arXiv subjects

D. Nanopoulos

Publications and source records attributed to D. Nanopoulos.

72 records · Page 4Linked to original sources

arXiv1993

Accurate Neutralino Relic Density Computations in Supergravity Models

We investigate the question of the proper thermal averaging of neutralino annihilation amplitudes which possess poles and thresholds, as they impact on the calculated neutralino relic density and therefore on the cosmological viability of supersymmetric models. We focus on two typical resonances, namely the $Z$ boson and the lightest Higgs boson ($h$). In the context of supergravity models with radiative electroweak symmetry breaking, an exploration of the whole parameter space of the model is possible and the overall relevance of these sophisticated analyses can be ascertained. As an example we chose the minimal $SU(5)$ supergravity model since the presence of such poles is essential to obtain a cosmologically acceptable model. We find that the proper thermal averaging is important for individual points in parameter space and that the fraction of cosmologically acceptable points is increased somewhat by the accurate procedure. However, qualitatively the new set of points is very similar to that obtained previously using the usual series approximations to the thermal average. We conclude that all phenomenological analyses based on the previously determined cosmologically allowed set remain valid.

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arXiv1993

Flipped Angles and Phases: A Systematic Study

We discuss systematically the fermion mass and mixing matrices in a generic \linebreak field-theoretical flipped $SU(5)$ model, with particular applications to neutrino and baryon number-changing physics. We demonstrate that the different quark flavour branching ratios in proton decay are related to the Cabibbo-Kobayashi-Maskawa angles, whereas the lepton flavour branching ratios are undetermined. The light neutrino mixing angles observable via oscillation effects are related to the heavy conjugate (right-handed) neutrino mass matrix, which also plays a key role in cosmological baryogenesis. The ratios of neutrino and charged lepton decay modes in baryon decay may also be related to neutrino oscillation parameters. Plausible Ansätze for the generation structure of coupling matrices motivate additional relations between physical observables, and yield a satisfactory baryon asymmetry.

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arXiv1993

Perturbative QCD data are consistent with light gluinos

We discuss systematically the consistency of light gluinos with data on perturbative QCD from deep inelastic scattering, quarkonia, jets at LEP, and the total hadronic cross-section in ${e^+ e^-}$ annihilation on the $Z$ peak and elsewhere. We demonstrate that, in addition to the well-known increase in the value of $α_s(m_Z)$ inferred from lower-energy data due to the slower running of $α_s$ in the presence of light gluinos, the value of $α_s(m_Z)$ extracted from the LEP data must also be $increased$, as a result of including the effects of virtual light gluinos. The effect of these increases in other estimates of $α_s(m_Z)$ is to make them more consistent with the value extracted from the total $e^+ e^-$ cross-section, which would otherwise appear distinctly higher. We discuss the possibility of looking for light gluinos in four-jet events at LEP, and their possible implications for scaling violations at HERA.

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arXiv1993

Sparticle and Higgs Production and Detection at LEPII in two Supergravity Models

We study the most promising signals for supersymmetry at LEPII in the context of two well motivated supergravity models: (i) the minimal $SU(5)$ supergravity model including the stringent constraints from proton stability and a not too young Universe, and (ii) a recently proposed string-inspired no-scale flipped $SU(5)$ supergravity model. Our computations span the neutralino, chargino, slepton, and Higgs sectors together with their interconnections in this class of models. We find that the number of `mixed' (1-lepton + 2-jets + $\slash{p}$) events occuring in the decay of pair-produced charginos ($χ^\pm_1$) is quite significant (per ${\cal L}=100\ipb$) for both models and that these predictions do not overlap. That is, if $m_{χ^\pm_1}<100\GeV$ then LEPII should be able to exclude at least one of the two models. In the no-scale flipped $SU(5)$ model we find that the number of acoplanar di-electron events from selectron pair production should allow for exploration of selectron masses up to the kinematical limit and chargino masses indirectly as high as 150 GeV. We find that the cross section $e^+e^-\to Z^*h$ deviates negligibly from the SM result in the minimal model, whereas it can be as much as $1/3$ lower in the flipped model. The usually neglected invisible mode $h\toχ^0_1χ^0_1$ can erode the preferred $h\to 2\,{\rm jets}$ signal by as much as $40\%$ in these models. We conclude that the charged slepton sector is a deeper probe than the chargino/neutralino or Higgs sectors of the flipped $SU(5)$ model at LEPII, while the opposite is true for the minimal $SU(5)$ model where the slepton sector is no probe at all.

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arXiv1993

Improved LEP lower bound on the lightest SUSY Higgs mass from Radiative Electroweak Breaking and its Experimental Consequences

We show that the present LEPI lower bound on the Standard Model Higgs boson mass ($M_H\gsim60\GeV$) applies as well to the lightest Higgs boson ($h$) of the minimal $SU(5)$ and no-scale flipped $SU(5)$ supergravity models. This result would persist even for the ultimate LEPI lower bound ($M_H\gsim70\GeV$). We show that this situation is a consequence of a decoupling phenomenon in the Higgs sector driven by radiative electroweak breaking for increasingly larger sparticle masses, and thus it should be common to a large class of supergravity models. A consequence of $m_h\gsim60\GeV$ in the minimal $SU(5)$ supergravity model is the exclusion from the allowed parameter space of `spoiler modes' ($χ^0_2\toχ^0_1 h$) which would make the otherwise very promising trilepton signal in $p\bar p\toχ^\pm_1χ^0_2X$ unobservable at Fermilab. Within this model we also obtain stronger upper bounds on the lighter neutralino and chargino masses, \ie, $m_{χ^0_1}\lsim50\GeV$, $m_{χ^0_2,χ^\pm_1}\lsim100\GeV$. This should encourage experimental searches with existing facilities.

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arXiv1992

Supersymmetry Tests at Fermilab: A Proposal

We compute the number of trilepton events to be expected at Fermilab as a result of the reaction $p\bar p\toχ^\pm_1χ^0_2X$, where $χ^\pm_1$ is the lightest chargino and $χ^0_2$ is the next-to-lightest neutralino. This signal is expected to have very little background and is the best prospect for supersymmetry detection at Fermilab if the gluino and squarks are beyond reach. We evaluate our expressions for all points in the allowed parameter space of two basic supergravity models: (i) the minimal $SU(5)$ supergravity model including the severe constraints from proton decay and a not too young Universe, and (ii) a recently proposed no-scale flipped $SU(5)$ supergravity model. We study the plausible experimental scenaria and conclude that a large portion of the parameter spaces of these models could be explored with $100\ipb$ of integrated luminosity. In the minimal $SU(5)$ supergravity model chargino masses up to $90-95\GeV$ could be probed. In the no-scale flipped model it should be possible to probe some regions of parameter space with $m_{χ^\pm_1}\lsim200\GeV$, therefore possibly doubling the reach of LEPII for chargino and neutralino (since $m_{χ^0_2}\approx m_{χ^\pm_1}$) masses. In both models such probes would indirectly explore gluino masses much beyond the reach of Fermilab.

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arXiv1992

Proposed Tests for Minimal SU(5) Supergravity at Fermilab, Gran Sasso, SuperKamiokande, and LEP

A series of predictions are worked out in order to put the minimal $SU(5)$ supergravity model under experimental test. Using the two-loop gauge coupling renormalization group equations, with the inclusion of supersymmetric threshold corrections, we calculate a new value for the proton decay rate in this model and find that SuperKamiokande and Gran Sasso should see the proton decay mode $p\to\barνK^+$ for most of the allowed parameter space. A set of physically sensible assumptions and the cosmological requirement of a not too young Universe give us a rather restrictive set of allowed points in the parameter space, which characterizes this model. This set implies the existence of interesting correlations among various masses: either the lightest chargino and the next-to-lightest neutralino are below $\approx100\GeV$ (and therefore observable at the Tevatron) or the lightest Higgs boson is below $\approx50\GeV$ (and therefore observable at LEP I-II). These tests are crucial steps towards selecting the correct low-energy effective supergravity model. We also comment on the compatibility of the model with $\sin^2θ_w(M_Z)$ measurements as a function of $α_3(M_Z)$.

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arXiv1992

The Simplest, String-Derivable, Supergravity Model and its Experimental Predictions

We present the simplest, string-derivable, supergravity model and discuss its experimental consequences. This model is a new string-inspired flipped $SU(5)$ which unifies at the string scale $M_U=10^{18}\GeV$ due to the introduction of an additional pair of \r{10},\rb{10} flipped $SU(5)$ representations which contain new intermediate scale `gap' particles. We study various model-building issues which should be addressed in string-derived incarnations of this model. We focus our study on the no-scale supergravity mechanism and explore thoroughly the three-dimensional parameter space of the model ($m_{\tilde g},m_t,\tanβ$), thus obtaining several simple relationships among the particle masses, such as: $m_{\tilde q}\approx m_{\tilde g}$, $m_{\tilde e_L}\approx m_{\tildeν}\approx0.30 m_{\tilde g}$, $m_{\tilde e_R}\approx0.18m_{\tilde g}$, and $m_{χ^0_2}\approx2m_{χ^0_1}\approx m_{χ^\pm_1}$. In a strict interpretation of the no-scale supergravity scenario we solve for $\tanβ$ as a function of $m_t$ and $m_{\tilde g}$, and show that $m_t$ determines not only the sign of the Higgs mixing parameter $μ$ but also whether the lightest Higgs boson mass is above or below $100\GeV$. We also find that throughout the parameter space the neutralino relic abundance is within observational bounds ($Ω_χh^2_0\lsim0.25$) and may account for a significant portion of the dark matter in the Universe.

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arXiv1992

Proton Decay and Cosmology Strongly Constrain the Minimal SU(5) Supergravity Model

We present the results of an extensive exploration of the five-dimensional parameter space of the minimal $SU(5)$ supergravity model, including the constraints of a long enough proton lifetime ($τ_p>1\times10^{32}\y$) and a small enough neutralino cosmological relic density ($Ω_χh^2_0\le1$). We find that the combined effect of these two constraints is quite severe, although still leaving a small region of parameter space with $m_{\tilde g,\tilde q}<1\TeV$. The allowed values of the proton lifetime extend up to $τ_p\approx1\times10^{33}\y$ and should be fully explored by the SuperKamiokande experiment. The proton lifetime cut also entails the following mass correlations and bounds: $m_h\lsim100\GeV$, $m_χ\approx{1\over2}m_{χ^0_2}\approx0.15\gluino$, $m_{χ^0_2}\approx m_{χ^+_1}$, and $m_χ<85\,(115)\GeV$, $m_{χ^0_2,χ^+_1}<165\,(225)\GeV$ for $α_3=0.113\,(0.120)$. Finally, the {\it combined} proton decay and cosmology constraints predict that if $m_h\gsim75\,(80)\GeV$ then $m_{χ^+_1}\lsim90\,(110)\GeV$ for $α_3=0.113\,(0.120)$. Thus, if this model is correct, at least one of these particles will likely be observed at LEPII.

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arXiv1992

New Constraints on Neutralino Dark Matter in the Supersymmetric Standard Model

We investigate the prospects for neutralino dark matter within the Supersymmetric Standard Model (SSM) including the constraints from universal soft supersymmetry breaking and radiative breaking of the electroweak symmetry. The latter is enforced by using the one-loop Higgs effective potential which automatically gives the one-loop corrected Higgs boson masses. We perform an exhaustive search of the allowed five-dimensional parameter space and find that the neutralino relic abundance $Ω_χh^2_0$ depends most strongly on the ratio $ξ_0\equiv m_0/m_{1/2}$. For $ξ_0\gg1$ the relic abundance is almost always much too large, whereas for $ξ_0\ll1$ the opposite occurs. For $ξ_0\sim1$ there are wide ranges of the remaining parameters for which $Ω_χ\sim1$. We also determine that $m_{\tilde q}\gsim250\GeV$ and $m_{\tilde l}\gsim100\GeV$ are necessary in order to possibly achieve $Ω_χ\sim1$. These lower bounds are much weaker than the corresponding ones derived previously when radiative breaking was {\it not} enforced.

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arXiv1992

The prospects for CHORUS and NOMAD in the light of COBE and GALLEX

The most natural MSW neutrino oscillation interpretation of the GALLEX and other solar neutrino data, which invokes $m_{ν_μ}\sim3\times10^{-3}\eV$, and a general GUT see-saw hierarchy of neutrino masses, $m_{ν_{e,μ,τ}}\sim (m_{u,c,t})^2/M_U$, suggest that $m_{ν_τ}\sim10\eV$ in agreement with the preference of COBE and other data on large-scale structure in the Universe for a hot component in the Dark Matter. The general see-saw model also suggests that neutrino mixing angles are related to quark mixing angles, which is also consistent with the oscillation interpretation of the solar neutrino data, and suggests that the forthcoming CHORUS and NOMAD experiments at CERN have a good chance of observing $ν_μ- ν_τ$ oscillations. We present a minimal realization of the general see-saw hierarchy in the context of flipped $SU(5)$.

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arXiv1992

Troubles with the Minimal SU(5) Supergravity Model

We show that within the framework of the minimal $SU(5)$ supergravity model, radiatively-induced electroweak symmetry breaking and presently available experimental lower bounds on nucleon decay, impose severe constraints on the available parameter space of the model which correspond to fine-tuning of the model parameters of over two orders of magnitude. Furthermore, a straightforward calculation of the cosmic relic density of neutralinos ($χ$) gives $Ω_χh^2\gg1$ for most of the allowed parameter space in this model, although small regions may still be cosmologically acceptable. We finally discuss how the {\it no-scale flipped $SU(5)$ supergravity model} avoids naturally the above troubles and thus constitutes a good candidate for the low-energy effective supergravity model.

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arXiv1992

Aspects of Radiative Electroweak Symmetry Breaking in Supergravity Models

We discuss several aspects of state-of-the-art calculations of radiative electroweak symmetry breaking in supergravity models. These models have a five-dimensional parameter space in contrast with the 21-dimensional one of the MSSM. We examine the Higgs one-loop effective potential $V_1=V_0+ΔV$, in particular how its renormalization-scale ($Q$) independence is affected by the approximations used to calculate $ΔV$ and by the presence of a Higgs-field-independent term which makes $V_1(0)\not=0$. We show that the latter must be subtracted out to achieve $Q$-independence. We also discuss our own approach to the exploration of the five-dimensional parameter space and the fine-tuning constraints within this approach. We apply our methods to the determination of the allowed region in parameter space of two models which we argue to be the prototypes for conventional (SSM) and string (SISM) unified models. To this end we impose the electroweak breaking constraint by minimizing the one-loop effective potential and study the shifts in $μ$ and $B$ relative to the values obtained using the tree-level potential. These shifts are most significant for small values of $μ$ and $B$, and induce corresponding shifts on the lightest $μ$- and/or $B$-dependent particle masses, \ie, those of the lightest stau, neutralino, chargino, and Higgs boson states. Finally, we discuss the predictions for the squark, slepton, and one-loop corrected Higgs boson masses.

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arXiv1992

The Search for a realistic flipped SU(5) string model

We present an extensive search for a general class of flipped $SU(5)$ models built within the free fermionic formulation of the heterotic string. We describe a set of algorithms which constitute the basis for a computer program capable of generating systematically the massless spectrum and the superpotential of all possible models within the class we consider. Our search through the huge parameter space to be explored is simplified considerably by the constraint of $N=1$ spacetime supersymmetry and the need for extra $Q,\bar Q$ representations beyond the standard ones in order to possibly achieve string gauge coupling unification at scales of ${\cal O}(10^{18}\GeV)$. Our results are remarkably simple and evidence the large degree of redundancy in this kind of constructions. We find one model with gauge group $SU(5)\times U(1)_\ty\times SO(10)_h\times SU(4)_h\times U(1)^5$ and fairly acceptable phenomenological properties. We study the $D$- and $F$-flatness constraints and the symmetry breaking pattern in this model and conclude that string gauge coupling unification is quite possible.

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arXiv1991

Quantum Mechanics and Black Holes in Four-Dimensional String Theory

In previous papers we have shown how strings in a two-dimensional target space reconcile quantum mechanics with general relativity, thanks to an infinite set of conserved quantum numbers, ``W-hair'', associated with topological soliton-like states. In this paper we extend these arguments to four dimensions, by considering explicitly the case of string black holes with radial symmetry. The key infinite-dimensional W-symmetry is associated with the $\frac{SU(1,1)}{U(1)}$ coset structure of the dilaton-graviton sector that is a model-independent feature of spherically symmetric four-dimensional strings. Arguments are also given that the enormous number of string {\it discrete (topological)} states account for the maintenance of quantum coherence during the (non-thermal) stringy evaporation process, as well as quenching the large Hawking-Bekenstein entropy associated with the black hole. Defining the latter as the measure of the loss of information for an observer at infinity, who - ignoring the higher string quantum numbers - keeps track only of the classical mass,angular momentum and charge of the black hole, one recovers the familiar a quadratic dependence on the black-hole mass by simple counting arguments on the asymptotic density of string states in a linear-dilaton background.

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arXiv1991

Modular invariance and nonrenormalizable interactions

We examine the modular properties of nonrenormalizable superpotential terms in string theory and show that the requirement of modular invariance necessitates the nonvanishing of certain Nth order nonrenormalizable terms. In a class of models (free fermionic formulation) we explicitly verify that the nontrivial structure imposed by the modular invariance is indeed present. Alternatively, we argue that after proper field redefinition, nonrenormalizable terms can be recast as to display their invariance under the modular group. We also discuss the phenomenological implications of the above observations.

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arXiv1991

Flipped SU(5): Origins and Recent Developments

We present an account of the early developments that led to the present form of the flipped $SU(5)$ string model. We focus on the method used to decide on this particular string model, as well as the basic steps followed in constructing generic models in the free fermionic formulation of superstrings in general and flipped $SU(5)$ in particular. We then describe the basic calculable features of the model which are used to obtain its low-energy spectrum: doublet and triplet Higgs mass matrices, fermion Yukawa matrices, neutrino masses, and the top-quark mass. We also review the status of proton decay in the model, as well as the hidden sector bound states called cryptons. Finally, we comment on the subject of string threshold corrections and string unification.

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arXiv1991

Gauge and Matter Condensates in Realistic String Models

We examine the inter-relationship of the superpotential containing hidden and observable matter fields and the ensuing condensates in free fermionic string models. These gauge and matter condensates of the strongly interacting hidden gauge groups play a crucial role in the determination of the physical parameters of the observable sector. Supplementing the above information with the requirement of modular invariance, we find that a generic model with only trilinear superpotential allows for a degenerate (and sometimes pathological) set of vacua. This degeneracy may be lifted by higher order terms in the superpotential. We also point out some other subtle points that may arise in calculations of this nature. We exemplify our observations by computing explicitly the modular invariant gaugino and matter condensates in the flipped SU(5) string model with hidden gauge group $SO(10)\times SU(4)$.

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