arXiv subjects
Publications and source records attributed to D. Nanopoulos.
We show that the gravitino contribution to (g-2)_μ is finite in many popular supergravity models, including no-scale supergravity and string and M-theory models. This contribution is greatly enhanced for very light gravitino masses, and leads to new upper bounds on the ratio of the smuon to the gravitino mass.
We compactify the M-theory proposed by Horava and Witten on a Calabi-Yau manifold with boundary $S_1/Z_2$. A no-scale-like Kähler potential, the superpotential, and the gauge kinetic function are obtained in this 4-dimensional $E_6\times E_8$ model. We also study the general phenomenological consequences of the resulting M-theory-inspired model, which may include very light gravitinos, axions and axinos.
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 Kähler 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.
We discuss the highlights of recent developments in no-scale supergravity and flipped SU(5), especially in the context of a very light gravitino scenario that may explain the puzzling CDF e+e+gamma+gamma+E_T,miss event. We update the status of both subjects and discuss the impact of the latest experimental constraints from LEP and the Tevatron. We also comment on a new form of cold dark matter that may solve this phenomenological `problem' of light gravitino models. [Lecture presented by D. V. Nanopoulos at the 34th International School of Subnuclear Physics "Effective theories and fundamental interactions", Erice, July 3-12, 1996.]
A previous study of supersymmetric models has indicated that they are unlikely to make a large contribution to R_b. We revisit this analysis, taking into account the improved lower limits on sparticle masses provided recently by LEP 2 and the Tevatron, finding that a generic supersymmetric model cannot contribute more than about one-and-a-half current experimental standard deviations to R_b. We then specialize this analysis to minimal supergravity models with universal high-energy boundary conditions, and find a much more stringent upper bound R^{susy}_b < 0.0003. We discuss in detail why such models can only attain values of R^{susy}_b that are considerably smaller even than those obtainable in more general supersymmetric models.
We study the single-photon signals expected at LEP in models with a very light gravitino. The dominant process is neutralino-gravitino production (e+e- -> chi+ G) with subsequent neutralino decay via chi->gamma+G, giving a gamma+E_miss signal. We first calculate the cross section at arbitrary center-of-mass energies and provide new analytic expressions for the differential cross section valid for general neutralino compositions. We then consider the constraints on the gravitino mass from LEP 1 and LEP161 single-photon searches, and possible such searches at the Tevatron. We show that it is possible to evade the stringent LEP 1 limits and still obtain an observable rate at LEP 2, in particular in the region of parameter space that may explain the CDF e+e+gamma+gamma+E_T,miss event. As diphoton events from neutralino pair-production would not be kinematically accessible in this scenario, the observation of whichever photonic signal will discriminate among the various light-gravitino scenarios in the literature. We also perform a Monte Carlo simulation of the expected energy and angular distributions of the emitted photon, and of the missing invariant mass expected in the events. Finally we specialize the results to the case of a recently proposed one-parameter no-scale supergravity model.
We explore and contrast the single-photon and diphoton signals expected at LEP 2, that arise from neutralino-gravitino (e^+ e^- -> chi + gravitino -> gamma + E_miss) and neutralino-neutralino (e^+ e^- -> chi + chi -> gamma + gamma + E_miss) production in supersymmetric models with a light gravitino. LEP 1 limits imply that one may observe either one, but not both, of these signals at LEP 2, depending on the values of the neutralino and gravitino masses: single-photons for m_chi > Mz and m_gravitino < 3 x 10^-5 eV; diphotons for m_chi < Mz and all allowed values of m_gravitino.
We propose an analysis of LEP constraints on radiative neutralino decays into a light gravitino, based on the plane of the Higgs mixing parameter mu and the SU(2) gaugino mass M_2. The preliminary LEP 2W constraints in the (mu, M_2) plane are considerably stronger than for supersymmetric models in which the lightest neutralino is stable. A significant portion of the parameter space in which chargino or selectron decay into a final state containing a light gravitino could provide an interpretation of the CDF ee gamma gamma + E_T,miss event can now excluded by the preliminary LEP 2W data.
We examine the possibility that some anomalous acoplanar diphoton events observed at LEP may be consistent with the predictions of our previously proposed one-parameter no-scale supergravity model with a light gravitino, via the process e^+e^- -> chi chi -> gamma gamma+E_miss. We find that one such event may indeed be consistent with the model predictions for m_chi \approx (60-70) GeV. This region of parameter space is also consistent with the selectron and chargino interpretations of the CDF ee gamma gamma + E_T,miss event.
We show that leptophobic $Z'$ gauge bosons occur naturally in flipped SU(5) and may shift $R_b$ in an interesting way without upsetting the good values of $Γ_{had}$ and $R_c$. Within a string-derived version of the model, we study three possible scenarios and the constraints imposed on model building that would allow the new symmetry to remain unbroken down to low energies. Such $Z'$ gauge boson has generation non-universal couplings to quarks that violate parity maximally in the up-quark sector, and may contribute significantly to spin asymmetries in polarized $pp$ scattering experiments now being prepared for RHIC.
We discuss the scattering of a light closed-string state off a $D$ brane, taking into account quantum recoil effects on the latter, which are described by a pair of logarithmic operators. The light-particle and $D$-brane subsystems may each be described by a world-sheet with an external source due to the interaction between them. This perturbs each subsystem away from criticality, which is compensated by dressing with a Liouville field whose zero mode we interpret as time. The resulting evolution equations for the $D$ brane and the closed string are of Fokker-Planck and modified quantum Liouville type, respectively. The apparent entropy of each subsystem increases as a result of the interaction between them, which we interpret as the loss of information resulting from non-observation of the other entangled subsystem. We speculate on the possible implications of these results for the propagation of closed strings through a dilute gas of virtual $D$ branes.
After a quick tour through the present status of the Standard Model, an attempt is made to set up a framework to discuss some presently available exotica, including $R_b$ and the CDF $eeγγ+E_T\hskip-13pt/\ $ ``event." Supersymmetry seems to be a key player in establishing a paradigm shift beyond the Standard Model. (XI Topical Workshop on Proton-antiproton Collider Physics, Abano Terme (Padua), Italy, 26 May-1 June, 1996.)
We explore two possible interpretations for the CDF $eeγγ+E_T\hskip-13pt/\quad$ event in the context of a recently proposed one-parameter no-scale supergravity model with a light gravitino. We delineate the region in parameter space consistent with the kinematics of the event interpreted either as selectron pair-production ($p\bar p\to\tilde e^+\tilde e^-\,X$, $\tilde e=\tilde e_R,\tilde e_L$) or as chargino pair-production ($p\bar p\toχ^+_1χ^-_1\,X$). In the context of this model, the selectron interpretation requires $\tilde e=\tilde e_R$ and predicts comparable rates for $\ell^\pmγγ+E_T\hskip-13pt/\quad$, whereas the chargino interpretation predicts comparable rates for $(\ell^\pm\ell^{'+}\ell^{'-},\ell^+\ell^-,\ell^+\ell^-jj)\,γγ+E_T\hskip-13pt/\quad$. We also consider the constraints from LEP~1.5 and the expectations for LEP~2. We point out that one of the acoplanar photon pairs observed by the OPAL collaboration at LEP~1.5 may be attributable to supersymmetry in the present model via $e^+e^-\toχ^0_1χ^0_1\to γγ+E_T\hskip-13pt/\quad$. We also show how similar future events at LEP~2 may be used to deduce the neutralino mass.
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.
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.