Phenomenology with supersymmetric flipped SU(6)

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The supersymmetric flipped SU(6) × U(1) gauge symmetry can arise through compactification of the ten-dimensional E 8 × E 8 superstring theory. We show how realistic phenomenology can emerge from this theory by supplementing it with the symmetry R × U(1), where R denotes a discrete ‘R’-symmetry. The well-known doublet-triplet splitting problem is resolved to ‘all orders’ via the pseudo-Goldstone mechanism, and the GUT scale arises from an interplay of the Planck and supersymmetry breaking scales. The symmetry R × U(1) is also important for understanding the fermion mass hierarchies as well as the magnitudes of the CKM matrix elements. Furthermore, the well-known MSSM parameter tan β is estimated to be of order unity, while the proton lifetime ( τ p ∼ 10 2 τ pSU(5) ) is consistent with observations. Depending on some parameters, p → Kμ + can be the dominant decay mode. Finally, the observed solar and atmospheric neutrino ‘anomalies’ requir us to introduce a ‘sterile’ neutrino state. Remarkably, the R × U(1) symmetry protects it from becoming heavy, so that maximal angle ν μ oscillations into a sterile state can explain the atmospheric anomaly, while the solar neutrino puzzle is resolved via the small angle ν e − ν τ MSW oscillations. The existence of some (∼ 15–20% of critical energy density) neutrino hot dark matter is also predicted.