arXiv subjects

J. Lopez

Publications and source records attributed to J. Lopez.

At least 91 records · Page 5Linked to original sources

arXiv1993

"Large $(g-2)_μ$ in SU(5)xU(1) supergravity models"

We compute the supersymmetric contribution to the anomalous magnetic moment of the muon within the context of $SU(5)\times U(1)$ supergravity models. The largest possible contributions to $a^{susy}_μ$ occur for the largest allowed values of $\tanβ$ and can easily exceed the present experimentally allowed range, even after the LEP lower bounds on the sparticle masses are imposed. Such $\tanβ$ enhancement implies that $a^{susy}_μ$ can greatly exceed both the electroweak contribution ($\approx1.95\times10^{-9}$) and the present hadronic uncertainty ($\approx\pm1.75\times10^{-9}$). Therefore, the new E821 Brookhaven experiment (with an expected accuracy of $0.4\times10^{-9}$) should explore a large fraction (if not all) of the parameter space of these models, corresponding to slepton, chargino, and squarks masses as high as 200, 300, and 1000 GeV respectively. Moreover, contrary to popular belief, the $a^{susy}_μ$ contribution can have either sign, depending on the sign of the Higgs mixing parameter $μ$: $a^{susy}_μ>0\,(<0)$ for $μ>0$ ($μ<0$). The present $a_μ$ constraint excludes chargino masses in the range $45-120\GeV$ depending on the value of $\tanβ$, although there are no constraints for $\tanβ\lsim8$. We also compute $a^{susy}_τ$ and find $|a^{susy}_τ|\approx(m_τ/m_μ)^2\,|a^{susy}_μ|\lsim10^{-5}$ and briefly comment on its possible observability.

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arXiv1993

SUSY GUTs: A practical introduction

An introduction to the most important concepts in the subject of supersymmetric unified theories is presented. The emphasis is on the practical aspects leading to state-of-the-art calculations in this renascent subject. The topics covered include: generalities of supersymmetric unified theories, gauge and Yukawa coupling unification including the most up-to-date numerical analyses, soft supersymmetry breaking, and radiative electroweak symmetry breaking enforced using the tree-level and one-loop effective potentials. This class of supersymmetric models can be described in terms of five parameters: the top-quark mass ($m_t$), the ratio of Higgs vacuum expectation values ($\tanβ$), and three universal soft-supersymmetry-breaking parameters ($m_{1/2},m_0,A$). Thus, highly correlated predictions can be expected for all conceivable experimental observables. In effect, these general models provide a basic framework upon which more constrained models can be built.

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arXiv1993

The strongest experimental constraints on SU(5)xU(1) supergravity models

We consider a class of well motivated string-inspired flipped $SU(5)$ supergravity models which include four supersymmetry breaking scenarios: no-scale, strict no-scale, dilaton, and special dilaton, such that only three parameters are needed to describe all new phenomena $(m_t,\tanβ,m_{\tilde g})$. We show that the LEP precise measurements of the electroweak parameters in the form of the $ε_1$ variable, and the CLEOII allowed range for $\bsg$ are at present the most important experimental constraints on this class of models. For $m_t\gsim155\,(165)\GeV$, the $ε_1$ constraint (at 90(95)\%CL) requires the presence of light charginos ($m_{χ^\pm_1}\lsim50-100\GeV$ depending on $m_t$). Since all sparticle masses are proportional to $m_{\tilde g}$, $m_{χ^\pm_1}\lsim100\GeV$ implies: $m_{χ^0_1}\lsim55\GeV$, $m_{χ^0_2}\lsim100\GeV$, $m_{\tilde g}\lsim360\GeV$, $m_{\tilde q}\lsim350\,(365)\GeV$, $m_{\tilde e_R}\lsim80\,(125)\GeV$, $m_{\tilde e_L}\lsim120\,(155)\GeV$, and $m_{\tildeν}\lsim100\,(140)\GeV$ in the no-scale (dilaton) flipped $SU(5)$ supergravity model. The $\bsg$ constraint excludes a significant fraction of the otherwise allowed region in the $(m_{χ^\pm_1},\tanβ)$ plane (irrespective of the magnitude of the chargino mass), while future experimental improvements will result in decisive tests of these models. In light of the $ε_1$ constraint, we conclude that the outlook for chargino and selectron detection at LEPII and at HERA is quite favorable in this class of models.

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arXiv1993

SU(5)xU(1): a string paradigm of a TOE and its experimental consequences

We present a string-inspired/derived supergravity model based on the flipped $SU(5)\times U(1)$ structure supplemented by a minimal set of additional matter representations such that unification occurs at the string scale ($\sim10^{18}\GeV$). This model is complemented by two string supersymmetry breaking scenaria: the $SU(N,1)$ no-scale supergravity model and a dilaton-induced supersymmetry breaking scenario. Both imply universal soft supersymmetry breaking parameters: $m_0=0, A=0$ and $m_0=\coeff{1}{\sqrt{3}}m_{1/2}, A=-m_{1/2}$ respectively. In either case the models depend on only three parameters: $m_t$, $\tanβ$, and $m_{\tilde g}$. We present a comparative study of the sparticle and Higgs spectra of both models and conclude that even though both can be partially probed at the Tevatron, LEPII, and HERA, a larger fraction of the parameter space of the no-scale model is actually accessible. In both cases there is a more constrained version which allows to determine $\tanβ$ in terms of $m_t,m_{\tilde g}$. In the strict no-scale case we find that the value of $m_t$ determines the sign of $μ$ ($μ>0:\,m_t\lsim135\GeV$, $μ<0:\,m_t\gsim140\GeV$) and whether the lightest Higgs boson mass is above or below $100\GeV$. In the more constrained version of the dilaton scenario, $\tanβ\approx1.4-1.6$ and $m_t\lsim155\GeV$, $61\GeV\lsim m_h\lsim91\GeV$ follow. Thus, continuing Tevatron top-quark searches and LEPI,II Higgs searches could probe this restricted scenario completely.

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arXiv1993

The SuperWorlds of SU(5) and SU(5)xU(1): A Critical Assessment and Overview

We present an overview of the simplest supergravity models which enforce radiative breaking of the electroweak symmetry, namely the minimal $SU(5)$ supergravity model and the class of string-inspired/derived supergravity models based on the flipped $SU(5)\times U(1)$ structure supplemented by a minimal set of additional matter representations such that unification occurs at the string scale ($\sim10^{18}\GeV$). These models can be fully parametrized in terms of the top-quark mass, the ratio $\tanβ=v_2/v_1$, and three supersymmetry breaking parameters ($m_{1/2},m_0,A$). The latter are chosen in the minimal $SU(5)$ model such that the stringent constraints from proton decay and cosmology are satisfied. In the flipped $SU(5)$ case we consider two string-inspired supersymmetry breaking scenaria: $SU(N,1)$ no-scale supergravity and a dilaton-induced supersymmetry breaking scenario. Both imply universal soft supersymmetry breaking parameters: $m_0=A=0$ and $m_0=\coeff{1}{\sqrt{3}}m_{1/2}, A=-m_{1/2}$ respectively. We present a comparative study of the sparticle and Higgs spectra of both flipped $SU(5)$ models and the minimal $SU(5)$ model and conclude that all can be partially probed at the Tevatron and LEPII (and the flipped models at HERA too). In both flipped $SU(5)$ cases there is a more constrained version which allows to determine $\tanβ$ in terms of $m_t,m_{\tilde g}$ and which leads to much sharper and readily accessible experimental predictions. We also discuss the prospects for indirect experimental detection: a non-trivial fraction of the parameter space of the flipped $SU(5)$ models is in conflict with the present experimental allowed range for the $b\to sγ$ rare decay mode, and the one-loop electroweak radiative corrections imply the 90\% CL upper bound

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arXiv1993

Towards a unified string supergravity model

We present a unified string supergravity model based on a string-derived $SU(5)\times U(1)$ model and a string-inspired supersymmetry breaking scenario triggered by the $F$-term of the universally present dilaton field. This model can be described by three parameters: $m_t$, $\tanβ$, and $m_{\tilde g}$. We work out the predictions for all sparticle and Higgs masses and discuss the prospects for their detection at the Tevatron, LEPI,II, and HERA. We find that the cosmological neutralino relic density is always within current expectations (\ie, $Ω_χh^2_0\lsim0.9$). We also consider a more constrained version of this supersymmetry breaking scenario where the $B$-term is specified. In this case $\tanβ$ can be determined ($\tanβ\approx1.4-1.6$) and implies $m_t\lsim155\GeV$ and $m_h\lsim91\GeV$. Thus, continuing Tevatron top-quark searches and LEPI,II Higgs searches could probe this restricted scenario completely.

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arXiv1993

LEP and radiative electroweak breaking close the light gluino window

We show that the LEP lower bound on the chargino mass, in conjunction with the well motivated theoretical requirement of radiative electroweak symmetry breaking, imply an upper bound on the lightest Higgs boson mass ($m_h\lsim62\GeV$) in a supersymmetry breaking scenario where the gluino mass is a few GeV. Moreover, Higgs searches at LEP in the context of this model require $m_h\gsim61\GeV$. The remaining experimentally allowed region in the five-dimensional parameter space of this light gluino model is severely fine-tuned (with $\tanβ=1.88-1.89$ and $m_t=114.0-114.3\GeV$) and cosmologically troublesome (with a neutralino relic abundance over {\it five-hundred times larger than allowed}). Modest improvements in sensitivity in LEP Higgs searches and Tevatron top-quark searches should soon exclude this theoretically disfavored model completely.

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arXiv1993

Precision Electroweak Tests of the Minimal and Flipped SU(5) Supergravity Models

We explore the one-loop electroweak radiative corrections in the minimal $SU(5)$ and the no-scale flipped $SU(5)$ supergravity models via explicit calculation of vacuum polarization contributions to the $ε_{1,2,3}$ parameters. Experimentally, $ε_{1,2,3}$ are obtained from a global fit to the LEP observables, and $M_W/M_Z$ measurements. We include $q^2$-dependent effects which have been neglected in most previous ``model-independent" analyses of this type. These effects induce a large systematic negative shift on $ε_{1,2,3}$ for light chargino masses ($m_{χ^\pm_1}\lsim70\GeV$). In agreement with previous general arguments, we find that for increasingly large sparticle masses, the heavy sector of both models rapidly decouples, \ie, the values for $ε_{1,2,3}$ quickly asymptote to the Standard Model values with a {\it light} Higgs ($m_{H_{SM}}\sim100\GeV$). Specifically, at present the $90\%$ CL upper limit on the top-quark mass is $m_t\lsim175\GeV$ in the no-scale flipped $SU(5)$ supergravity model. These bounds can be strengthened for increasing chargino masses in the $50-100\GeV$ interval. In particular, for $m_t\gsim160\GeV$, the Tevatron may be able to probe through gluino($\tilde g$) and squark($\tilde q$) production up to $m_{\tilde g}\approx m_{\tilde q}\approx250\GeV$, exploring at least half of the parameter space in this model.

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arXiv1993

Probing supergravity models with the $b\to sγ$ microscope

We present a calculation of the branching ratio $\brbsg$ in two well motivated supersymmetric models: the minimal $SU(5)$ and the no-scale flipped $SU(5)$ supergravity models. We find that the improved CLEO upper bound ($\brbsg<5.4\times10^{-4}$ at 95\% CL) does not yet constrain the minimal $SU(5)$ supergravity model, where $\brbsg_{minimal}=(2.3-3.6)\times10^{-4}$. In the flipped $SU(5)$ model the CLEO bound is constraining, although still not very significantly, even for light charged Higgs masses. An improvement in sensitivity by a factor of two will probe all (more than half) of the parameter space of the minimal (flipped) $SU(5)$ supergravity model. This ``resolution" of the $\bsg$ microscope far surpasses that of present collider experiments. In the flipped model there exists a significant region of parameter space where $\brbsg$ is highly suppressed due to a new phenomenon involving a complicated cancellation against the QCD correction parameter.

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arXiv1993

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

Sparticle production and detection at HERA are studied within the recently proposed no-scale flipped $SU(5)$ 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: $e^-p\to \tilde e^-_{L,R}χ^0_i+X, \tilde ν_eχ^-_1+X$, where $χ^0_i(i=1,2)$ are the two lightest neutralinos and $χ^-_1$ is the lightest chargino. We study the elastic and deep-inelastic contributions to the cross sections using the Weizsäcker-Williams approximation. We find that the most promising supersymmetric production channel is right-handed selectron ($\tilde e_{R}$) plus first neutralino ($χ^0_1$), with one hard electron and missing energy signature. The $\tildeν_eχ^-_1$ 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 $m_{\tilde e_R}, m_{\tildeν_e},m_{χ^0_1}$ 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.

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arXiv1993

Accurate Neutralino Relic Density Computations in Supergravity Models

We investigate the question of the proper thermal averaging of neutralino annihilation amplitudes which possess poles and thresholds, as they impact on the calculated neutralino relic density and therefore on the cosmological viability of supersymmetric models. We focus on two typical resonances, namely the $Z$ boson and the lightest Higgs boson ($h$). In the context of supergravity models with radiative electroweak symmetry breaking, an exploration of the whole parameter space of the model is possible and the overall relevance of these sophisticated analyses can be ascertained. As an example we chose the minimal $SU(5)$ supergravity model since the presence of such poles is essential to obtain a cosmologically acceptable model. We find that the proper thermal averaging is important for individual points in parameter space and that the fraction of cosmologically acceptable points is increased somewhat by the accurate procedure. However, qualitatively the new set of points is very similar to that obtained previously using the usual series approximations to the thermal average. We conclude that all phenomenological analyses based on the previously determined cosmologically allowed set remain valid.

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arXiv1993

Flipped Angles and Phases: A Systematic Study

We discuss systematically the fermion mass and mixing matrices in a generic \linebreak field-theoretical flipped $SU(5)$ model, with particular applications to neutrino and baryon number-changing physics. We demonstrate that the different quark flavour branching ratios in proton decay are related to the Cabibbo-Kobayashi-Maskawa angles, whereas the lepton flavour branching ratios are undetermined. The light neutrino mixing angles observable via oscillation effects are related to the heavy conjugate (right-handed) neutrino mass matrix, which also plays a key role in cosmological baryogenesis. The ratios of neutrino and charged lepton decay modes in baryon decay may also be related to neutrino oscillation parameters. Plausible Ansätze for the generation structure of coupling matrices motivate additional relations between physical observables, and yield a satisfactory baryon asymmetry.

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arXiv1993

Sparticle and Higgs Production and Detection at LEPII in two Supergravity Models

We study the most promising signals for supersymmetry at LEPII in the context of two well motivated supergravity models: (i) the minimal $SU(5)$ supergravity model including the stringent constraints from proton stability and a not too young Universe, and (ii) a recently proposed string-inspired no-scale flipped $SU(5)$ supergravity model. Our computations span the neutralino, chargino, slepton, and Higgs sectors together with their interconnections in this class of models. We find that the number of `mixed' (1-lepton + 2-jets + $\slash{p}$) events occuring in the decay of pair-produced charginos ($χ^\pm_1$) is quite significant (per ${\cal L}=100\ipb$) for both models and that these predictions do not overlap. That is, if $m_{χ^\pm_1}<100\GeV$ then LEPII should be able to exclude at least one of the two models. In the no-scale flipped $SU(5)$ model we find that the number of acoplanar di-electron events from selectron pair production should allow for exploration of selectron masses up to the kinematical limit and chargino masses indirectly as high as 150 GeV. We find that the cross section $e^+e^-\to Z^*h$ deviates negligibly from the SM result in the minimal model, whereas it can be as much as $1/3$ lower in the flipped model. The usually neglected invisible mode $h\toχ^0_1χ^0_1$ can erode the preferred $h\to 2\,{\rm jets}$ signal by as much as $40\%$ in these models. We conclude that the charged slepton sector is a deeper probe than the chargino/neutralino or Higgs sectors of the flipped $SU(5)$ model at LEPII, while the opposite is true for the minimal $SU(5)$ model where the slepton sector is no probe at all.

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arXiv1993

Improved LEP lower bound on the lightest SUSY Higgs mass from Radiative Electroweak Breaking and its Experimental Consequences

We show that the present LEPI lower bound on the Standard Model Higgs boson mass ($M_H\gsim60\GeV$) applies as well to the lightest Higgs boson ($h$) of the minimal $SU(5)$ and no-scale flipped $SU(5)$ supergravity models. This result would persist even for the ultimate LEPI lower bound ($M_H\gsim70\GeV$). We show that this situation is a consequence of a decoupling phenomenon in the Higgs sector driven by radiative electroweak breaking for increasingly larger sparticle masses, and thus it should be common to a large class of supergravity models. A consequence of $m_h\gsim60\GeV$ in the minimal $SU(5)$ supergravity model is the exclusion from the allowed parameter space of `spoiler modes' ($χ^0_2\toχ^0_1 h$) which would make the otherwise very promising trilepton signal in $p\bar p\toχ^\pm_1χ^0_2X$ unobservable at Fermilab. Within this model we also obtain stronger upper bounds on the lighter neutralino and chargino masses, \ie, $m_{χ^0_1}\lsim50\GeV$, $m_{χ^0_2,χ^\pm_1}\lsim100\GeV$. This should encourage experimental searches with existing facilities.

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arXiv1992

Supersymmetry Tests at Fermilab: A Proposal

We compute the number of trilepton events to be expected at Fermilab as a result of the reaction $p\bar p\toχ^\pm_1χ^0_2X$, where $χ^\pm_1$ is the lightest chargino and $χ^0_2$ is the next-to-lightest neutralino. This signal is expected to have very little background and is the best prospect for supersymmetry detection at Fermilab if the gluino and squarks are beyond reach. We evaluate our expressions for all points in the allowed parameter space of two basic supergravity models: (i) the minimal $SU(5)$ supergravity model including the severe constraints from proton decay and a not too young Universe, and (ii) a recently proposed no-scale flipped $SU(5)$ supergravity model. We study the plausible experimental scenaria and conclude that a large portion of the parameter spaces of these models could be explored with $100\ipb$ of integrated luminosity. In the minimal $SU(5)$ supergravity model chargino masses up to $90-95\GeV$ could be probed. In the no-scale flipped model it should be possible to probe some regions of parameter space with $m_{χ^\pm_1}\lsim200\GeV$, therefore possibly doubling the reach of LEPII for chargino and neutralino (since $m_{χ^0_2}\approx m_{χ^\pm_1}$) masses. In both models such probes would indirectly explore gluino masses much beyond the reach of Fermilab.

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arXiv1992

Proposed Tests for Minimal SU(5) Supergravity at Fermilab, Gran Sasso, SuperKamiokande, and LEP

A series of predictions are worked out in order to put the minimal $SU(5)$ supergravity model under experimental test. Using the two-loop gauge coupling renormalization group equations, with the inclusion of supersymmetric threshold corrections, we calculate a new value for the proton decay rate in this model and find that SuperKamiokande and Gran Sasso should see the proton decay mode $p\to\barνK^+$ for most of the allowed parameter space. A set of physically sensible assumptions and the cosmological requirement of a not too young Universe give us a rather restrictive set of allowed points in the parameter space, which characterizes this model. This set implies the existence of interesting correlations among various masses: either the lightest chargino and the next-to-lightest neutralino are below $\approx100\GeV$ (and therefore observable at the Tevatron) or the lightest Higgs boson is below $\approx50\GeV$ (and therefore observable at LEP I-II). These tests are crucial steps towards selecting the correct low-energy effective supergravity model. We also comment on the compatibility of the model with $\sin^2θ_w(M_Z)$ measurements as a function of $α_3(M_Z)$.

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arXiv1992

The Simplest, String-Derivable, Supergravity Model and its Experimental Predictions

We present the simplest, string-derivable, supergravity model and discuss its experimental consequences. This model is a new string-inspired flipped $SU(5)$ which unifies at the string scale $M_U=10^{18}\GeV$ due to the introduction of an additional pair of \r{10},\rb{10} flipped $SU(5)$ representations which contain new intermediate scale `gap' particles. We study various model-building issues which should be addressed in string-derived incarnations of this model. We focus our study on the no-scale supergravity mechanism and explore thoroughly the three-dimensional parameter space of the model ($m_{\tilde g},m_t,\tanβ$), thus obtaining several simple relationships among the particle masses, such as: $m_{\tilde q}\approx m_{\tilde g}$, $m_{\tilde e_L}\approx m_{\tildeν}\approx0.30 m_{\tilde g}$, $m_{\tilde e_R}\approx0.18m_{\tilde g}$, and $m_{χ^0_2}\approx2m_{χ^0_1}\approx m_{χ^\pm_1}$. In a strict interpretation of the no-scale supergravity scenario we solve for $\tanβ$ as a function of $m_t$ and $m_{\tilde g}$, and show that $m_t$ determines not only the sign of the Higgs mixing parameter $μ$ but also whether the lightest Higgs boson mass is above or below $100\GeV$. We also find that throughout the parameter space the neutralino relic abundance is within observational bounds ($Ω_χh^2_0\lsim0.25$) and may account for a significant portion of the dark matter in the Universe.

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arXiv1992

Proton Decay and Cosmology Strongly Constrain the Minimal SU(5) Supergravity Model

We present the results of an extensive exploration of the five-dimensional parameter space of the minimal $SU(5)$ supergravity model, including the constraints of a long enough proton lifetime ($τ_p>1\times10^{32}\y$) and a small enough neutralino cosmological relic density ($Ω_χh^2_0\le1$). We find that the combined effect of these two constraints is quite severe, although still leaving a small region of parameter space with $m_{\tilde g,\tilde q}<1\TeV$. The allowed values of the proton lifetime extend up to $τ_p\approx1\times10^{33}\y$ and should be fully explored by the SuperKamiokande experiment. The proton lifetime cut also entails the following mass correlations and bounds: $m_h\lsim100\GeV$, $m_χ\approx{1\over2}m_{χ^0_2}\approx0.15\gluino$, $m_{χ^0_2}\approx m_{χ^+_1}$, and $m_χ<85\,(115)\GeV$, $m_{χ^0_2,χ^+_1}<165\,(225)\GeV$ for $α_3=0.113\,(0.120)$. Finally, the {\it combined} proton decay and cosmology constraints predict that if $m_h\gsim75\,(80)\GeV$ then $m_{χ^+_1}\lsim90\,(110)\GeV$ for $α_3=0.113\,(0.120)$. Thus, if this model is correct, at least one of these particles will likely be observed at LEPII.

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