GeV. Large Yukawa effects in the high energy region arrest the growth of the QCD coupling near M_X making the evolution flat. The renormalization effects of the vector-like generations on soft mass parameters has important effects on the charge and color breaking(CCB)minima. We will show that there exists parameter space where there is no charge and color breaking. We will also demonstrate that there exists minima of the Higgs potential which satisfies the mass of the Z boson but avoid CCB. Upper limits on the mass of the lightest Higgs boson from the one-loop effective scalar potential is obtained for sets of universal soft supersymmetry breaking mass parameters.Comment: LaTeX, 38 pages and 7 eps figures. Many revisions include

    Asymptotic safety, hypergeometric functions, and the Higgs mass in spectral action models

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    We study the renormalization group flow for the Higgs self coupling in the presence of gravitational correction terms. We show that the resulting equation is equivalent to a singular linear ODE, which has explicit solutions in terms of hypergeometric functions. We discuss the implications of this model with gravitational corrections on the Higgs mass estimates in particle physics models based on the spectral action functional.Comment: 25 pages, LaTeX, 8 PDF figure

    Supersymmetry Breakdown at a Hidden Wall

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    We consider hidden sector supersymmetry breakdown in the strongly coupled heterotic E8×E8E_8\times E_8 theory of Ho\v{r}ava and Witten. Using effective field theory methods in four dimensions, we can show that gravitational interactions induce soft breaking terms in the observable sector that are of order of the gravitiono mass. We apply these methods to the mechanism of gaugino condensation at the hidden wall. Although the situation is very similar to the weakly coupled case, there is a decisive difference concerning the observable sector gaugino mass; with desirable phenomenological as well as cosmological consequences.Comment: 29 pages, latex, no figures, To appear in Nuclear Physics B. Minor change

    Strong Unification

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    We investigate the possibility that unification occurs at strong coupling. We show that, despite the fact the couplings pass through a strong coupling regime, accurate predictions for their low energy values are possible because the couplings of the theory flow to infrared fixed points. We determine the low-energy QCD coupling in a favoured class of strong coupling models and find it is reduced from the weak coupling predictions, lying close to the experimentally measured value. We extend the analysis to the determination of quark and lepton masses and show that (even without Grand Unification) the infra-red fixed point structure may lead to good predictions for the top mass, the bottom to tau mass ratio and tanβ\tan \beta . Finally we discuss the implications for the unification scale finding it to be increased from the MSSM value and closer to the heterotic string prediction.Comment: 12 pages, LateX, no figure

    String-Unification, Universal One-Loop Corrections and Strongly Coupled Heterotic String Theory

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    We derive the universal threshold corrections in heterotic string theory including a continuous Wilson line. Unification of gauge and gravitational couplings is shown to be possible even within perturbative string theory. The relative importance of gauge group dependent and independent thresholds on unification is clarified. Equipped with these results we can then attempt an extrapolation to the strongly coupled heterotic string -- M-theory. We argue that such an extrapolation might be meaningful because of the holomorphic structure of the gauge coupling function and the close connection of the threshold corrections to the anomaly cancelation mechanism.Comment: 28 LaTex pages with 2 fig

    Time variation of fundamental couplings and dynamical dark energy

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    Scalar field dynamics may give rise to a nonzero cosmological variation of fundamental constants. Within different scenarios based on the unification of gauge couplings, the various claimed observations and bounds may be combined in order to trace or restrict the time history of the couplings and masses. If the scalar field is responsible for a dynamical dark energy or quintessence, cosmological information becomes available for its time evolution. Combining this information with the time variation of couplings, one can determine the interaction strength between the scalar and atoms, which may be observed by tests of the Weak Equivalence Principle. We compute bounds on the present rate of coupling variation from experiments testing the differential accelerations for bodies with equal mass and different composition and compare the sensitivity of various methods. In particular, we discuss two specific models of scalar evolution: crossover quintessence and growing neutrino models.Comment: 26 pages, 2 figures; minor typos & added references, to be published in JCA

    Grand Unification at Intermediate Mass Scales through Extra Dimensions

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    One of the drawbacks of conventional grand unification scenarios has been that the unification scale is too high to permit direct exploration. In this paper, we show that the unification scale can be significantly lowered (perhaps even to the TeV scale) through the appearance of extra spacetime dimensions. Such extra dimensions are a natural consequence of string theories with large-radius compactifications. We show that extra spacetime dimensions naturally lead to gauge coupling unification at intermediate mass scales, and moreover may provide a natural mechanism for explaining the fermion mass hierarchy by permitting the fermion masses to evolve with a power-law dependence on the mass scale. We also show that proton-decay constraints may be satisfied in our scenario due to the higher-dimensional cancellation of proton-decay amplitudes to all orders in perturbation theory. Finally, we extend these results by considering theories without supersymmetry; experimental collider signatures; and embeddings into string theory. The latter also enables us to develop several novel methods of explaining the fermion mass hierarchy via DD-branes. Our results therefore suggest a new approach towards understanding the physics of grand unification as well as the phenomenology of large-radius string compactifications.Comment: 65 pages, LaTeX, 20 figure
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