arXiv subjects

A. Zichichi

Publications and source records attributed to A. Zichichi.

70 records · Page 4Linked to original sources

arXiv1993

First Constraints on SU(5)xU(1) Supergravity from Trilepton Searches at the Tevatron

We present the first constraints on the parameter space of $SU(5)\times U(1)$ supergravity (in both no-scale and dilaton scenarios) which arise from the recently announced limits on trilepton searches at the Tevatron. The trilepton rate has been calculated for those points in parameter space which satisfy not only the minimal theoretical and experimental LEP constraints, but also the {\em combined} effect of the following indirect experimental constraints: (i) the CLEO limits on the $b\to sγ$ rate, (ii) the long-standing limit on the anomalous magnetic moment of the muon, (iii) the non-observation of anomalous muon fluxes in underground detectors (``neutrino telescopes"), and (iv) the electroweak LEP high-precision measurements in the form of the $ε_{1},ε_b$ parameters. For $m_t=150\GeV$, the trilepton constraint rules out some regions of parameter space with chargino masses as high as $m_{χ^\pm_1}\approx105\GeV$, although it is not possible to establish a new absolute lower bound on the chargino mass. For $m_t=170\GeV$, the simultaneous imposition of {\em all} of the above constraints excludes the dilaton scenario completely, and leaves only a few allowed points in parameter space in the no-scale scenario (with $m_{\tilde q}\approx m_{\tilde g}\lsim285\GeV$). The five-fold increase in integrated luminosity expected in the upcoming Tevatron run should probe some regions of parameter space with chargino masses much beyond the reach of LEPII.

hep-ph↗

arXiv1993

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

We explore the one-loop electroweak radiative corrections in $SU(5)\times U(1)$ supergravity via explicit calculation of vacuum-polarization and vertex-correction contributions to the $ε_1$ and $ε_b$ parameters. Experimentally, these parameters are obtained from a global fit to the set of observables $Γ_{l}, Γ_{b}, A^{l}_{FB}$, and $M_W/M_Z$. We include $q^2$-dependent effects, which induce a large systematic negative shift on $ε_{1}$ for light chargino masses ($m_{χ^\pm_1}\lsim70\GeV$). The (non-oblique) supersymmetric vertex corrections to $\Zbb$, which define the $ε_b$ parameter, show a significant positive shift for light chargino masses, which for $\tanβ\approx2$ 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 $ε_1$ and $ε_b$ do not constrain in any way $SU(5)\times U(1)$ 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 $\tanβ\gsim4$, remains allowed and corresponds to light chargino masses ($m_{χ^\pm_1}\lsim70\GeV$). Thus $SU(5)\times U(1)$ supergravity combined with high-precision LEP data would suggest the presence of light charginos if the top quark is not detected at the Tevatron.

hep-ph↗

arXiv1993

A Layman's guide to SUSY GUTs

The determination of the most straightforward evidence for the existence of the Superworld requires a guide for non-experts (especially experimental physicists) for them to make their own judgement on the value of such predictions. For this purpose we review the most basic results of Super-Grand unification in a simple and clear way. We focus the attention on two specific models and their predictions. These two models represent an example of a direct comparison between a traditional unified-theory and a string-inspired approach to the solution of the many open problems of the Standard Model. We emphasize that viable models must satisfy {\em all} available experimental constraints and be as simple as theoretically possible. The two well defined supergravity models, $SU(5)$ and $SU(5)\times U(1)$, can be described in terms of only a few parameters (five and three respectively) instead of the more than twenty needed in the MSSM model, \ie, the Minimal Supersymmetric extension of the Standard Model. A case of special interest is the strict no-scale $SU(5)\times U(1)$ supergravity where all predictions depend on only one parameter (plus the top-quark mass). A general consequence of these analyses is that supersymmetric particles can be at the verge of discovery, lurking around the corner at present and near future facilities. This review should help anyone distinguish between well motivated predictions and predictions based on arbitrary choices of parameters in undefined models.

hep-ph↗

arXiv1993

Scrutinizing Supergravity Models through Neutrino Telescopes

Galactic halo neutralinos ($χ$) captured by the Sun or Earth produce high-energy neutrinos as end-products of various annihilation modes. These neutrinos can travel from the Sun or Earth cores to the neighborhood of underground detectors (``neutrino telescopes") where they can interact and produce upwardly-moving muons. We compute these muon fluxes in the context of the minimal $SU(5)$ supergravity model, and the no-scale and dilaton $SU(5)\times U(1)$ supergravity models. At present, with the Kamiokande 90\% C.L. upper limits on the flux, only a small fraction of the parameter space of the $SU(5)\times U(1)$ models is accessible for $m_χ\sim m_{\rm Fe}$, which in turn implies constraints for the lightest chargino mass around 100 GeV for a range of $\tanβ$ values. We also delineate the regions of parameter space that would be accessible with the improvements of experimental sensitivity expected in the near future at Gran Sasso, Super-Kamiokande, and other facilities such as DUMAND and AMANDA, currently under construction. We conclude that if neutralinos are present in the halo, then this technique can be used to eventually explore more than half of the allowed parameter space of these specific models, and more generally of a large class of supergravity models, in many ways surpassing the reach of traditional collider experiments.

astro-ph↗

arXiv1993

A search for exact superstring vacua

We investigate $2d$ sigma-models with a $2+N$ dimensional Minkowski signature target space metric and Killing symmetry, specifically supersymmetrized, and see under which conditions they might lead to corresponding exact string vacua. It appears that the issue relies heavily on the properties of the vector $M_μ$, a reparametrization term, which needs to possess a definite form for the Weyl invariance to be satisfied. We give, in the $n = 1$ supersymmetric case, two non-renormalization theorems from which we can relate the $u$ component of $M_μ$ to the $β^G_{uu}$ function. We work out this $(u,u)$ component of the $β^G$ function and find a non-vanishing contribution at four loops. Therefore, it turns out that at order $α^{\prime 4}$, there are in general non-vanishing contributions to $M_u$ that prevent us from deducing superstring vacua in closed form.

hep-th↗

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.

hep-ph↗

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.

hep-ph↗

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

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

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 $α_3(M_Z)$.

hep-ph↗

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.

hep-ph↗

arXiv1992

Troubles with the Minimal SU(5) Supergravity Model

We show that within the framework of the minimal $SU(5)$ supergravity model, radiatively-induced electroweak symmetry breaking and presently available experimental lower bounds on nucleon decay, impose severe constraints on the available parameter space of the model which correspond to fine-tuning of the model parameters of over two orders of magnitude. Furthermore, a straightforward calculation of the cosmic relic density of neutralinos ($χ$) gives $Ω_χh^2\gg1$ for most of the allowed parameter space in this model, although small regions may still be cosmologically acceptable. We finally discuss how the {\it no-scale flipped $SU(5)$ supergravity model} avoids naturally the above troubles and thus constitutes a good candidate for the low-energy effective supergravity model.

hep-ph↗