Kajia Yuan

A standard-like model in the four-dimensional free fermionic string formulation

Abstract We show how the Standard Model may be derived from the heterotic string with no intermediate energy scale by using the free fermionic formulation. An explicit model is presented, with the following properties: (i) The observable gauge group is SU(3)C × SU(2)L × U(1)n where n = 1 or 2, after a possible symmetry breaking at the string level. (ii) The complete massless spectrum is derived and is shown to be anomaly free apart from a single “anomalous” U(1). In the observable sector there are only three generations of chiral fermions with their superpartners. (iii) There are enough scalar doublets and singlets to break the gauge symmetry in a realistic way. (iv) All trilinear superpotential couplings have been calculated. (v) The weak hypercharge is uniquely defined, including a possible new, generation independent, U(1) symmetry, which may remain unbroken down to low energies, and prevents fast proton decay.

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

Abstract We present an extensive search for a 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, Q representations beyond the standard ones in order to possibly achieve string gauge coupling unification at scales of 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)× U (1) Y × SO (10) h × SU (4) h × 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.

Aspects of radiative electroweak breaking in supergravity models

Abstract 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 approximation used to calculate ΔV and by the presence of a Higgs-field-independent term which makes V 1 (0) ≠ 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 -independent particle masses, i.e., 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.

Best fit to the gluino mass from quarkonia decays.

Assuming that perturbative QCD is the dominant explanation for the narrowness of the vector quarkonia, we perform a [chi][sup 2] minimization analysis of their hadronic decays as a function of two parameters: the mass of the gluino and the value of [alpha][sub 3]([ital M][sub [ital Z]]). A value below 1 GeV for the gluino mass is strongly preferred. Consequences for supersymmetry-breaking scenarios are discussed.

SUSY GUTs dark matter

We examine the cosmology of the lightest neutralino (χ) in a minimal SUSY GUT model. Our analysis incorporates the ensuing relations among the squark, slepton, and gaugino masses, and takes into account the recent experimental constraints on the parameter space. We consider a general neutralino state and compute its cosmological relic density resulting from pair annihilation into all allowed final states. Our analysis reveals a rich structure of resonances due to s-channel exchanges, and shows that some previous studies using pure neutralino states in “nearly” pure regions of parameter space give rather poor estimates for mχ < MW. We show that in the regions of parameter space favored by cosmology, the neutralino is a “nearly” pure bino state with mχ⪆ 100 GeV. The SUSY GUTs mass relations then imply m∼l ⪆ 200 GeV and mq,g ⪆ 600 GeV, all well within the reach of future hadron colliders.

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 [ital Z] boson and the lightest Higgs boson ([ital 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 choose 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.

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. The radiative breaking constraint imposes novel correlations between the traditional relic density variables and makes the Higgs-boson masses variable and generally heavy throughout the allowed parameter space. We perform an exhaustive search of this five-dimensional space and find that the neutralino relic abundance

Moduli and Kähler potential in fermionic strings.

{\ensuremath{\Omega}}_{\ensuremath{\chi}}{h}_{0}^{2}

SL (∞,R) symmetry of quantum W∞ gravity

depends most strongly on the ratio

Higgs boson masses in no-scale supergravity

{\ensuremath{\xi}}_{0}=\frac{{m}_{0}}{{m}_{\frac{1}{2}}}