Analysis of running coupling constant unification in string theory

Gaillard, Mary K.; Xiu, Rulin

doi:10.1016/0370-2693(92)90805-e

Cited by 39 publications

(35 citation statements)

References 25 publications

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“…We now turn to the effects induced by additional matter below the string scale [17,27,54]. As we shall see, such matter appears naturally in the string models we have examined.…”

Section: Intermediate Matter Thresholdsmentioning

confidence: 90%

Gauge coupling unification in realistic free-fermionic string models

1995

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We discuss the unification of gauge couplings within the framework of a wide class of realistic free-fermionic string models which have appeared in the literature, including the flipped SU (5), SO(6) × SO(4), and various SU (3) × SU (2) × U (1) models. If the matter spectrum below the string scale is that of the Minimal Supersymmetric Standard Model (MSSM), then string unification is in disagreement with experiment. We therefore examine several effects that may modify the minimal string predictions. First, we develop a systematic procedure for evaluating the one-loop heavy string threshold corrections in free-fermionic string models, and we explicitly evaluate these corrections for each of the realistic models. We find that these string threshold corrections are small, and we provide general arguments explaining why such threshold corrections are suppressed in string theory. Thus heavy thresholds cannot resolve the disagreement with experiment. We also study the effect of non-standard hypercharge normalizations, light SUSY thresholds, and intermediate-scale gauge structure, and similarly conclude that these effects cannot resolve the disagreement with low-energy data. Finally, we examine the effects of additional color triplets and electroweak doublets beyond the MSSM. Although not required in ordinary grand unification scenarios, such states generically appear within the context of certain realistic free-fermionic string models. We show that if these states exist at the appropriate thresholds, then the gauge couplings will indeed unify at the string scale. Thus, within these string models, string unification can be in agreement with low-energy

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“…We now turn to the effects induced by additional matter below the string scale [17,27,54]. As we shall see, such matter appears naturally in the string models we have examined.…”

Section: Intermediate Matter Thresholdsmentioning

confidence: 90%

Gauge coupling unification in realistic free-fermionic string models

1995

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“…Indeed, it was found by Gaillard and Xiu in Ref. [67] that (3 +3, 2) representations with hypercharge Y = ±1/6 were necessary to string scale unification, while Faraggi achieved string unification in Ref. [68] in a model where the only colored exotics were (3 +3, 1) states.…”

Section: Gauge Coupling Unificationmentioning

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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“…With the field content of Table I it is well known that gauge coupling unification cannot be achieved consistent with the measured low-energy values of α 3 , α em and sin 2 θ W . This can be easily remedied by the introduction of additional fields in an anomaly-free manner [12,49,50]. As this is imthe E 6 relations between the Higgs and D, D c Yukawa couplings must not be respected in order to avoid rapid proton decay.…”

Section: A General Frameworkmentioning

confidence: 99%

Theory and phenomenology of exotic isosinglet quarks and squarks

2008

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Extensions of the MSSM often predict the existence of new fermions and their scalar superpartners which are vectorlike with respect to the standard model gauge group but may be chiral under additional gauge factors. In this paper we explore the production and decay of an important example, i.e., a heavy isosinglet charge −1/3 quark and its scalar partner, using the charge assignments of a 27-plet of E6 for illustration. We emphasize that, depending on the symmetries of the low energy theory, such exotic particles may decay by the mixing of the fermion with the d, s, or b quarks; may decay by leptoquark or diquark couplings (which may nevertheless preserve a form of R-parity); or may be stable with respect to renormalizable couplings but decay by higher-dimension operators on cosmological times scales. We discuss the latter two possibilities in detail for various assumptions concerning the relative masses of the exotic fermions, scalars, and the lightest neutralino, and emphasize the necessity of considering the collider signatures in conjunction with the normal MSSM processes. Existing and projected constraints from colliders, indirect experiments, proton decay, and big bang nucleosynthesis are considered.

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Analysis of running coupling constant unification in string theory

Cited by 39 publications

References 25 publications

Gauge coupling unification in realistic free-fermionic string models

Gauge coupling unification in realistic free-fermionic string models

Spectra in Standard-like Z3 Orbifold Models

Theory and phenomenology of exotic isosinglet quarks and squarks

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