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.Comment: 25 pages, Latex, 11 figures (not included), CERN-TH.7077/93, CTP-TAMU-65/93. A complete ps file (1.31MB) with embedded figures is available by request from [email protected]

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 $b\to s\gamma$ 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 $\epsilon_{1},\epsilon_b$ parameters. For m_t=150\GeV, the trilepton constraint rules out some regions of parameter space with chargino masses as high as m_{\chi^\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.Comment: 9 pages, 5 figures, latex. Figures available as 0.540MB uuencoded file from [email protected]. CERN-TH.7107/93, CTP-TAMU-72/9

RbR_b in supergravity models

We compute the supersymmetric contribution to $R_{b}\equiv \Gamma (Z\to b{\bar b})/\Gamma (Z\to {\rm hadrons})$ in a variety of supergravity models. We find R^{\rm susy}_b\lsim0.0004, which does not shift significantly the Standard Model prediction ($R^{\rm SM}_b=0.2162$ for m_t=160\GeV). An improvement in experimental precision by a factor of four would be required to be sensitive to such an effect.Comment: 5 pages, LaTeX, 1 figure (included)

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 $(m_t,\tan\beta,m_{\tilde g})$. We show that the LEP precise measurements of the electroweak parameters in the form of the $\epsilon_1$ 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 $\epsilon_1$ constraint (at 90(95)\%CL) requires the presence of light charginos (m_{\chi^\pm_1}\lsim50-100\GeV depending on $m_t$). Since all sparticle masses are proportional to $m_{\tilde g}$, m_{\chi^\pm_1}\lsim100\GeV implies: m_{\chi^0_1}\lsim55\GeV, m_{\chi^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_{\tilde\nu}\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 $(m_{\chi^\pm_1},\tan\beta)$ 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 $\epsilon_1$ constraint, we conclude that the outlook for chargino and selectron detection at LEPII and at HERA is quite favorable in this class of models.Comment: CTP-TAMU-40/93, Latex, 13 pages, 10 figures (available as uuencoded 0.963MB file from [email protected]

A New Supersensitive Flame Detector and its Use for Early Forest Fire Detection

A new flame detector, three orders of magnitude more powerful than the existing ones, is presented. This detector needs to be mass-produced for its use in order to be incorporated in an early forest fire detection system. A project able to implement its use to overcome the forest fire emergency is described

The top-quark mass in SU(5)xU(1) supergravity

We show that the currently experimentally preferred values of the top-quark mass (\ie, 130\lsim m_t\lsim180\GeV) are naturally understood in the context of string models, where the top-quark Yukawa coupling at the string scale is generically given by $\lambda_t={\cal O}(g)$, with $g$ the unified gauge coupling. A detailed study of the Yukawa sector of $SU(5)\times U(1)$ supergravity shows that the ratio of the bottom-quark to tau-lepton Yukawa couplings at the string scale is required to be in the range 0.7\lsim\lambda_b/\lambda_\tau\lsim1, depending on the values of $m_t$ and $m_b$. This result is consistent with $SU(5)\times U(1)$ symmetry, which does {\em not} require the equality of these Yukawa couplings in the unbroken symmetry phase of the theory. As a means of possibly predicting the value of $m_t$, we propose a procedure whereby the size of the allowed parameter space is determined as a function of $m_t$, since all sparticle and Higgs-boson masses and couplings depend non-trivially on $m_t$. At present, no significant preference for particular values of $m_t$ in $SU(5)\times U(1)$ supergravity is observed, except that high-precision LEP data requires m_t\lsim180\GeV.Comment: latex, 10 pages, 4 figures (included), CERN-TH.7138/94, CTP-TAMU-05/9

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 $\epsilon_1$ and $\epsilon_b$ parameters. Experimentally, these parameters are obtained from a global fit to the set of observables $\Gamma_{l}, \Gamma_{b}, A^{l}_{FB}$, and $M_W/M_Z$. We include $q^2$-dependent effects, which induce a large systematic negative shift on $\epsilon_{1}$ for light chargino masses (m_{\chi^\pm_1}\lsim70\GeV). The (non-oblique) supersymmetric vertex corrections to \Zbb, which define the $\epsilon_b$ parameter, show a significant positive shift for light chargino masses, which for $\tan\beta\approx2$ 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 $\epsilon_1$ and $\epsilon_b$ 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 \tan\beta\gsim4, remains allowed and corresponds to light chargino masses (m_{\chi^\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.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

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: $m_t$, $\tan\beta$, 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 $\tan\beta$ 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 $\mu$ (\mu>0:\,m_t\lsim135\GeV, \mu<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, $\tan\beta\approx1.4-1.6$ 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.Comment: CERN-TH.6926/93, CTP-TAMU-33/93, LaTeX, 33 pages, 11 embedded figures. Significant numerical correction to the dilaton scenario. Complete ps file (7943 blocks,3.33MB) available via anonymous ftp from site tamsun.tamu.edu in directory incoming, filename: CTP-TAMU-33-93.p

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 $\epsilon_{1,2,3}$ parameters. Experimentally, $\epsilon_{1,2,3}$ 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 $\epsilon_{1,2,3}$ for light chargino masses (m_{\chi^\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 $\epsilon_{1,2,3}$ 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.Comment: 15 pages,(6 ps figures available upon request), TeX(harvmac), CTP-TAMU-19/93, ACT-07/9