arXiv subjects

Oleg Lebedev

Publications and source records attributed to Oleg Lebedev.

At least 73 records · Page 4Linked to original sources

arXiv2010

Heterotic mini-landscape (II): completing the search for MSSM vacua in a Z_6 orbifold

We complete our search for MSSM vacua in the Z_6-II heterotic orbifold by including models with 3 Wilson lines. We estimate the total number of inequivalent models in this orbifold to be 10^7. Out of these, we find almost 300 models with the exact MSSM spectrum, gauge coupling unification and a heavy top quark. Models with these features originate predominantly from local GUTs. The scale of gaugino condensation in the hidden sector is correlated with properties of the observable sector such that soft masses in the TeV range are preferred.

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arXiv2010

On Inflation in the Presence of a Gaugino Condensate

We study the effect of inflation on gaugino condensation in supergravity. Unless the Hubble scale H is significantly below the gaugino condensation scale, the gaugino condensate is a dynamical variable which cannot be integrated out. For a sufficiently high H, the gaugino condensate evolves to zero which in turn leads to dilaton/moduli destabilization. In practice, this often occurs at the Hubble rate about an order of magnitude below the gaugino condensation scale. This effect is independent of the specifics of moduli stabilization and thus places model independent constraints on inflationary scenarios. It also applies more generally to any periods of fast expansion in the early Universe.

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arXiv2009

The NMSSM and String Theory

We study the possibility of constructing the NMSSM from the heterotic string. String derived NMSSMs are much more rare than MSSMs due to the extra requirement that there exist a light singlet which couples to the Higgs pairs. They share the common feature that the singlet self-interactions are typically suppressed, leading to either the "decoupling" or the Peccei-Quinn limit of the NMSSM. In the latter case, the spectrum contains a light pseudoscalar which may be relevant to the MSSM fine-tuning problem. We provide a Z_6 heterotic orbifold example of the NMSSM with approximate Peccei-Quinn symmetry, whose origin lies in the string selection rules combined with our choice of the vacuum configuration.

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arXiv2009

Gravitino Dark Matter in Gravity Mediation

We study general conditions for the gravitino to be the lightest supersymmetric particle (LSP) in models with gravity mediated supersymmetry breaking. We find that the decisive quantities are the Kaehler potential K and the gauge kinetic function f. In constrained MSSM (CMSSM) type models, the gravitino LSP occurs if the gaugino mass at the GUT scale is greater than approximately 2.5 gravitino masses. This translates into \sqrt{K"}/f' < 0.2, where the derivatives are taken with respect to the dominant SUSY breaking field. This requirement can easily be satisfied in string-motivated setups.

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arXiv2009

Higgs-dependent Yukawa couplings

We consider the possibility that the Yukawa couplings depend on the Higgs field, with the motivation of generating the fermion mass hierarchy through appropriate powers of the Higgs vacuum expectation value. This leads to drastic modifications of the Higgs branching ratios, new Higgs contributions to various flavor-violating processes, and observable rates for the top quark decay t-> hc. The underlying flavor dynamics must necessarily appear at the TeV scale and is within the reach of the LHC.

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arXiv2008

Against Tachyophobia

We examine the possible extension of the parameter space of the minimal supersymmetric extension of the Standard Model (MSSM), as expressed via the renormalization-group equations in terms of universal soft supersymmetry-breaking terms at the unification scale, to include tachyonic input scalar masses. Many models with negative masses-squared for scalars at the unification scale may be viable because the small sizes of the masses-squared allow them to change signs during the renormalization-group evolution to the electroweak scale. However, in many cases, there is, in addition to the electroweak vacuum, a much deeper high-scale vacuum located along some F- and D-flat direction in the effective potential for the MSSM. We perform a numerical search for such vacua in both the CMSSM and the NUHM. We discuss the circumstances under which the existence of such a deep charge- and color-breaking vacuum is consistent with standard cosmology. A crucial role is played by the inflation--induced scalar masses, whereas thermal effects are often irrelevant.

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arXiv2007

The Seesaw with Many Right-Handed Neutrinos

There are no upper limits on the possible number of massive, singlet (right--handed) neutrinos that may participate in the seesaw mechanism, and some string constructions motivate seesaw models with up to O(100) right--handed neutrinos. In this case, the seesaw mass scale can be significantly higher than that in the traditional scheme with just 3 right--handed neutrinos. We consider the possible phenomenological implications of such models, in particular, for lepton-flavour violation and electric dipole moments. Since the neutrino masses depend on the Majorana mass scale linearly, while supersymmetric loop corrections depend on it logarithmically, the magnitude of lepton-flavour- and CP-violating transitions may increase with the multiplicity of the right--handed neutrinos and may be enhanced by orders of magnitude. We also point out that, in the context of leptogensis, the bounds on the reheating temperature and the lightest neutrino mass get relaxed compared to those in the case of 3 right--handed neutrinos.

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arXiv2007

The Heterotic Road to the MSSM with R parity

In a previous paper, referred to as a "Mini-Landscape" search, we explored a "fertile patch" of the heterotic landscape based on a Z6-II orbifold with SO(10) and E6 local GUT structures. In the present paper we extend this analysis. We find many models with the minimal supersymmetric standard model spectra and an exact R parity. In all of these models, the vector-like exotics decouple along D flat directions. We present two "benchmark" models which satisfy many of the constraints of a realistic supersymmetric model, including non-trivial Yukawa matrices for 3 families of quarks and leptons and Majorana neutrino masses for right-handed neutrinos with non-trivial See-Saw masses for the 3 light neutrinos. In an appendix we comment on the important issue of string selection rules and in particular the so-called "gamma-rule".

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arXiv2007

Supersymmetric Standard Model from the Heterotic String (II)

We desribe in detail a Z_6 orbifold compactification of the heterotic E_8 x E_8 string which leads to the (supersymmetric) standard model gauge group and matter content. The quarks and leptons appear as three 16-plets of SO(10), two of which are localized at fixed points with local SO(10) symmetry. The model has supersymmetric vacua without exotics at low energies and is consistent with gauge coupling unification. Supersymmetry can be broken via gaugino condensation in the hidden sector. The model has large vacuum degeneracy. Certain vacua with approximate B-L symmetry have attractive phenomenological features. The top quark Yukawa coupling arises from gauge interactions and is of the order of the gauge couplings. The other Yukawa couplings are suppressed by powers of standard model singlet fields, similarly to the Froggatt-Nielsen mechanism.

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arXiv2007

Seesaw Neutrinos from the Heterotic String

We study the possibility of realizing the neutrino seesaw mechanism in the E8XE8 heterotic string. In particular, we consider its Z_6 orbifold compactifications leading to the supersymmetric standard model gauge group and matter content. We find that these models possess all the necessary ingredients for the seesaw mechanism, including the required Dirac Yukawa couplings and large Majorana mass terms. We argue that this situation is quite common in heterotic orbifolds. In contrast to the conventional seesaw of grand unified theories (GUTs), no large GUT representations are needed to generate the Majorana mass terms. The total number of right--handed neutrinos can be very large, up to O(100).

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arXiv2007

Low Energy Supersymmetry from the Heterotic Landscape

We study possible correlations between properties of the observable and hidden sectors in heterotic string theory. Specifically, we analyze the case of the Z6-II orbifold compactification which produces a significant number of models with the spectrum of the supersymmetric standard model. We find that requiring realistic features does affect the hidden sector such that hidden sector gauge group factors SU(4) and SO(8) are favoured. In the context of gaugino condensation, this implies low energy supersymmetry breaking.

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arXiv2007

Supersymmetric Jarlskog Invariants: the Neutrino Sector

We generalize the notion of the Jarlskog invariant to supersymmetric models with right--handed neutrinos. This allows us to formulate basis--independent necessary and sufficient conditions for CP conservation in such models.

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arXiv2007

Metastable Vacua in Flux Compactifications and Their Phenomenology

In the context of flux compactifications, metastable vacua with a small positive cosmological constant are obtained by combining a sector where supersymmetry is broken dynamically with the sector responsible for moduli stabilization, which is known as the F-uplifting. We analyze this procedure in a model-independent way and study phenomenological properties of the resulting vacua.

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arXiv2006

A Mini-Landscape of Exact MSSM Spectra in Heterotic Orbifolds

We explore a ``fertile patch'' of the heterotic landscape based on a Z_6-II orbifold with SO(10) and E_6 local GUT structures. We search for models allowing for the exact MSSM spectrum. Our result is that of order 100 out of a total 3\times 10^4 inequivalent models satisfy this requirement.

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arXiv2006

SUSY Phenomenology of KKLT Flux Compactifications

We study SUSY phenomenology of the KKLT (Kachru-Kallosh-Linde-Trivedi) scenario of string theory compactifications with fluxes. This setup leads to a specific pattern of soft masses and distinct phenomenological properties. In particular, it avoids the cosmological gravitino/moduli problems. Remarkably, the model allows for the correct abundance of SUSY dark matter consistently with all experimental constraints including the bound on the Higgs mass, b-->s gamma, etc. This occurs for both small and large tan beta, and requires the SUSY spectrum above 1 TeV.

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arXiv2006

Supersymmetric Standard Model from the Heterotic String

We present a Z_6 orbifold compactification of the E_8xE_8 heterotic string which leads to the (supersymmetric) Standard Model gauge group and matter content. The quarks and leptons appear as three 16-plets of SO(10), whereas the Higgs fields do not form complete SO(10) multiplets. The model has no exotic states at low energies and is consistent with gauge coupling unification. The top quark Yukawa coupling arises from gauge interactions and is of the order of the gauge couplings, whereas the other Yukawa couplings are suppressed.

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arXiv2006

De Sitter Vacua from Matter Superpotentials

Consistent uplifting of AdS vacua in string theory often requires extra light degrees of freedom in addition to those of a (Kaehler) modulus. Here we consider the possibility that de Sitter and Minkowski vacua arise due to hidden sector matter interactions. We find that, in this scheme, the hierarchically small supersymmetry breaking scale can be explained by the scale of gaugino condensation and that interesting patterns of the soft terms arise. In particular, a matter-dominated supersymmetry breaking scenario and a version of the mirage mediation scheme appear in the framework of spontaneously broken supergravity.

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arXiv2006

Local Grand Unification

In the standard model matter fields form complete representations of a grand unified group whereas Higgs fields belong to incomplete `split' multiplets. This remarkable fact is naturally explained by `local grand unification' in higher-dimensional extensions of the standard model. Here, the generations of matter fields are localized in regions of compact space which are endowed with a GUT gauge symmetry whereas the Higgs doublets are bulk fields. We realize local grand unification in the framework of orbifold compactifications of the heterotic string, and we present an example with SO(10) as a local GUT group, which leads to the supersymmetric standard model as an effective four-dimensional theory. We also discuss different orbifold GUT limits and the unification of gauge and Yukawa couplings.

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