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]

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

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: $e^-p\to \tilde e^-_{L,R}\chi^0_i+X, \tilde \nu_e\chi^-_1+X$, where $\chi^0_i(i=1,2)$ are the two lightest neutralinos and $\chi^-_1$ 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 ($\tilde e_{R}$) plus first neutralino ($\chi^0_1$), with one hard electron and missing energy signature. The $\tilde\nu_e\chi^-_1$ 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 $m_{\tilde e_R}, m_{\tilde\nu_e},m_{\chi^0_1}$ 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

M-theory Inspired No-scale Supergravity

We propose a supergravity model that contains elements recently shown to arise in the strongly-coupled limit of the $E_8\times E_8$ 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

New Precision Electroweak Tests in Supergravity Models

We update the analysis of the precision electroweak tests in terms of 4 epsilon parameters, $\epsilon_{1,2,3,b}$, to obtain more accurate experimental values of them by taking into account the new LEP data released at the 28th ICHEP (1996, Poland). We also compute $\epsilon_1$ and $\epsilon_b$ in the context of the no-scale $SU(5)\times U(1)$ supergravity model to obtain the updated constraints by imposing the correlated constraints in terms of the experimental ellipses in the $\epsilon_1-\epsilon_b$ plane and also by imposing the new bound on the lightest chargino mass, $m_{\chi^\pm_1}\gtrsim 79$ $GeV$. Upon imposing these new experimental results, we find that the situations in the no-scale model are much more favorable than those in the standard model, and if $m_t\gtrsim 170$ $GeV$, then the allowed regions at the 95% C.~L. in the no-scale model are $\tan\beta\gtrsim 4$ and $m_{\chi^\pm_1}\lesssim 120 (82)$ $GeV$ for $\mu>0 (\mu<0)$, which are in fact much more stringent than in our previous analysis. Therefore, assuming that $m_t\gtrsim 170$ $GeV$, if the lightest chargino mass bound were to be pushed up only by a few GeV, the sign on the Higgs mixing term $\mu$ in the no-scale model could well be determined from the $\epsilon_1-\epsilon_b$ constraint to be positive at the 95% C.~L. At any rate, better accuracy in the measured $m_t$ from the Tevatron in the near future combined with the LEP data is most likely to provide a decisive test of the no-scale $SU(5)\times U(1)$ supergravity model.Comment: 15 pages, REVTEX, 1 figure (not included but available as a ps file from [email protected]