arXiv subjects

D. Nanopoulos

Publications and source records attributed to D. Nanopoulos.

At least 37 records · Page 2Linked to original sources

arXiv1995

Flipped SU(5): a Grand Unified Superstring Theory (GUST) Prototype

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.)

hep-ph↗

arXiv1995

Test of CPT Symmetry and Quantum Mechanics with Experimental data from CPLEAR

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.

hep-ex↗

arXiv1995

Lowering alpha_s by flipping SU(5)

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 $p\to\barνK^+$.

hep-ph↗

arXiv1995

Vanishing Str M^2 in the presence of anomalous U_A(1)

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.

hep-ph↗

arXiv1995

The Missing Doublet Model Revamped

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.

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arXiv1995

Non-Critical String Theory Formulation of Microtubule Dynamics and Quantum Aspects of Brain Function

Microtubule (MT) networks, subneural paracrystalline cytosceletal structures, seem to play a fundamental role in the neurons. We cast here the complicated MT dynamics in the form of a $1+1$-dimensional non-critical string theory, thus enabling us to provide a consistent quantum treatment of MTs, including enviromental {\em friction} effects. We suggest, thus, that the MTs are the microsites, in the brain, for the emergence of stable, macroscopic quantum coherent states, identifiable with the {\em preconscious states}. Quantum space-time effects, as described by non-critical string theory, trigger then an {\em organized collapse} of the coherent states down to a specific or {\em conscious state}. The whole process we estimate to take ${\cal O}(1\,{\rm sec})$, in excellent agreement with a plethora of experimental/observational findings. The {\em microscopic arrow of time}, endemic in non-critical string theory, and apparent here in the self-collapse process, provides a satisfactory and simple resolution to the age-old problem of how the, central to our feelings of awareness, sensation of the progression of time is generated.

hep-ph↗

arXiv1995

$R_b$ in supersymmetric models

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.

hep-ph↗

arXiv1995

Theory of Brain Function, Quantum Mechanics and Superstrings

Recent developments/efforts to understand aspects of the brain function at the {\em sub-neural} level are discussed. MicroTubules (MTs) participate in a wide variety of dynamical processes in the cell, especially in bioinformation processes such as learning and memory, by possessing a well-known binary error-correcting code with 64 words. In fact, MTs and DNA/RNA are unique cell structures that possess a code system. It seems that the MTs' code system is strongly related to a kind of ``Mental Code" in the following sense. The MTs' periodic paracrystalline structure make them able to support a superposition of coherent quantum states, as it has been recently conjectured by Hameroff and Penrose, representing an external or mental order, for sufficient time needed for efficient quantum computing. Then the quantum superposition collapses spontaneously/dynamically through a new, string-derived mechanism for collapse proposed recently by Ellis, Mavromatos, and myself. At the moment of collapse, organized quantum exocytosis occurs, and this is how a ``{\em mental order}" may be translated into a ``{\em physiological action}". Our equation for quantum collapse, tailored to the MT system, predicts that it takes 10,000 neurons ${\cal O}(1\,{\rm sec})$ to dynamically collapse (process and imprint information). Different observations/experiments and various schools of thought are in agreement with the above numbers concerning ``{\em conscious events}". If indeed MTs, may be considered as the {\em microsites of consciousness}, then several unexplained properties of consciousness/awareness, get easily explained, including ``{\em backward masking}", ``{\em referal backwards in time}". The {\em non-locality} in the cerebral cortex of neurons related to particular missions, and the related {\em unitary sense of self} as well as

hep-ph↗

arXiv1995

Precision Tests of CPT Symmetry and Quantum Mechanics in the Neutral Kaon System

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.

hep-ph↗

arXiv1995

A string no-scale supergravity model and its experimental consequences

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.

hep-ph↗

arXiv1995

String no-scale supergravity

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.

hep-ph↗

arXiv1995

A new cosmological constant model

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.

hep-ph↗

arXiv1995

$R_b$ in supergravity models

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.

hep-ph↗

arXiv1994

Supersymmetry dileptons and trileptons at the Tevatron

We consider the production of supersymmetry neutralinos and charginos in $p\bar p$ collisions at the Tevatron, and their subsequent decay via hadronically quiet dileptons and trileptons. We perform our computations in the context of a variety of supergravity models, including generic four-parameter supergravity models, the minimal $SU(5)$ supergravity model, and $SU(5)\times U(1)$ supergravity with string inspired two- and one-parameter moduli and dilaton scenarios. Our results are contrasted with estimated experimental sensitivities for dileptons and trileptons for integrated luminosities of $100\ipb$ and $1\ifb$, which should be available in the short and long terms. We find that the dilepton mode is a needed complement to the trilepton signal when the latter is suppressed by small neutralino leptonic branching ratios. The estimated reaches in chargino masses can be as large as $100\,(150)\GeV$ for $100\ipb\,(1\ifb)$. We also discuss the task left for LEPII once the Tevatron has completed its short-term search for dilepton and trilepton production.

hep-ph↗

arXiv1994

Direct detection of dark matter in SU(5)xU(1) supergravity

We compute the scattering rates for the lightest neutralino $χ^0_1$ in the forthcoming germanium $(^{73}{\rm Ge}+^{76}{\rm Ge})$ detector and a proposed lead detector $(^{207}{\rm Pb})$, within the framework of $SU(5)\times U(1)$ supergravity. We find that in only a small portion ($\lsim10\%$) of the parameter spaces of this class of models, are the rates in the germanium detector above the expected initial experimental sensitivity of 0.1 events/kg/day. However, a much larger portion ($\lsim40\%$) of the parameter spaces could be probed with an improved background rejection capability (0.01 events/kg/day) and/or a more sensitive detector $(^{207}{\rm Pb})$.

hep-ph↗

arXiv1994

The march towards no-scale supergravity

The different steps that led us to the discovery of {\em no-scale supergravity} are discussed from a very personal point of view. No-scale supergravity has been heralded as the most promising effective theory that describes physics below the Planck scale. In its string-derived form it holds the potential for a Dynamical Determination Of Everything (DDOE). (Invited talk presented at the International Conference on: ``The History of Original Ideas and Basic Discoveries in Particle Physics", Erice, Italy, 29 July - 4 August 1994.)

hep-ph↗

arXiv1994

New phenomena in the standard no-scale supergravity model

We revisit the no-scale mechanism in the context of the simplest no-scale supergravity extension of the Standard Model. This model has the usual five-dimensional parameter space plus an additional parameter $ξ_{3/2}\equiv m_{3/2}/m_{1/2}$. We show how predictions of the model may be extracted over the whole parameter space. A necessary condition for the potential to be stable is ${\rm Str}{\cal M}^4>0$, which is satisfied if $\bf m_{3/2}\lsim2 m_{\tilde q}$. Order of magnitude calculations reveal a no-lose theorem guaranteeing interesting and potentially observable new phenomena in the neutral scalar sector of the theory which would constitute a ``smoking gun'' of the no-scale mechanism. This new phenomenology is model-independent and divides into three scenarios, depending on the ratio of the weak scale to the vev at the minimum of the no-scale direction. We also calculate the residual vacuum energy at the unification scale ($C_0\, m^4_{3/2}$), and find that in typical models one must require $C_0>10$. Such constraints should be important in the search for the correct string no-scale supergravity model. We also show how specific classes of string models fit within this framework.

hep-ph↗

arXiv1994

Experimental consequences of one-parameter no-scale supergravity models

We consider the experimental predictions of two {\em one-parameter} no-scale $SU(5)\times U(1)$ supergravity models with string-inspired moduli and dilaton seeds of supersymmetry breaking. These predictions have been considerably sharpened with the new information on the top-quark mass from the Tevatron, and the actual measurement of the $B(b\to sγ)$ branching ratio from CLEO. In particular, the sign of the Higgs mixing parameter $μ$ is fixed. A more precise measurement of the top-quark mass above (below) $\approx160\GeV$ would disfavor the dilaton (moduli) scenario. Similarly a measurement of the lightest Higgs-boson mass above 90 GeV (below 100 GeV) would disfavor the dilaton (moduli) scenario. At the Tevatron with $100\ipb$, the reach into parameter space is significant only in the dilaton scenario ($m_{χ^\pm_1}\lsim80\GeV$) via the trilepton and top-squark signals. At LEPII the dilaton scenario could be probed up the kinematical limit via chargino and top-squark pair production, and the discovery of the lightest Higgs boson is guaranteed. In the moduli scenario only selectron pair production looks promising. We also calculate the supersymmetric contribution to the anomalous magnetic moment of the muon.

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