arXiv subjects
Publications and source records attributed to J. Lopez.
We summarize recent developments in the prediction for $α_s(M_Z)$, self-consistent string unification and the dynamical determination of mass scales, and leptophobic $Z'$ 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).]
We present a unified no-scale supergravity model with a light gravitino that can naturally explain the observed $eeγγ$ event at CDF via right-handed selectron pair-production. The full spectrum of our model can be described in terms of a single parameter and can be distinguished from alternative proposals in the literature. Ongoing and future runs at LEP 2 should be able to probe the full allowed parameter space via acoplanar diphoton events from $e^+e^-\toχ^0_1χ^0_1$ production.
We calculate the one-loop supersymmetric electroweak-like and QCD-like corrections to the top-quark pair-production cross section at the Tevatron, including the important effects of non-degenerate top-squarks and left-right top-squark mixing. The largest electroweak effects yield a negative shift in the cross section and are enhanced right below the threshold for top-quark decay into top-squark and higgsino-like neutralino, and can be as large as $-35\%$. The largest QCD effects are positive and are enhanced for light top-squark masses, and can be as large as $20\%$. Such shifts greatly exceed the present theoretical uncertainty in the Standard Model prediction, and therefore may be experimentally observable. We also explore the one-loop shifts in scenarios containing light top-squarks and higgsino-like neutralinos that have been recently proposed to explain various apparent experimental anomalies.
We re-examine the phenomenological aspects of a recently proposed string-derived SU(5)$\times$U(1) one-parameter supergravity model, and explore the sensitivity of the model predictions to variations in the strong coupling and the string unification scale. We also perform an analysis of the constraints on the parameter space of the model in light of the recent Tevatron trilepton data and the LEP~1.5 chargino and slepton data. We obtain $m_{χ^\pm_1}\gsim70\GeV$, which excludes one-third of the parameter space of the model. The remainder of the parameter space should be probed by ongoing analyses of the Tevatron trilepton data and forthcoming LEP~2 runs.
The physics of supersymmetry is reviewed from the perspective of physics at ever increasing energies. Starting from the minimal supersymmetric extension of the Standard Model at the electroweak scale, we proceed to higher energies seeking to understand the origin of the many model parameters. Supersymmetric grand unification, supergravity, and superstrings are introduced sequentially, and their contribution to the sought explanations is discussed. Typical low-energy supersymmetric models are also presented, along with their possible experimental consequences via direct and indirect processes at high-energy physics experimental facilities. Contents 1. Introduction 2. Low-energy Supersymmetry 3. Supersymmetric Grand Unification 4. Supergravity 5. Superstrings 6. Dynamics 7. Experimental Prospects 8. Conclusions
We re-examine the possible magnitude of the supersymmetric contribution to $R_b\equivΓ(Z^0\to\bar b b)/Γ(Z^0\to{\rm all})$ in the light of the constraints imposed by the absence of light charginos at LEP~1.5, implementing also other available phenomenological constraints. We find the supersymmetric contribution to be $R^{\rm susy}_b < 0.0017$, and discuss the extent to which this upper bound could be strengthened by future constraints on the chargino and top-squark masses. Such values of $R^{\rm susy}_b$ tend to disfavor a supersymmetry explanation of the apparent $R_b$ discrepancy.
We present a new scenario for gauge coupling unification in flipped SU(5) string models, which identifies the $M_{32}$ scale of SU(3) and SU(2) unification with the empirical $M_{\rm LEP}\sim10^{15-16}$~GeV scale, and the $M_{51}$ scale of SU(5) and U(1) unification with the theoretical $M_{\rm string}\sim5\times10^{17}$~GeV string unification scale. The vacuum shift necessary for the cancellation of the anomalous $\rm U_A(1)$ and an SU(4) hidden sector with fractionally-charged particles, play a crucial role in the dynamical determination of all intermediate mass scales in this scenario.
In this Lecture we first review the basic properties that make flipped SU(5) a very economical and well motivated unified model, including some recent developments regarding the prediction for $α_s$ and new implications for proton decay. Then we sketch the derivation of flipped SU(5) from strings, stressing some new results concerning the cosmological constant, the stability of the no-scale mechanism, and a new mechanism for generating the ``LEP" scale $M_{\rm LEP}\sim10^{16}\GeV$ in string models. Finally we present a sample supersymmetry breaking scenario, where all sparticle masses depend on a single parameter, and discuss the experimental signatures at LEP and the Tevatron. This scenario also predicts the top-quark mass to be close to 175 GeV. (Lecture presented by D.~V.~Nanopoulos at the 33rd International School of Subnuclear Physics ``Vacuum and vacua: the physics of nothing", Erice, July 2-10, 1995.)
We use fits to recent published CPLEAR data on neutral kaon decays to $π^+π^-$ and $πeν$ to constrain the CPT--violation parameters appearing in a formulation of the neutral kaon system as an open quantum-mechanical system. The obtained upper limits of the CPT--violation parameters are approaching the range suggested by certain ideas concerning quantum gravity.
We show that the prediction for $α_s(M_Z)$ in flipped SU(5) is naturally lower than in minimal SU(5), and that the former can accommodate the full range of $α_s(M_Z)$ presently allowed by experiment. Our computations include two-loop ($δ_{\rm 2loop}$) and light ($δ_{\rm light}$) and heavy ($δ_{\rm heavy}$) threshold effects. Unlike minimal SU(5), in flipped SU(5) the heavy threshold effects can naturally decrease the predicted value of $α_s(M_Z)$. We also show that the value of the proton lifetime into the dominant channel $p\to e^+π^0$ is within the observable range at SuperKamiokande, and should discriminate against minimal supersymmetric SU(5), where the dominant mode is $p\to\barνK^+$.
We show that the presence of an anomalous $\rm U_A(1)$ factor in the gauge group of string-derived models may have the new and important phenomenological consequence of allowing the vanishing of ${\rm Str}\,{\cal M}^2$ in the ``shifted" vacuum, that results in the process of cancelling the anomalous $\rm U_A(1)$. The feasibility of this effect seems to be enhanced by a vanishing vacuum energy, and by a ``small" value of ${\rm Str}\,{\cal M}^2$ 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 ${\rm Str}\,{\cal M}^2>0$ in the original vacuum. A vanishing ${\rm Str}\,{\cal M}^2$ ameliorates the cosmological constant problem and is a necessary element in the stability of the no-scale mechanism.
We revisit the Missing Doublet Model (MDM) as a means to address the apparent difficulties of the minimal $SU(5)$ supergravity model in dealing with the doublet-triplet splitting problem, the prediction of $α_3(M_Z)$, and the proton lifetime. We revamp the original MDM by extending its observable sector to include fields and interactions that naturally suppress the dimension-five proton decay operators and that allow see-saw neutrino masses. We also endow the model with a hidden sector which (via gaugino condensation) triggers supersymmetry breaking of the desired magnitude, and (via hidden matter condensation) yields a new dynamical intermediate scale for the right-handed neutrino masses ($\sim10^{10}\GeV$), and provides an effective Higgs mixing parameter $μ$. The model is consistent with gauge coupling unification for experimentally acceptable values of $α_3(M_Z)$, and with proton decay limits even for large values of $\tanβ$. The right-handed neutrinos can be produced subsequent to inflation, and their out-of-equilibrium decays induce a lepton asymmetry which is later reprocessed (via sphaleron interactions) into a baryon asymmetry at the electroweak scale. The resulting see-saw neutrino masses provide a candidate for the hot dark matter component of the Universe ($m_{ν_τ}\sim{\cal O}(10\eV)$) and are consistent with the MSW solution to the solar neutrino problem. We finally compare the features of this traditional GUT model with that of the readily string-derivable $SU(5)\times U(1)$ model, and discuss the prospects of deriving the revamped MDM from string theory.
We compute the supersymmetric contribution to $R_{b}\equiv Γ(Z\to b{\bar b})/Γ(Z\to {\rm hadrons})$ in a variety of supersymmetric models. In the context of supergravity models with universal soft-supersymmetry-breaking and radiative electroweak breaking we find $R^{\rm susy}_b\lsim0.0004$, which does not shift significantly the Standard Model prediction ($R^{\rm SM}_b=0.2157$ for $m_t=175\GeV$; $R^{\rm exp}_b=0.2204\pm0.0020$). We also compute $R_b$ in the minimal supersymmetric standard model (MSSM), and delineate the region of parameter space which yields interestingly large values of $R_b$. This region entails light charginos and top-squarks, but is {\em strongly} restricted by the {\em combined} constraints from $B(b\to sγ)$ and a not-too-large invisible top-quark branching ratio: only a few percent of the points with $R^{\rm susy}_b>0.0020\,(1σ)$ are allowed.
We present a systematic phenomenological analysis of the tests of CPT symmetry that are possible within an {\em open} quantum-mechanical description of the neutral kaon system that is motivated by arguments based on quantum gravity and string theory. We develop a perturbative expansion in terms of the three small CPT-violating parameters admitted in this description, and provide expressions for a complete set of $K \rightarrow 2π, 3π$ and $π\ellν$ decay observables to second order in these small parameters. We also illustrate the new tests of CPT symmetry and quantum mechanics that are possible in this formalism using a regenerator. Indications are that experimental data from the CPLEAR and previous experiments could be used to establish upper bounds on the CPT-violating parameters that are of order $10^{-19}$ GeV, approaching the order of magnitude that may be attainable in quantum theories of gravity.
We propose a string-derived model based on the gauge group $SU(5)\times U(1)$ which satisfies the stringent constraints from no-scale supergravity, allows gauge coupling unification at the string scale, and entails previously unexplored correlations among various sectors of the model. All supersymmetric observables are given in terms of a single mass parameter with self-consistency of the model determining the rest, including $\tanβ=2.2-2.3$ and $m_t\approx175\GeV$. A small non-universality of the scalar masses at the string scale produces a downward shift in the right-handed slepton masses at the electroweak scale, such that for $m_{1/2}\gsim180\GeV$ these particles become lighter than the lightest neutralino. This cutoff in the parameter space entails the imminent discovery of charginos at the Tevatron via trilepton events ($m_{χ^\pm_1}<90\GeV$). Also, the lightest Higgs boson ($m_h<90\GeV$), the lightest chargino, and the right-handed sleptons ($m_{\tilde\ell_R}<50\GeV$) should be readily observable at LEPII. We also discuss the model predictions for $B(b\to sγ)$, $(g-2)_μ$, $R_b$, and the prospects for direct neutralino dark matter detection.
We explore the postulates of string no-scale supergravity in the context of free-fermionic string models. The requirements of vanishing vacuum energy, flat directions of the scalar potential, and stable no-scale mechanism impose strong restrictions on possible string no-scale models, which must possess only two or three moduli, and a constrained massless spectrum. The soft-supersymmetry-breaking parameters involving all twisted and untwisted fields are given explicitly. This class of models contain no free parameters, \ie, in principle all supersymmetric particle masses and interactions are completely determined. A computerized search for free-fermionic models with the desired properties yields a candidate $SU(5)\times U(1)$ model containing extra (\r{10},\rb{10}) matter representations that allow gauge coupling unification at the string scale. Our candidate model possesses a bening non-universal assignment of supersymmetry breaking scalar masses, which may have interesting low-energy experimental consequences.
We propose a new cosmological model with a time-dependent cosmological constant ($Λ\propto 1/t^2$), which starting at the Planck time as $Λ_{Pl}\sim M^2_{Pl}$, evolves to the present-day allowed value of $Λ_0\sim10^{-120}M^2_{Pl}$. This scenario is supported by non-critical string theory considerations. We compute the age of the Universe and the time-dependence of the scale factor in this model, and find general agreement with recent determinations of the Hubble parameter for substantial values of $Ω_{\rm Λ}$. This effectively low-density open Universe model differs from the traditional cosmological constant model, and has observable implications for particle physics and cosmology.
We compute the supersymmetric contribution to $R_{b}\equiv Γ(Z\to b{\bar b})/Γ(Z\to {\rm hadrons})$ 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.