4,263 research outputs found

    String Unification and Leptophobic Z′Z' in Flipped SU(5)

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    We summarize recent developments in the prediction for őĪs(MZ)\alpha_s(M_Z), self-consistent string unification and the dynamical determination of mass scales, and leptophobic Z‚Ä≤Z' gauge bosons in the context of stringy flipped SU(5). [To appear in Proceedings of Fourth International Conference on Supersymmetry (SUSY96), University of Maryland (May 1996).]Comment: 5 pages, LaTeX (uses espcrc2.sty), 5 figures (included

    M-theory Inspired No-scale Supergravity

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    We propose a supergravity model that contains elements recently shown to arise in the strongly-coupled limit of the E8√óE8E_8\times E_8 heterotic string (M-theory), including a no-scale--like K\"ahler potential, the identification of the string scale with the gauge coupling unification scale, and the onset of supersymmetry breaking at an intermediate scale determined by the size of the eleventh dimension of M-theory. We also study the phenomenological consequences of such scenario, which include a rather constrained sparticle spectrum within the reach of present-generation particle accelerators.Comment: 8 pages, LaTeX, 3 figures (included

    Moduli and K\"ahler potential in fermionic strings

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    We study the problem of identifying the moduli fields in fermionic four-dimensional string models. We deform a free-fermionic model by introducing exactly marginal operators in the form of Abelian Thirring interactions on the world-sheet, and show that their couplings correspond to the untwisted moduli fields. We study the consequences of this method for simple free-fermionic models which correspond to Z2√óZ2Z_2\times Z_2 orbifolds and obtain their moduli space and K\"ahler potential by symmetry arguments and by direct calculation of string scattering amplitudes. We then generalize our analysis to more complicated fermionic structures which arise in constructions of realistic models corresponding to asymmetric orbifolds, and obtain the moduli space and K\"ahler potential for this case. Finally we extend our analysis to the untwisted matter sector and derive expressions for the full K\"ahler potential to be used in phenomenological applications, and the target space duality transformations of the corresponding untwisted matter fields.Comment: 27pp Latex text, no figs, CERN-TH.7259/94, CTP-TAMU-14/94 and ACT-06/9

    Vanishing Str M^2 in the presence of anomalous U_A(1)

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    We show that the presence of an anomalous UA(1)\rm U_A(1) factor in the gauge group of string-derived models may have the new and important phenomenological consequence of allowing the vanishing of Str‚ÄČM2{\rm Str}\,{\cal M}^2 in the ``shifted" vacuum, that results in the process of cancelling the anomalous UA(1)\rm U_A(1). The feasibility of this effect seems to be enhanced by a vanishing vacuum energy, and by a ``small" value of Str‚ÄČM2{\rm Str}\,{\cal M}^2 in the original vacuum. In the class of free-fermionic models with vanishing vacuum energy that we focus on, a necessary condition for this mechanism to be effective is that Str‚ÄČM2>0{\rm Str}\,{\cal M}^2>0 in the original vacuum. A vanishing Str‚ÄČM2{\rm Str}\,{\cal M}^2 ameliorates the cosmological constant problem and is a necessary element in the stability of the no-scale mechanism.Comment: 7 pages, no figures, LaTe

    New Precision Electroweak Tests of SU(5) x U(1) Supergravity

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    We explore the one-loop electroweak radiative corrections in SU(5)√óU(1)SU(5)\times U(1) supergravity via explicit calculation of vacuum-polarization and vertex-correction contributions to the ŌĶ1\epsilon_1 and ŌĶb\epsilon_b parameters. Experimentally, these parameters are obtained from a global fit to the set of observables őďl,őďb,AFBl\Gamma_{l}, \Gamma_{b}, A^{l}_{FB}, and MW/MZM_W/M_Z. We include q2q^2-dependent effects, which induce a large systematic negative shift on ŌĶ1\epsilon_{1} for light chargino masses (m_{\chi^\pm_1}\lsim70\GeV). The (non-oblique) supersymmetric vertex corrections to \Zbb, which define the ŌĶb\epsilon_b parameter, show a significant positive shift for light chargino masses, which for tan‚Ā°ő≤‚Čą2\tan\beta\approx2 can be nearly compensated by a negative shift from the charged Higgs contribution. We conclude that at the 90\%CL, for m_t\lsim160\GeV the present experimental values of ŌĶ1\epsilon_1 and ŌĶb\epsilon_b do not constrain in any way SU(5)√óU(1)SU(5)\times U(1) supergravity in both no-scale and dilaton scenarios. On the other hand, for m_t\gsim160\GeV the constraints on the parameter space become increasingly stricter. We demonstrate this trend with a study of the m_t=170\GeV case, where only a small region of parameter space, with \tan\beta\gsim4, remains allowed and corresponds to light chargino masses (m_{\chi^\pm_1}\lsim70\GeV). Thus SU(5)√óU(1)SU(5)\times U(1) supergravity combined with high-precision LEP data would suggest the presence of light charginos if the top quark is not detected at the Tevatron.Comment: LaTeX, 11 Pages+4 Figures(not included), the figures available upon request as an uuencoded file(0.4MB) or 4 PS files from [email protected], CERN-TH.7078/93, CTP-TAMU-68/93, ACT-24/9

    SUSY signals at HERA in the no-scale flipped SU(5) supergravity model

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    Sparticle production and detection at HERA are studied within the recently proposed no-scale flipped SU(5)SU(5) supergravity model. Among the various reaction channels that could lead to sparticle production at HERA, only the following are within its limit of sensitivity in this model: e‚ąíp‚Üíe~L,R‚ąíŌái0+X,őĹ~eŌá1‚ąí+Xe^-p\to \tilde e^-_{L,R}\chi^0_i+X, \tilde \nu_e\chi^-_1+X, where Ōái0(i=1,2)\chi^0_i(i=1,2) are the two lightest neutralinos and Ōá1‚ąí\chi^-_1 is the lightest chargino. We study the elastic and deep-inelastic contributions to the cross sections using the Weizs\"acker-Williams approximation. We find that the most promising supersymmetric production channel is right-handed selectron (e~R\tilde e_{R}) plus first neutralino (Ōá10\chi^0_1), with one hard electron and missing energy signature. The őĹ~eŌá1‚ąí\tilde\nu_e\chi^-_1 channel leads to comparable rates but also allows jet final states. A right-handedly polarized electron beam at HERA would shut off the latter channel and allow preferentially the former one. With an integrated luminosity of {\cal L}=100\ipb, HERA can extend the present LEPI lower bounds on me~R,mőĹ~e,mŌá10m_{\tilde e_R}, m_{\tilde\nu_e},m_{\chi^0_1} by \approx25\GeV, while {\cal L}=1000\ipb will make HERA competitive with LEPII. We also show that the Leading Proton Spectrometer (LPS) at HERA is an excellent supersymmetry detector which can provide indirect information about the sparticle masses by measuring the leading proton longitudinal momentum distribution.Comment: 11 pages, 8 figures (available upon request as uuencoded file or separate ps files), tex (harvmac) CTP-TAMU-15/93, CERN/LAA/93-1

    Estimación de la conductividad eléctrica del tejido humano en la terapia de hipertermia por radiofrecuencia

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    The use of mathematical models to study complex systems such as physical and biological phenomena allows understanding their behavior, specifically regarding variables and parameters that are difficult to obtain. Additionally, studying optimization techniques has made it possible to approximate the characteristics of these systems by correlating numerical simulations and experimentation. Radiofrequency hyperthermia therapy for cancer treatment is currently under consideration for future medical applications. However, some of its properties are complex to measure, which could prevent their control. This is the case of electrical conductivity, which depends on the induction frequency and the tissue characteristics. In this paper, radiofrequency hyperthermia therapy was simulated via the finite element method. Then, an estimation of the electrical conductivity involved in the treatment was performed using the particle swarm optimization method. The execution time and the difference between the estimated parameter and the exact value were evaluated and compared with those obtained using the Levenberg-Marquardt method. The results indicate a significant agreement between the estimated and exact values in three different cases. The Levenberg-Marquardt method has a difference of 0,1942% and a performance time of 22 minutes, whereas the particle swarm optimization method has a difference of 0,0967% and a performance time of 327 minutes. The latter performs better in terms of parameter value estimation, whereas the former has better computational times. These techniques may help medical doctors to prescribe treatment protocols and may open the possibility of devising control strategies for hyperthermia therapy as a cancer treatment.El uso de modelos matem√°ticos para el estudio de sistemas complejos como los fen√≥menos f√≠sicos y biol√≥gicos permite comprender su comportamiento, espec√≠ficamente con respecto a variables y par√°metros dif√≠ciles de obtener. Adicionalmente, el estudio de t√©cnicas de optimizaci√≥n ha permitido aproximar las caracter√≠sticas de estos sistemas por medio de la correlaci√≥n de simulaciones num√©ricas y la experimentaci√≥n. La terapia de hipertermia por radiofrecuencia para el tratamiento del c√°ncer est√° actualmente en consideraci√≥n para su futura aplicaci√≥n m√©dica. Sin embargo, algunas de sus propiedades son dif√≠ciles de medir, lo cual impedir√≠a su control. Este es el caso de la conductividad el√©ctrica, que depende de la frecuencia de inducci√≥n y de las caracter√≠sticas del tejido. En este art√≠culo se simul√≥ la terapia de hipertermia por radiofrecuencia mediante el m√©todo de elementos finitos. Luego se realiz√≥ una estimaci√≥n de la conductividad el√©ctrica en el tratamiento mediante el m√©todo de optimizaci√≥n por enjambres de part√≠culas. Se evaluaron el tiempo de ejecuci√≥n y la diferencia del valor estimado con respecto al valor exacto, y se compararon sus valores estimados con los obtenidos mediante el m√©todo de Levenberg-Marquardt. Los resultados indican una concordancia significativa entre los valores estimados y los exactos en tres casos diferentes. El m√©todo de Levenberg-Marquardt tiene una diferencia de 0,1942% y un tiempo de ejecuci√≥n de 22 minutos, mientras que el m√©todo de optimizaci√≥n de enjambres de part√≠culas tiene una diferencia de 0,0967% y un tiempo de ejecuci√≥n de 327 minutos. Este √ļltimo tiene un mejor rendimiento en t√©rminos de estimaci√≥n del valor de los par√°metros, mientras que el otro tiene un mejor tiempo de ejecuci√≥n computacional. Estas t√©cnicas podr√≠an ayudar a los m√©dicos a prescribir protocolos de tratamiento y abrir la posibilidad de dise√Īar estrategias de control para la terapia de hipertermia como tratamiento para el c√°ncer.Fil: Lopez Perez, Jorge Iv√°n. Consejo Nacional de Investigaciones Cient√≠ficas y T√©cnicas. Centro Cient√≠fico Tecnol√≥gico Conicet - Bah√≠a Blanca. Instituto de Ciencias e Ingenier√≠a de la Computaci√≥n. Universidad Nacional del Sur. Departamento de Ciencias e Ingenier√≠a de la Computaci√≥n. Instituto de Ciencias e Ingenier√≠a de la Computaci√≥n; Argentina. Universidad Santiago de Cali; ColombiaFil: Bermeo Var√≥n, L. A.. Universidad Santiago de Cali; Colombi
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