arXiv subjects

J. Lopez

Publications and source records attributed to J. Lopez.

98 records · Page 6Linked to original sources

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.

hep-ph↗

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 $SU(5)$.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

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.

hep-th↗

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.

hep-th↗

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.

hep-th↗

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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