String theory and the path to unification: A review of recent developments

Dienes, Keith R.

doi:10.1016/s0370-1573(97)00009-4

Cited by 296 publications

(308 citation statements)

References 250 publications

(295 reference statements)

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“…However, it is not possible to obtain small enough k Y , in the range of 1.4 to 1.5, to achieve string scale unification with only the MSSM field content-a string unification scenario studied in Refs. [71,59] and reviewed in [61].…”

Section: Discussionmentioning

confidence: 99%

Spectra in Standard-like Z3 Orbifold Models

Giedt

2002

Annals of Physics

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General features of the spectra of matter states in all 175 models found in a previous work by the author are discussed. Only twenty patterns of representations are found to occur. Accomodation of the Minimal Supersymmetric Standard Model (MSSM) spectrum is addressed. States beyond those contained in the MSSM and nonstandard hypercharge normalization are shown to be generic, though some models do allow for the usual hypercharge normalization found in SU (5) embeddings of the Standard Model gauge group. The minimum value of the hypercharge normalization consistent with accomodation of the MSSM is determined for each model. In some cases, the normalization can be smaller than that corresponding to an SU (5) embedding of the Standard Model gauge group, similar to what has been found in free fermionic models. Bizzare hypercharges typically occur for exotic states, allowing for matter which does not occur in the decomposition of SU (5) representations-a result which has been noted many times before in four-dimensional string models. Only one of the twenty patterns of representations, comprising seven of the 175 models, is found to be without an anomalous U (1). The sizes of nonvanishing vacuum expectation values induced by the anomalous U (1) are studied. It is found that large radius moduli stabilization may lead to the breakdown of σ-model perturbativity. Various quantities of interest in effective supergravity model building are tabulated for the set of 175 models. In particular, it is found that string moduli masses appear to be generically quite near the gravitino mass. String scale gauge coupling unification is shown to be possible, albeit contrived, in an example model. The intermediate scales of exotic particles are estimated and the degree of fine-tuning is studied.

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Section: Discussionmentioning

confidence: 99%

Spectra in Standard-like Z3 Orbifold Models

Giedt

2002

Annals of Physics

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“…Note that these gauge couplings run logarithmically over this range precisely because v 6 ∼ Oð1Þ, so that the theory is effectively four-dimensional below M s . Thus, for v 6 ∼ Oð1Þ, the usual logarithmic gauge coupling unification can be preserved and naturally embedded into string theory [31].…”

Section: Gut Precursors and Entwined Susymentioning

confidence: 97%

GUT precursors and entwined SUSY: The phenomenology of stable nonsupersymmetric strings

Abel

Mavroudi

2018

Phys. Rev. D

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Recent work has established a method of constructing nonsupersymmetric string models that are stable, with near-vanishing one-loop dilaton tadpoles and cosmological constants. This opens up the tantalizing possibility of realizing stable string models whose low-energy limits directly resemble the Standard Model rather than one of its supersymmetric extensions. In this paper we consider the general structure of such strings and find that they share two important phenomenological properties. The first is a so-called "GUT-precursor" structure in which new GUT-like states appear with masses that can be many orders of magnitude lighter than the scale of gauge coupling unification. These states allow a parametrically large compactification volume, even in weakly coupled heterotic strings, and in certain regions of parameter space can give rise to dramatic collider signatures which serve as "smoking guns" for this overall string framework. The second is a residual "entwined-SUSY" (or e-SUSY) structure for the matter multiplets in which different multiplet components carry different horizontal Uð1Þ charges. As a concrete example and existence proof of these features, we present a heterotic string model that contains the fundamental building blocks of the Standard Model such as the Standard-Model gauge group, complete chiral generations, and Higgs fields-all without supersymmetry. Even though massless gravitinos and gauginos are absent from the spectrum, we confirm that this model has an exponentially suppressed one-loop dilaton tadpole and displays both the GUT-precursor and e-SUSY structures. We also discuss some general phenomenological properties of e-SUSY, such as cancellations in radiative corrections to scalar masses, the possible existence of a corresponding approximate moduli space, and the prevention of rapid proton decay.

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“…In our search for the upper Higgs mass limit, we do not restrict ourselves to these possibilities but consider other options. Although split SU (5) representations are difficult to arrange in GUT models, they can easily arise in string models and may even help in solving the mismatch [10] between the MSSM unification scale, M G ϳ 2 3 10 16 GeV, and the string scale, M S ϳ 5 3 10 17 GeV [degenerate full SU (5) …”

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confidence: 99%

Gauge Unification and the Supersymmetric Light Higgs Mass

1998

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We consider general supersymmetric models with (a) arbitrary matter content and (b) gauge coupling unification near the string scale ϳ10 17 GeV, and derive the absolute upper limit on the mass of the lightest Higgs boson. For models without Higgs couplings to extra SU(2) triplets the bound is about 155 GeV. The bound is maximized for models with such triplets and can be as high as 205 GeV, significantly larger than previous estimates partly due to a normalization error.[ S0031-9007(98) In this context, the sector of the theory responsible for electroweak symmetry breaking has a special status. While all superpartners of the known SM particles can be made heavy by simply rising soft SUSY-breaking mass parameters in the model, the Higgs sector necessarily contains a physical Higgs scalar whose mass does not depend sensitively on the details of soft masses but is fixed by the scale of electroweak symmetry breaking. This important fact follows simply from the spontaneous breaking of the electroweak gauge symmetry [3,4] and it is not specific to supersymmetric models. More precisely, the general statement is that some Higgs boson must exist whose mass squared satisfies m 2 h # ly 2 , where y is the electroweak scale (y 174.1 GeV) and l is the dimensionless quartic coupling of some Higgs state in the model. In other words, the mass of the light Higgs can be made heavy only at the expense of making the coupling l very strong. The role of supersymmetry is to fix l in some models. For example, in the minimal supersymmetric standard model (MSSM), which includes two Higgs doublets,to give masses to quarks and leptons, l is related to the SU͑2͒ L 3 U͑1͒ Y gauge couplings (g and g 0 , respectively) and the following (tree-level) bound on the mass of the lightest Higgs boson holdswhere tan b ͗H 0 2 ͗͘͞H 0 1 ͘. This represents a very stringent prediction which, as is well known, gets significantly relaxed when radiative corrections to l are included [5][6][7]. These corrections depend logarithmically on the soft masses and push the upper mass limit for the lightest Higgs boson of the MSSM up to 125 GeV (for a top-quark mass M t 175 GeV and M SUSY , 1 TeV). The fact that l is calculable in terms of gauge couplings in the MSSM is due to supersymmetry and to the fact that the superpotential does not contain cubic terms of the form W h X XH i H j (with i, j 1, 2) as no X field exists with the appropriate quantum numbers to form a gauge-invariant object. In extended models, the presence of such fields and couplings modifies the quartic Higgs self-interactions, which can ultimately have an impact on the tree-level upper bound on m 2 h , which receives corrections proportional to h 2 X y 2 . The Yukawa coupling h X is unknown but asymptotically nonfree, and so it can be bounded from above if it is further required to remain in the perturbative regime up to some large energy scale [where grand unified theory (GUT), string, or Planck physics takes over].As there is no reason to believe that low-energy supersymmetry is realized in Natur...

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String theory and the path to unification: A review of recent developments

Cited by 296 publications

References 250 publications

Spectra in Standard-like Z3 Orbifold Models

Spectra in Standard-like Z3 Orbifold Models

GUT precursors and entwined SUSY: The phenomenology of stable nonsupersymmetric strings

Gauge Unification and the Supersymmetric Light Higgs Mass

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